Excerpt
Contents
ACKNOWLEDGMENT
LIST OF TABLES
List of Figures
NOMENCLATURE
CHAPTER 1 Introduction
1.1 MOTIVATION
1.2 APPLICATION OF WATER OVER WIND TUNNELS
1.3 BASIC REQUIREMENT OF THE WATER TUNNEL
CHAPTER 2 Literature Review
CHAPTER 3 Theoretical Studies
3.1Basic Definitions
3.1.1 Basic Formulae’s
3.2 Inlet Plenum
3.2.1 Baffle
3.2.2 Honeycomb
3.3 Convergent Section
3.4 Test section
3.5 Diffuser
3.5.1 Wide Angle Diffuser
3.6 Outlet Module
3.7 Computer Aided Design Model
CHAPTER 4 Fabrication Process
4.1 Base
4.2 Side Walls
4.2.1 Manufacturing of Polymer Sheets
4.2.2 Reaction with Polymer
4.2.3 Properties of General Purpose BQTN (Bayesian quantitative trait nucleotide)
4.3 Test Section
4.3.1 Material of Acrylic Glass
4.3.2 Advantages of Acrylic Glass
4.4 Baffle
4.5 Honeycomb Structure
4.6 Fabricated model
4.7 Tools and Machinery
4.8 Water Pump and Plumbing
4.8.1 Plumbing
4.8.2 Working
4.9 Accessories
4.9.1 Dye Bottles and Stand
4.9.2 Dye Injection System and Stand
4.9.3 Camera Stand
4.9.4 Test Model mount
4.9.5 Manometer
4.9.6 Tachometer
CHAPTER 5 Experimental Results
5.1 Flow Visualization
5.1.1 Injecting External Material to the Flowing Fluid
5.1.2 Optical Method for Compressible Flow
5.1.3 Introducing Heat or Electrical Energy to the Fluid
5.2 Dye Injection System
5.2.1 Ratio and Proportion
5.3Model Testing
5.3.1 Procedure 1: (Dye Making)
5.3.2 Procedure 2: (Model Test)
5.4 Flow Visualization on different model
5.4.1 Cylinder
5.4.2 Eppler E854
5.4.3 NACA 2412
5.4.4 Delta
5.4.5 Nozzle Section
5.4.6 Diffuser Section
5.4.7 C – D Section
5.4.8 Propeller
CHAPTER 6 Computational Results
6.1 2D Analysis
6.1.1 Grid Generation
6.1.1.1 Cylinder
6.1.1.2 Eppler E854
6.1.1.3 NACA 2412
6.1.1.4 Delta
6.1.1.5 Nozzle Section
6.1.1.6 Diffuser Section
6.1.1.7 C – D Section
6.1.2 Computational Results of 2D Models
6.1.2.1 Cylinder
6.1.2.2 Eppler E854
6.1.2.3NACA 2412
6.1.2.4 Delta
6.1.2.5 Nozzle
6.1.2.6 Diffuser
6.1.2.7 C-D Section
CHAPTER 7 Comparison of Experimental and Computational Results
7.1 Cylinder
7.2 Eppler E854
7.3 NACA 2412
7.4 Delta
7.5 Nozzle Section
7.6 Diffuser Section
7.7 C –D Section
CHAPTER 8 Conclusion
8.1 Future Scope
BIBLIOGRAGHY
ACKNOWLEDGMENT
We wish to express our profound sense of deepest gratitude and sincere thanks to my honourable and esteemed guide Asst. Prof. Pawan Kumar Karn of Department of Aeronautical Engineering for his exemplary guidance, encouragement and untiring support. His trust and support inspired us in the most important moments of making right decisions to us and we thank him from the bottom of our heart.
We would like to thank Prof P B Khope, Head of Aeronautical Engineering Department for availing us all the necessary facilities for this work.
We convey our sincere thanks to all the faculties of Aeronautical Engineering Department especially Asst. Prof. Manoj Kharbade who have enlightened us during our studies. The facilities and co-operation received from the technical and non-technical staff is thankfully acknowledged.
Last, but not least, we would like to thank the authors of various research articles and books whose work has been consulted, utilized and cited in our project work.
LIST OF TABLES
3.1 Parameters of Converging Section
3.2 Co – ordinates of Walls of Converging section
3.3 Shows the Results of Reynolds and Frounde Number
3.4 Shows Cavitation Number
4.1 Specifications of Water Pump
5.1 Types of Flow Visualization Methods
5.2 Observation Table
5.3 Coefficient of Drag for Cylinder
5.4 Cylinder (δ, Δ and θ)
5.5 Coefficient of Drag for Eppler E
5.6 Eppler E854 (δ, Δ and θ)
5.7 Coefficient of Drag for NACA
5.8 NACA 2412 (δ, Δ and θ)
5.9 Coefficient of Drag for Delta
5.10 Delta (δ, Δ and θ)
List of Figures
3.1 Sketch of Honeycomb Cells
3.2 Velocity Profiles for a convergent Section
3.3 Schematics of Contraction Shape
3.4 Parameters of Wide Angle Diffuser
3.5 Schematic of CAD model
3.6 Top view of CAD model
3.7 Side view of CAD model
3.8 CAD model of Baffle
3.9 CAD model of Honeycomb Structure
3.10 CAD model of Bottle Stand
3.11 CAD model of Mobile Stand
3.12 CAD model of Model Mount Stand
4.1 Blue print of Base of Water tunnel
4.2 Base with PVC sheet
4.3 Polymer sheet
4.4 Baffle and Honeycomb Structure
4.5 Actual Fabricated model
4.6 RPM vs. Velocity
4.7 Discharge vs. Velocity
4.8 Water Pump
4.9 Block diagram of regulator switches
4.10 Dye Bottle and Stand
4.11 Dye injection stand
4.12 Camera stand
4.13 Model mounting stand
4.14 Manometer
4.15 Tachometer
5.1 Cylinder
5.2 Eppler E854
5.3 NACA 2412
5.4 Delta
5.5 Nozzle Section
5.6 Diffuser Section
5.7 C – D Section
5.8 Propeller
6.1 Mesh over Cylinder
6.2 Mesh over Eppler E854
6.3 Mesh over NACA 2412
6.4 Mesh Over Delta
6.5 Mesh Inside the Nozzle
6.6 Mesh Inside the Diffuser
6.7 Mesh Inside the C – D Section
6.8 Velocity Streamline for Cylinder
6.9 Dynamic Pressure Contour Of Cylinder
6.10 Velocity Streamline for Eppler E854
6.11 Dynamic Pressure Contour for Eppler E854
6.12 Velocity Streamline for NACA 2412
6.13 Dynamic Pressure Contour for NACA 2412
6.14 Velocity streamline for Delta
6.15 Dynamic Pressure Contour of Delta
6.16 Velocity Streamline in Nozzle
6.17 Dynamic Pressure Contour of Nozzle
6.18 Velocity Streamline in Diffuser
6.19 Dynamic Pressure Contour of Diffuser
6.20 Velocity Streamline in C-D Section
6.21 Dynamic Pressure Contour of C – D Section
6.22 Shows the open surface model for analysis
6.23 Shows the closed surface model modified for analysis
6.24 Volumes Created for Meshing
6.25 Shows the Unstructured Mesh
6.26 Shows Velocity Magnitude vs. Position Graph
6.27 Shows Velocity Streamline Pattern
7.1 Comparison of Cylinder
7.2 Comparison of Eppler E854
7.3 Comparison of NACA 2412
7.4 Comparison of Delta
7.5 Comparison of Nozzle Section
7.6 Comparison of Diffuser Section
7.7 Comparison of C - D Section
NOMENCLATURE
Abbildung in dieser Leseprobe nicht enthalten
CHAPTER 1 Introduction
A water tunnel is an experimental setup for studying the hydrodynamic behavior of the submerged bodies in the flowing water. It is analogous to that of the wind tunnel except that the working fluid is water. Water tunnels are used in place of the wind tunnels to calculate different forces such as lift and drag on different test bodies. But water tunnels are widely used for flow visualization over the submerged bodies. It is also experimentally used for measurements of particle image velocimetry (PIV) as it is easy and sophisticated to implement in water rather than in any other working fluid. As long as the Reynolds number is controlled and well within the limits, the results are valid to calculate details same as that in air for most of the cases. For low Reynolds number flows, tunnels can be made to run with oil instead of water. The advantage is that the increased kinematic viscosity will allow the flow to be a faster speed for a lower Reynolds number.
Water tunnels belong to the category of experimental aerodynamics, as almost universally they are of scale and scheme adequate with what one finds in university laboratories, rather than showpieces of government or industry installations. Water-tunnel models can also be built quicker and cheaper than wind-tunnel models.[30]
1.1 MOTIVATION
Flow visualization on different bodies such as airfoil, cylinder, propeller etc. Plays a very dominant role in studying behaviour of flow over a body for aero dynamist and hydro dynamist. Wind tunnel and water tunnel provides easy solution for it. Wind tunnel is suitable for the high speed flow visualization but requires a good technology for visualization such as high speed cameras. Sometimes it is not possible to properly visualise flow over body at very low speed, it is where water tunnel can be use.
Water tunnel is very useful in analysis the behaviour of micro aerial vehicles due to their low velocities, hence because easy to study the dynamics behaviour. Flow of fluid over a model such as airfoil and cylinder can be very easily and at a very low cost can be seen with less effort and technology. Behavior of various submerged bodies (in water) such as submarine and ship propellers can be studied using water tunnel.[11],[30]
1.2 APPLICATION OF WATER OVER WIND TUNNELS
As with wind tunnels, two water-tunnel arrangements are possible: the open-circuit type and the closed-circuit, or return-flow type. Practically, only the return-flow type, where pumps or propellers are used to circulate the flow through a closed loop, is important. Open circuits are practical only where large volumes of water under suitable head can be circulated through the device and wasted. The simplicity associated with the open-circuit wind tunnel, where air is drawn from the atmosphere and returned to the atmosphere, is difficult to duplicate for the water tunnel.
Water-tunnel working sections, again like wind tunnels, can be of the "open jet" type or "closed jet" type. In the "open jet" type a jet discharges submerged into a water-filled chamber whose dimensions normal to the jet axis are somewhat larger than the jet thickness. After travelling a certain distance submerged, the jet water enters a "gathering" nozzle and is conducted away. The body under test is supported in the jet. This arrangement is designed to give a constant static pressure over the length of the working section. By contrast, in the "closed jet" type the water merely flows through a tube of rectangular or circular cross section. The test body is again supported in the stream. This arrangement provides steadier flow conditions, but with an appreciable pressure drop in the direction of motion.[11],[30]
1.3 BASIC REQUIREMENT OF THE WATER TUNNEL
Basically, the water tunnel is a device for simulating the conditions of relative motion obtained when free bodies move in an infinite body of fluid. As developed to date it is used almost exclusively under steady flow conditions. There are several requirements of the flow circuit as follows: First, a uniform velocity distribution is necessary throughout the working section space to be occupied by the model. Second, provision must be made for maintaining steady flow in the working section and, while not absolutely necessary; means of varying the flow velocity is a practical essential. Third, of course, some means must be provided for supporting the test model in place in such a manner that the support offers a minimum of "interference" to the flow about the body. Fourth, the feature that distinguishes the water tunnel from the ordinary flume, and makes possible the most important applications is the provision for controlling the pressure in the working section.
The method of obtaining a uniform velocity distribution in the working section is essentially the same for all tunnels. First an effort is made to assure a flow free from eddies. Steady flow in the working section requires that complete stability of the flow in the water-tunnel circuit and within the circulating pump or propeller be met jointly. First, then, the circuit design must eliminate possible causes of unsteady conditions such as zones of eddy formation in the converging section, test section, diffusers and various joints of plumbing systems. Design of these items requires careful consideration of effects of turbulence, boundary layer growth, and even cavitation on flow separation.
The provision for supporting the test model depends on the kind of model and the kind of information required. The axial shaft required for propellers is also useful for measuring the drag (thrust) on some other types of bodies. When more than one component of the hydrodynamic forces is to be measured the model is usually supported from the side or bottom by struts or wires brought in normal to the flow. In order that the flow about the free body is reproduced as faithfully as possible all supports must be "streamlined" or surrounded by streamlined shields so as to disturb the flow as little as possible.[11],[30]
CHAPTER 2 Literature Review
In the past, water tunnels have primarily been used to carry out detailed flow-visualization studies using scaled models, such as aircraft. They are better suited to such studies than are wind tunnels, due to water having a higher density and lower mass diffusivity than air, and the fact that the free-stream velocities used in water tunnels are generally substantially less than those used in wind tunnels. Flow-visualization can give an insight into the complex flow behavior around models, enabling researchers to obtain an understanding of the fluid dynamics of the flow.
Harshad Kalyankar, Rishabh Melwanki, Dinesh Choudhary, Siddharth Jethwa and Dipak Chaudhari designed and fabricated water tunnel with a test section of dimension 10 x 15 x 70 cm. The setup was capable of generating a flow speed between 0cm/s to 21cm/s. Flow visualization was carried out using external dye injection technique. A Kirloskar Brothers Ltd. KDS-225++,with flow rate 19.08m3/hr, and maximum pressure head of 26m is used as the pump for recirculating the water in the setup. They also proposed other suitable dye injection techniques. The flow in the setup was analysed using CFD simulation. The setup was thus optimized to persist laminar flow throughout the water tunnel. For a particular set of input parameters, the desired velocity of 8cm/s was achieved.[2]
James H. Bell and Rabindra D. Mehta has developed an iterative design procedure for three-dimensional contractions installed on small, low-speed wind tunnels. The procedure consists of first computing the potential flow field and hence the pressure distributions along the walls of a contraction of given size and shape using a three-dimensional numerical panel method. For small, low-speed contractions it is shown that the assumption of a laminar boundary layer originating from stagnation conditions at the contraction entry and remaining laminar throughout passage through the “successful” designs is justified. This hypothesis was confirmed by comparing the predicted boundary layer data at the contraction exit with measured data in existing wind tunnels. From the contraction wall shapes investigated, the one based on a fifth-order polynomial was selected for installation on a newly designed mixing layer wind tunnel. [3]
Ranjan Basak, Debojyoti Mitra and Asis Mazumdar designed various components of an open Circuit blower tunnel without exit diffuser. They have performed aerodynamic design of open circuit wind tunnel driven by a centrifugal blower connected to the settling chamber by a wide angle diffuser. The design or choice of blower, diffuser, screens, contraction and working section are described. They have a strong influence on tunnel performance. The design rules and suggestions are mainly based on data collected from successful blower tunnel design. It is more feasible and sensible to predict design boundaries based on the data from existing tunnels which are known to perform satisfactorily. [4]
CHAPTER 3 Theoretical Studies
3.1Basic Definitions
- Viscosity: Viscosity is defined as the property of a fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid.
- Kinematic Viscosity: It is defined as the ratio between the dynamic viscosity and density of fluid.
- Vapour pressure: Vapour pressure is defined the when vaporization of a fluid takes place, the molecules of escape from the free surface of the liquid surface of the liquid. These vapours molecular get accumulated in the space between the free liquid surface and the top of the vessel. These accumulated vapours exert a pressure at which the liquid is converted into vapours.
- Cavitation: Cavitation is defined as if the pressure at any point in the flowing liquid becomes equal to or less than the vapour pressure, the vaporization of the liquid starts. The bubbles of these vapours are carried by the flowing liquid into region of high pressure where they collapse, giving rise to high impact pressure. The pressure developed by the collapsing bubbles is so high that the material from the adjoining boundaries gets eroded and cavities are formed on them. This phenomenon is known as cavitation. [24], [26], [27]
3.1.1 Basic Formulae’s
- Mass Flow Rate: Mass flow rate is defined as the mass of a substance which passes per unit of time.
M = ρ A V
(3.1)
- Reynolds’s Number : Reynolds’s number is defined as the ratio of inertia force to viscous force. Mathematically it is represent as
Abbildung in dieser Leseprobe nicht enthalten
(3.2)
- Boundary Layer Thickness: Boundary layer thickness defined as the distance from the boundary of the solid body measured in y direction to the point, where the velocity of the fluid is approximately equal to 0.99 times the free stream velocity. Mathematically it is represent as
Abbildung in dieser Leseprobe nicht enthalten
(3.3)
- Displacement Thickness: Displacement thickness is defined as the distance, measured perpendicular to the boundary of the solid body, by which the boundary should be displaced to compensate for reduction in flow rate on account of boundary layer formation. Mathematically it is represent as,
Abbildung in dieser Leseprobe nicht enthalten
(3.4)
- Momentum Thickness: Momentum thickness is defined as the distance, measured perpendicular to the solid body, by which the boundary should be displaced to compensate for the reduction in momentum of the flowing fluid on account of boundary layer formation. Mathematically it is represent as,
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- Cavitation Number: Cavitation number defined as it is difference between point pressure and vapour pressure of fluid to its dynamic force. Mathematically it is represent as,
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- Froude number: The Froude number is a dimensionless number defined as the model or body’s inertia force to gravitational force. Mathematically it is represent as,
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- Epoxide number: The Epoxide number is defined as number of Epoxide equivalents in 1kg of resin or as the equivalent weight, which is the weight of Epoxide (g/mole). The equivalent weight or Epoxide number is used to calculate the amount hardener during making the mixture of resin and hardener.
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- Hydrostatic law: It states that rate of increase of pressure in a vertical direction is equal to weight density of the fluid at that point. Mathematically it is represents as,
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3.2 Inlet Plenum
Inlet plenum acts as a reservoir tank, main function of the inlet plenum is to store the sufficient amount of water. When water pumps out from inlet through pipe then water is collected in inlet plenum, because of area of inlet plenum increases drastically the velocity of flow reduces which is higher than the diversion section. [2], [8]
Inlet plenum consist of two section
- Settling Chamber: Settling Chamber reduces flow velocity and turbulence intensity by providing space for water molecules. Settling chamber is rectangular in shape. Dimension of settling chamber is given below.
Abbildung in dieser Leseprobe nicht enthalten
To provide sufficient amount discharge of water to converging section we had retain some parameter such as:
1. Width of the settling chamber must be same as that of width of converging section
2. Length of the settling chamber must be 2 times of the inlet diameter of test section, but we have preferred more than it to occupy more amount of water.
- Semicircular Section: Main function of the semicircular section is to guide the backward flow which is coming out from the inlet towards the honeycomb structure to reduce the turbulence of water through the circular junctions. Semicircular section is having the radius of 33cm which is more than the length of the settling chamber. Amount of water store in semicircular section is 17.10 litres.
3.2.1 Baffle
“Baffle” is the concept which is used to cover inlet delivery point of the water tunnel. It is useful to overcome the problems like overflow of the water. When it gushing upwards in the inlet piping system and entering the inlet plenum of the setup. When water coming out from inlet delivery with high velocity and high energy at that time also it is helpful to slowdown the energy and velocity of water. In the result, it’s reduces the turbulence energy of the incoming water and it gives us laminar and smooth flow which is very required while analysis in water tunnel. We have used the baffle, which is made up of the PVC (Polyvinyl chloride) pipe with diameter of 20 cm, thickness of 0.5 cm and the height of the baffle is 20 cm. Holes were drilled with standard 0.5 cm drill bit and 0.25 cm drill bit for effective performance. The space between two successive holes is 0.5 cm. [2]
3.2.2 Honeycomb
The purpose of the “Honeycomb” is also the same as the baffle, but it can’t be said that they both are same. Honeycomb is the special shape of cardboard and it is the structure made up of a many adjacent hollow tubes or cells through which the fluid constrained to flow. There purpose is to convert the turbulence flow to laminar flow and reduce the transverse component of velocity fluctuation. The cell size depends on the scale of transverse disturbances and the optimum proportions appear to be obtained with length/diameter ratio of 6 or 8. It is necessary to obtain the laminar flow while analyzing the models. The need for free stream turbulence level in water tunnel has increased over the last few years. Honeycomb must be place before converging section to obtain the laminar flow in the test section. [1], [9]
As discussed earlier, the straight rectangular structure (duct) of honeycomb must be installed before the convergent section. A distance of 30-40 the cell sizes between the honeycomb and contraction is sufficient for the turbulence decay. The average cell size M of honeycomb was estimated to be: (Refer Fig. 3.1)
Abbildung in dieser Leseprobe nicht enthalten
(3.10)
Where,
= 0.459” is the average height of the cell, and
= 0.571” is the average width of cell.
Thus, approximately 20 inches (L/M≈40) had to be allocated for the straight duct. The original test section length was considered reasonable in order to observe the flow, downstream of a typical sized object placed in it
As per our above discussion and dimensions of the convergent section we have used the honeycomb of the dimension of length is 11 cm, height of 20 cm and width of 68 cm.
3.3 Convergent Section
A recirculating type of water tunnel is designed to produce a steady stream of fluid having uniform velocity, pressure and turbulence characteristics in the test section. It is however evident that the test stream as it leaves the test section and enters any conventional type of channel recirculating system will progressively deteriorate in the uniformity of energy distribution, due to growth of boundary layer generated by action of fluid and channel walls. The eventual result of such boundary layer growth will be evidenced by velocity profiles processing low values near the walls and high values in the core of the flow, a characteristic which will require major correction before returning the flow to the test section. Therefore it is necessary to provide some flow device which will accelerate the velocity from low value employed for economy in recirculation to high value required in the test section and at the same time provide unifying counteractive influence on the velocity profile. (Refer Fig. 3.2)
The turbulence characteristics of the open water conditions which the tunnel test stream attempts to stimulate are not too well known. In general turbulence with equal strength in all directions and low relative values is to be expected in the usual natural flow fields, except in boundary regions. [6]
Although test stream value is largely reliant on achieving a highest contraction ratio, it is evident that financial side and other factors may limit optimal values to more practical ranges. In an attempt to select an area ratio providing all necessary flow quality together with realistic concern of physical size and cost, the area ratios of existing wind and water tunnels were reviewed. The results of this comparison indicate that low turbulence wind tunnels with ratios as high as 25 have been employed, but the ratios for general purpose tunnels are more usually limited to the range from 5 to 10. In the case of water tunnels, ratios as large as 26 have been employed, but conventional design lies between values of 6 and 9. Because of this and the interest of obtaining high velocity uniformity with low turbulence and reasonable construction costs, an area ratio of 6 has been capriciously designated for application in this case. [3]
The main requirements were that a laminar boundary layer at low. This implies that the contraction length had to be minimized. This was in addition to the usual requirements of wanting to evade boundary layer separation on the walls and obtaining realistic mean flow uniformity at the convergent exit. The flow uniformity at the contraction outlet improves as the length is increased. This is not surprising since the radii of curvature decreases as the length is increased. (Refer Fig. 3.3)
Abbildung in dieser Leseprobe nicht enthalten
(3.11)
The 5th order contraction shape separated near the inlet where is 0.667. However, boundary layer separation is also predicted if the contraction is too long (= 1.79). This separation occurred due to the region of adverse pressure gradient near the contraction exit . A contraction that is too long allows the boundary layer to thicken (or its thickness is not reduced as much) and hence makes it more susceptible to separation near the exit. If the design is too short then boundary layer separation would also tend to occur, but this time near the inlet. The contraction wall shape satisfying most of the requirements is clearly the one given by the 5th order polynomial Eq. (3.11).
On the other hand, based on the calculated available length L/D ratios of more than 1.5 would require a shorter test section and/or smaller distance between the honeycomb and the contraction. [1]
The working and manufacturing dimensions and various other parameters are shown in table 3.1 and 3.2:
3.4 Test section
Test section is a major part of water tunnel. Following parameters were considered while designing of the test section:
1. Test section length
2. Velocity in test section
3. Strength
Parameter of test section:
1. Shape: Square test section was used to obtain equal pressure intensity as well as equal velocity distribution. It is easy do proper fitting with convergent and divergent section with no leakages, and it is easy for calculation. Length of the test section must two times of a model length hence test section length is 55cm.
2. Space of 10 cm is kept for dye injection apparatus.
3. The value of velocity which is getting in the range of 2 to 8 cm/s. While designing we considered a flow velocity of 2.5cm/s because at 2.5 cm/s and we getting laminar flow. However it is verified by Reynolds number calculation.
4. Dimensions: Length: 55 cm
Width: 14 cm
Height: 14 cm
5. Calculation for Reynolds, Froude’s and Cavitaion Number at a working Height of 11 cm is shown in table 3.3 and 3.4.
3.5 Diffuser
Diffusers are mainly used to increase the pressure in a ductwork or pipe work system containing air or water. The Fig. (3.4) indicates the two different areas, 1 and 2 i.e. the entry and exit areas respectively. The length of the diverging section is the distance between the parallel section 1 and 2. Diffusers may be rectangular in cross-section, square or circular. They may have a straight diverging section or this may be bell shaped in order to guide the flow by making it more streamlined.
The divergent section is downstream of the test section in the experimental setup; it is the region of increasing area and decreasing velocity. The divergent section acts as a diffuser for the subsonic tunnel, thus it reduces the velocity of the flow after the test section and prevents the backpressure of the flow as water do not expand much compared to gases the divergent section will have low diverging angle thus increasing its length comparatively.
3.5.1 Wide Angle Diffuser
A “wide-angle” diffuser is defined as a diffuser in which the cross- sectional area increases so swiftly that separation can be avoided only by using boundary layer control. A wide angle diffuser should be regarded as means of reducing the length of a diffuser of given area ratio. Uniformity and steadiness of the flow at the diffuser exit are of prime concern since this affects the performance of vital component downstream. [4]
The three most important parameters in wide angle diffuser are:
1. Area Ratio, A
2. The diffuser angle, 2θ
3. The total pressure drop coefficient, K
Design rules collected from the existing wind tunnels for wide angle diffuser, the area ratio should not be more than 4:1 and equivalent cone angle is normally taken as 45°. While designing wide angle diffuser it is assume that the total pressure drop coefficient K is 2 and diffuser angle 2θ is 60°. [4]
Abbildung in dieser Leseprobe nicht enthalten
(3.12)
For the assume value of K we have found area ratio as 3.28.
Abbildung in dieser Leseprobe nicht enthalten
The various design parameters of the diffuser are as follows:
Abbildung in dieser Leseprobe nicht enthalten
3.6 Outlet Module
The purpose of the outlet module is to connect the wide angle diffuser with the piping system to drain out the flow. The geometry of the outlet module should be design properly because if any irregularities occur in the model it leads to the reverse flow or back pressure in the system which will disturb the results in the test section. Outlets with two opening are designed so that the water flow is distributed equally. The curved surface after the diverging section guides the flow towards the holes thus making a smooth exit of the flow and prevents any back pressure in the system. [2]
When the setup was tested for the flow visualization, at the outlet module it was found that swirls where produced at the outlet. This swirl causes the air to mix up with the flow getting in the inlet of the motor. This in turn reduces the efficiency of motor and also disturbs the incoming flow at some scale. In order to overcome this problem we have used small baffles over the outlet ends. The baffle eliminates the swirl and makes the flow smooth and laminar at the outlet module.
The design parameters of the outlet module for our experimental setup are given below:
Exit diameter: 2.5 cm
Outer curve diameter: 25 cm
Distance between the two curves: 20 cm
3.7 Computer Aided Design Model
On the basis of above mention design criteria CAD model of each component was design in commercial CAD software. Each component was designed section wise in part design and was assembled as one product. Fig 3.5 shows the isometric view and schematics of CAD model. Fig 3.6 shows the top view. Fig. 3.7 shows the side views. Fig. 3.8 shows baffle. Fig. 3.9 shows honeycomb structure. Fig. 3.10 shows bottle stand. Fig. 3.11 shows camera stand. Fig. 3.12 shows model mounting stand.
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Figure 3.1 Sketch of Honeycomb Cells
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Figure 3.2. Velocity Profiles for a convergent Section
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Figure 3.3Schematics of Contraction Shape
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Figure 3.4 Parameters of Wide Angle Diffuser
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Figure 3.5 Schematics of CAD Model
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Figure 3.6 Top View of CAD Model
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Figure 3.7 Side View of CAD Model
Figure 3.8 CAD Model of Baffle
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Figure 3.9 CAD Model of Honeycomb Structure
Figure 3.10 CAD Model of Bottle Stand
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Figure 3.11 CAD Model of Mobile Stand
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Figure 3.12 CAD Model of Model Mount Stand
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Table 3.1 Parameters of Converging Section
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Table 3.2 Co – ordinates of Walls of Converging section
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Table 3.3 Shows the Results of Reynolds and Frounde Number
Abbildung in dieser Leseprobe nicht enthalten
Table 3.4 Shows Cavitation Number
CHAPTER 4 Fabrication Process
A detail description on the fabrication process and material used to manufacture different parts of water tunnel which are as follows:
1. Base
2. Side Walls
3. Test section
4. Baffle
5. Honeycomb.
4.1 Base
Base is the structure on which the whole water tunnel is constructed and supported. Since it had to bear various loads such as load of flowing water and structural load, it has to be rigid and strong. Hence, base is made up of plywood to give enough rigidity, but since water has to be use in water tunnel plywood proves to be a bad material. To overcome this problem plywood was cover with 2mm thick PVC sheet to make it water proof and durable. PVC sheet and plywood are glued together using Synthetic Rubber Adhesive (SR).
Base layout was drawn in CAD designing software and was printed on sheet of paper. Further the plywood was properly cut from the edges of the layout drawing to give the desired shape and size. Figure 4.1 (a) and (b) shows the CAD design and the actual cut-out base of water tunnel.
4.2 Side Walls
Due to the curvature of the wall it was not possible to use plywood hence composite material was used and given desired shape.
4.2.1 Manufacturing of Polymer Sheets
The process of manufacturing composite wall (polymer) is as follows:
- The general purpose BQTN polymer was mixed with hardener and accelerator in the proportion of 40:3:1.
- A layer of flexible PVC sheet fixed to the plywood borders such that a guiding wall was ready for polymer solution could be attached to it just before it hardens completely.
- The prepared solution of general purpose resin, hardener and accelerator was poured into the glass mould made separately is coated from inside with oil so that polymer should not stick with the wall of the mould.
- A net of fine grid with the dimension of mould was placed inside the polymer to provide it extra support and strength.
- When the solution was in a state of semi-solid, it was taken out of the mould and fixated on the PVC wall of the water tunnel.
- To keep the polymer sheets in place, sun mica was placed adjacent to polymer sheet.
- The acrylic sheets were then attached to the test section and are allowed to dry for at least 24 hours before any further was carried out. [17]
4.2.2 Reaction with Polymer
- Hardener make cross bond with resin. Petroleum is used as raw material for resin. Epoxide number is a major criteria during the composition and the reactor (hardener) amount is depends upon the Epoxide number.
- Accelerator does not take part in the chemical reaction but acts as the catalyst.
- The molecular chain of the resin (uncured) is given below:
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- The molecular chain of resin (with cured) is given below:
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- Chemical and Proportion (in percentage)
- Accelerator: (Cobalt Octoate) = 2%
- Hardener: (Methyl Ethyl Ketone): Its concentration depends upon the Epoxide number[19]
4.2.3 Properties of General Purpose BQTN (Bayesian quantitative trait nucleotide)
- Outstanding mechanical strength
- Good impact strength and rigidity
- Light weight
- Excellent durability
- Corrosion resistive
- Extremely low water absorption
- Used widely in making of FRP products like canoe, storage tank, boat, telephone booth, dustbin, seat, bus shelter, bath tub, etc.
- It can be moulded into any required shape.
4.3 Test Section
Test section is a part where the major tests on various models are carried out and flow can be seen over a model. Hence test section was made up of 2mm thick Acrylic glass. Acrylic glass is a very strong and rigid material with a transparency same as that of normal glass. The main advantage of it is that it does not break easily on normal load impact. It is screwed to the base of the model and the small gap between them is filled with SR to prevent leakage of water from the joints. Measuring scale is also attached to the test section to measure the level of water in it.
4.3.1 Material of Acrylic Glass
1. IUPAC Name: Methyl 2- Methyl Propenote.
2. CAS Number: 9011-14.7
3. Chemical formula: (C5O2H8)n
4. Density :1.18 g/cm3
5. Melting point: 160°C
4.3.2 Advantages of Acrylic Glass
1. Acrylic glass transmits more light than normal glass.
2. The value of thermal conductivity is much higher than the glass.
3. It is easy to clean in order to remove dirt.
4. Acrylic glass has higher impact strength than normal ordinary glass.
5. It sustain high pressure load.
4.4 Baffle
We used the baffle, made up of PVC pipe of diameter 20 cm, thickness 0.5 cm and height of 14cm. Holes of diameter 2.5cm and 5.0cm are drilled at an equal interval of 2cm and adjacent to each other as shown in figure 4.4.
4.5 Honeycomb Structure
Honeycomb structure is made up of water proof material so that it does not get wet in water and loose its effectiveness. A frame of plywood supports the honeycomb structure at its desired position and maintains its shape. Wire mesh of very fine grid strains any kind of impurities which could damage the test model and flow over its surface. Its dimensions are 68 x 11 x 14 cm (L x W x H).
4.6 Fabricated model
The figure 4.5 shows the actual model fabricated as discussed above for all the different parts of the water tunnel.
Leaks were prevented by injecting SR between the joints of different section such as test section (Acrylic glass), converging section, joints of diffuser section etc. Also M-seal was also use to further eliminate any leakages. The whole model was supported on the metal frame table with extra plywood as a support. Table, water tunnel and plywood are placed at a proper place by using nut and bolts. The supporting plywood was also made water-proof by applying a thin layer of polymer over its exposed surface.
4.7 Tools and Machinery
Various tools and machines used during the fabrication process are as follows:
- Cutter machine
- Drill machine
- Grinder
- Hammer
- Chisel
- Plier
- Screwdriver
- Adjustable wrenches
- Hack saw
- Sheet metal cutter
4.8 Water Pump and Plumbing
Water pump is use as a water circulating medium in the water tunnel between the inlet and the outlet of close circuit setup. For the low speed water tunnel the pump can achieve the maximum water velocity of 8cm/sec in the test-section where as the minimum water velocity was found to be 2 cm/sec by means of voltage regulator of 2000kW. Fig 4.6 shows change in velocity of the test section vs. change in RPM. Fig 4.7 shows change in velocity of the test section vs. discharge from pump.
4.8.1 Plumbing
Motor is connected to inlet and outlet by means of the flexible green pipes. Two pipes are connected to the inlet section of water pump for the suction whereas one delivery pipe is connected to the motor. Delivery pipe delivers water to the baffle connected at the water outlet i.e. before the converging section. M-seal is used to prevent leakages from the pipe.
GI metal bends are used at the corners and plastic connectors are use to attach the pipes to the water tunnel. Flexible green agricultural pipes are use for the connections because of their durability and semi-rigid nature. Flexibility of the pipe was found to be very effective in reducing the effects of vibration of the motor in test section and in whole apparatus. Water pump is mounted on separate stand with rigid base so as to avoid any interference of vibration with the test apparatus.
Inlet diameter of pipe
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4.8.2 Working
Water is suck from the two suctions provided at the outlet module aft of the test section. Two suctions were found to be more effective than one suction since it increases the water inlet to the motor and equal distribution of flow aft the test section, hence the level of turbulence was reduce to the greater level (Chap 3.7). Water is then discharge through a single outlet in the baffle to reduce discharge velocity and turbulence (Chap 3.2). Water is recirculated between the inlet and outlet continuously in close circuit.
The flow discharge and velocity of water in the test section is control by voltage regulator hence allows us to carry out test at various velocities ranging from 2 cm/sec to 8 cm/sec in the test section. By controlling the voltage to the motor, RPM of the motor is control, resulting into control in water discharge through outlet and velocity.
4.9 Accessories
Different accessories and equipments while performing the experiments are as follows:
4.9.1 Dye Bottles and Stand
Dye injection system consists of dye bottles and a stand. Bottles use for the dye injection is general saline bottles use in hospital with I.V. kit. Stand is made up of stainless steel and iron. The length of stand is 60cm in height and 4 or 5 bottles can be mounted at a time.
4.9.2 Dye Injection System and Stand
For the purpose of dye injection 3 syringe needles of 1.2mm diameter gauge and of length 38mm are use with their tips fin finely grinded to make a perfect circular section as shown in the figure. Needles are connected to the bottles with IV kit. Stand for mounting needled is made up of iron strip of 2mm thickness and 16cm height. Needles are placed at a mid-section of test-section.
4.9.3 Camera Stand
For the purpose of mounting camera to shoot the flow visualization while performing the practical flow analysis, mobile camera is position to provide the top view of test model. Mobile camera stand is made up of aluminum with camera mount.
4.9.4 Test Model mount
Models in the test section are mounted on a stand made up of aluminum. It allows test model to set at various angle of attack and rotate at an angle of 180 ͦ.Stand also facilitates model to position at any desire position or length along test section (up and down and to and fro).
4.9.5 Manometer
Manometer is used to measure pressure reading on the test model. The manometer used is inclined type manometer since it is very sensitive towards very small pressure change.
4.9.6 Tachometer
Tachometer is used to measure RPM of the motor to obtain various relations between motor and test section such as mass flow rate, velocity change, etc. in the test section.
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Figure 4.1 Blue Print of Base of Water Tunnel
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Figure 4.2 Base with PVC Sheet
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Figure 4.3 Shows Polymer Sheet
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Figure 4.4 Shows Baffle and Honeycomb Structure
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Figure 4.5 Actual Fabricated Model
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Figure 4.6 Shows RPM vs. Velocity
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Figure 4.7 Shows Discharge vs. Velocity
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Figure 4.8 Shows Water Pump
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Figure 4.9 Block Diagram of Regulator Switches
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Figure 4.10 Shows Dye Bottle and Stand
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Figure 4.11 Shows Dye Injection System
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Figure 4.12 Shows Camera Stand
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Figure 4.13 Shows Model Mounting Stand
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Figure 4.14 Shows Manometer
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Figure 4.15 Shows Tachometer
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Table 4.1 Specifications of the Water Pump
CHAPTER 5 Experimental Results
5.1 Flow Visualization
Flow visualization is a process of making flow patterns visible to the human eye, in order to study the flow properties of the fluid (gases, liquids). One can see such patterns in a coffee while stirring it or in any fluid, but to make the flow patterns visible flow visualization techniques are used and studied in wide range in different fields.
Engineers and scientists have discovered many techniques that allows flow pattern clearly visible to naked eye. The basic advantage of flow visualization is that certain flow properties of the fluid are directly visible and can be applied to many studies. Mainly there are two different regimes in a flow, one is laminar flow and other one is turbulent flow. The regime where laminar is converted into turbulent flow is called as transition regime, where we can see the flow patterns changing from laminar to turbulent. Flow separation and vortex generation are also observed and studied in flow visualization.
There are various techniques used for flow visualization:
1. Injecting external material to the flowing fluid.
2. Optical method for compressible flow.
3. Introducing heat or electrical energy to the fluid, etc.
These techniques are very useful for flow visualization and make our work simple. Fluids like water and air are transparent, hence the above techniques can help us to study and analyses the flow patterns on different models.[21],[22]
The different types of flow visualization techniques used in visualization of most of the fluids as they are highly used by many researchers and scientists to analyses the fluid flow properties.
5.1.1 Injecting External Material to the Flowing Fluid
In this technique dye is use as an external component to the fluid. Food liquid dye can be used, which gives beautiful flow patterns of the flowing fluid and also helps us to visualize the marked trajectories. Dye is injected adjacent to the testing model and boundary layer in the flow is studied. To show the streamlines of the flow, dye can be injected through a syringe or a needle which must be aligned to the local flow velocity. The flow velocity of the dye must match with the flow velocity of the water. Mostly contrast dye colors like red and blue are used to show flow patterns clearly .The viscosity of the dye used affects the flow over a body; hence suitable viscous dye should be used. The vortex shedding patterns and the boundary layer will be clearly visible in the flow. There should be no disturbance or vibration in the water as it may affect the streamlines of the flow. This technique is widely used by many researchers in the fluid dynamic experiments.[21]
5.1.2 Optical Method for Compressible Flow
This technique consists of optical method technique which uses the changes in refractive index of air caused by changes in density. This technique can also be used to investigate the flow in boundary layers. Schileren Method is one of such type. In Schileren Method, there is a light source and three lenses placed parallel to each other upon which light beam is passed and image is produced on a photographic plate. Sometimes mirrors are used instead of lens. Perhaps, we need high quality optical components, this method becomes a lot easier and cheaper which will give a good quality optical performance with mirrors than lens.[23],[25]
5.1.3 Introducing Heat or Electrical Energy to the Fluid
This technique includes hydrogen bubble technique in which bubbles are produced on the surface of thin platinum wire. The wire is used as cathode and metal or carbon is used as anode, which are placed at certain distance in the fluid. This technique is highly expensive. In this technique two electrodes are used which are immersed in water and a voltage is applied due to which hydrogen bubbles are developed near cathode. Typically the voltage ranges from 50-70 volt with a current of one ampere is usually applied. Salt or sodium sulphate is used as an electrolyte to enhance the formation of bubbles. A DC voltage is applied generating a current which passes through the water. These small hydrogen bubbles are trace down the model which gives flow patterns over a test model.[18]
5.2 Dye Injection System
Dye can be used to observe and visualize the streamlines of the flow. As it is the easiest and cheapest method to show flow patterns accurately. To see the flow patterns, dye is passed adjacent to the model. Dye injection system was used in the project. Food color is readily available in the market with different colors. Food color or food dye primarily consists of propylene glycol having chemical formula C3H8O2.The propylene glycol is a viscous liquid having density of 1.036 gram per meter cube. Dye is injected through a syringe having length of 38mm and diameter of orifice as 1.2mm. This dimension syringe was used to visualize the dye streamlines clearly and properly. This particular type of syringe is suggested by trial and error method and we found appropriate vortex shedding patterns. Three syringes with three different dye colors were used which showed three laminar streamlines. We used three syringes with three different colors to show the vortices and the wake regions near the test model which gave a clear view of the boundary layer in the flow.
The RL bottle and infusion IV set with a stand was used to hold the bottle. A velocity regulator was also used which controls the flow velocity of the dye. The set up used is same as the set up used to inject a person in a hospital. A portable stand was made to hold the syringe to a particular location without any disturbance .The viscosity of the dye or the food color used is 42 centipoises and velocity of the water was 8 cm per second.[16],[21]
5.2.1 Ratio and Proportion
Before using dye to visualize a flow over a body, dye has to be prepared by mixing the appropriate quantity of water. The water to dye proportion was taken as 8:1 ratio for the total dye contain of 2.5% in liquid food dye. Contain of water changes with the change in total dye contain in liquid dye. Total dye contain are printed on the bottle itself. Steps for preparation of dye is as follows
5.3Model Testing
Model testing is the most important step of the whole project. It is where flow over different models are carried out. But prior to model testing it is very essential to prepare dye and the whole setup to carry out test effectively. Procedure 1 and 2 listed below gives different steps to be carried out for dye preparation and model testing.
5.3.1 Procedure 1: (Dye Making)
1. Take 200ml of water in a beaker.
2. Add 25ml of colour liquid food dye to it (for 2.5% dye contain in the liquid dye).
3. Steer the mixture well to get the uniform concentration of dye.
(Note:-Dye contain of the liquid dye must be consider while making dye for test run i.e. 200ml of water for 2.5% total dye contain).
5.3.2 Procedure 2: (Model Test)
1. Level the test apparatus by bringing the bubble at centre of level indicator.
2. Switch on the main motor switch.
3. Fill the water to the desired level in test section.
4. Allow water to settle down (i.e. without turbulence) in test section before starting the test.
5. Attach the model to be tested in the stand.
6. Start the flow of dye from the needles.
7. Set the RPM of the motor to the desired value from the voltage regulator.
8. Observe the flow pattern on the body.
9. Tabulate the required data and manometer reading.
10. Drain all the water from the water-tunnel when experiment is over.
11. Table 5.2 shows the observation table.
5.4 Flow Visualization on different model
Flow visualization over different models is mention in following section:
5.4.1 Cylinder
When the fluid is passed over the cylinder as shown in fig. 5.1initially flow is laminar. Due to abrupt resistance provided by model the flow gets turbulent. As the flow pass over cylinder the energy of the particle decreases due to friction between fluid molecules and surface of the model. Swirls are formed behind the cylinder. Calculations for coefficient of drag are shown in Table 5.3 and calculation of boundary layer thickness, momentum thickness and displacement thickness are shown in table no. 5.4
5.4.2 Eppler E854
From the fig. 5.2 it can be seen that the flow reaching the leading edge of the airfoil is smooth and laminar. Two dyes are injected to observe the flow over the airfoil. The green dye is allowed to pass from the lower surface of the airfoil and the yellow dye from the upper surface. The green dye seems to be more laminar than the yellow one and the flow at the bottom is attached over a large surface compared to the upper one. The separation of flow before the expected point is due to the manufacturing defect of the airfoil model. The boundary layer effect over the height of the test section can be observed with help of the variation of the flow over the height represented by the increasing turbulence in the dyes across the height of the test section. Calculation of coefficient of drag are shown in Table 5.5 and calculation of boundary layer thickness, momentum thickness and displacement thickness are shown in table no. 5.6
5.4.3 NACA 2412
As it can be seen from the fig. 5.3 red dye is injected from the needle at a height of 5.5 cm from the bottom. Laminar flow can be seen on the upper surface of an airfoil up to the maximum camber. Flow separation takes place near the trailing edge due to loss of kinetic energy of a fluid on the body. It is observed that flow separates near the leading edge as we increase angle of attack. Calculation of coefficient of drag are shown in Table 5.7 and calculation of boundary layer thickness, momentum thickness and displacement thickness are shown in table no. 5.8
5.4.4 Delta
As shown in fig. 5.4 when fluid is passed over a delta, flow get separate from its leading edge as flow passes over the model velocity of flow get reduced. When flow leaves the tailing edge, vortices are formed which increases dead space behind the model. It is the area of the model where there is the greater reduction in flow velocity and also dynamic pressure takes place. Calculation of coefficient of drag are shown in Table 5.9 and calculation of boundary layer thickness, momentum thickness and displacement thickness are shown in table no. 5.10
5.4.5 Nozzle Section
Fig. 5.5 shows the variation of flow over different cross-section of converging section/ nozzle. Flow follows converging path and distorted flow can be seen at the minimum area which is the result of increase in velocity. Generally due to the increase in the velocity at the exit section increase in Reynolds number takes. As the Reynolds number increases flow becomes turbulent.
5.4.6 Diffuser Section
Exact opposite flow can be seen in the diverging section as shown in Fig. 5.6 Reduction in the velocity takes place as the flow pass the minimum area section. As the velocity reduces flow becomes more turbulent in nature and dye disperses in the outlet section. It is due to the fact that as the kinetic energy of the flow reduces flow does not have enough energy to pass down but incoming flow continuously exerts pressure on it and flow becomes turbulent.
5.4.7 C – D Section
Two dyes of red and yellow color are injected in the C-D section as shown in Fig. 5.7 Initially the velocity of flow is less in the converging section and as the flow progress velocity gradually increases till the throat section. The velocity of flow is maximum in the throat area and as the flow pass the throat velocity again decreases till the exit section.
5.4.8 Propeller
Fig.5.8 shows the flow visualization over the mini propeller blade. As the flow passes through the propeller we can see the turbulence behind the propeller. It is the flow pattern that the flow follows when a liquid fluid passes through it. Two color dyes can be seen mixing with each other behind the moving propeller.
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Figure 5.1 Cylinder
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Figure 5.2 Eppler E854
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Figure 5.3 NACA 2412
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Figure 5.4 Delta
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Figure 5.5 Nozzle Section
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Figure 5.6 Diffuser Section
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Figure 5.7 C-D Section
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Figure 5.8 Propeller
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Table 5.1 Types of Flow Visualization Methods
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Table 5.2 Observation Table
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Table 5.3 Coefficient of Drag for Cylinder
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Table 5.4 Cylinder (δ, Δ and θ)
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Table 5.5 Coefficient of Drag for Eppler E854
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Table 5.6 Eppler E854 (δ, Δ and θ)
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Table 5.7 Coefficient of Drag for NACA 2412
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Table 5.8 NACA 2412 (δ, Δ and θ)
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Table 5.9 Coefficient of Drag for Delta
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Table 5.10 Delta (δ, Δ and θ)
CHAPTER 6 Computational Results
6.1 2D Analysis
Test over different models such as cylinder, nozzle, diffuser, converging-diverging section, wedge etc. are carried out in commercial software to analyses flow pattern over different shapes and compare them with the actual test carried out in water tunnel. The models which are analyzed are as follows with velocity, dynamic pressure and static pressure contour.
The basic step is to create a model in commercial CAD software and save it in the .igs file format and then import the same geometry in the pre-processing software to generate the mesh. After importing the geometry check for multiple edges and curves since they will generate errors while creating faces on the geometry. Once the multiple edges and curves are deleted, split the curve of the domain and body to obtain multiple face regimes.
Nodes must be generated on the edges, adjust the nodes amount while adjusting nodes we have to provide high concentration where we want to catch boundary layer and other minute parameter. Concentration of nodes depends upon the model length.
While creating the mesh on the different faces, the number of elements on the opposite face must be equal to generate the proper mesh. Aspect ratio must be within 100 for the low subsonic speed of fluid over the body. Give different boundary condition to different sections.
The boundary conditions for the mesh model were defined as follows for external flow:
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As the velocity of water is at the inlet of the test section. As no conditions at the outlet face were known so outflow condition was define to it. And walls were defined with wall conditions. After defining the boundary conditions export the file to the commercial processing software and save the file with extension as msh.
In processing software various conditions are define which are essential for performing correct iterations. The steps that were followed before processing for iterations were as follows:
1. Read the mesh file which was saved during pre-processing.
2. Check the quality of the mesh created.
3. Scale the model if required in desire units
4. Define viscous-laminar flow.
5. Define material water – liquid (H2O) and its density and viscosity for 30 ͦC.
6. Define the cell zone conditions and select the fluid as water – liquid.
7. In operating conditions define the operating pressure at the height of 5 cm in addition to atmospheric pressure. (101815 Pa)
8. Define the boundary conditions which were earlier define during pre – processing software
a. Walls: Must be defined with specified shear in case of internal flow or else no slip condition must be defined.
b. Inlet: In Velocity Inlet, velocity of water must be defined which lies between 0.02 – 0.05 m/s in the test section.
c. Outflow: As no parameters were practically calculated at the outlet face.
9. Define the Reference Values:
a. Compute all the calculation from Inlet
b. Define area of model
c. Take the default values of Density, Enthalpy, Velocity and Viscosity.
d. Define the actual length of model.
e. Define the operating Pressure in Pa.
f. Define the temperature of fluid.
g. Define the ratio of specific Heats as 1.3 for water.
h. Select reference zone as fluid.
10. Define Solution Methods
1. Pressure Velocity Coupling
a. Select the scheme for pressure – velocity coupling as SIMPLE (Semi – Implicit Method for Pressure Linked Equations) because it is robust
b. Take the default values for skewness corrections as zero.
2. Spatial Discretization
a. Gradient: Select Least Squares Cell Based as it is recommended for unstructured mesh and it is known for its more accuracy and computationally less intensive.
b. Pressure: Select Standard.
c. Momentum: Select QUICK (Quadratic Upwind Interpolation) since it is suitable for Quad/Hex and Hybrid meshes and useful for rotating/ swirling flows.
11. Define the Solution Controls to its default value.
12. Edit Residuals monitor option by unchecking the convergence option or increase its absolute criteria so that it will be helpful for performing more iteration.
13. Create drag force residual monitor to find the drag coefficient on the body at different velocities.
14. Initialize the solution by using standard initialization method and compute all parameters from Inlet face. Let all values set to be as default values.
15. Auto save every iterations at the interval of 250 or depending upon the configuration PC.
16. Check the case, Make the corrections if recommended and define number of iterations that need to be performed and then calculate.
17. Stop the iterations when any of the residuals is converge.
18. Save the results as case and data.
Now the last step of 2D – Computational analysis is post processing. In post processing the computational results are verified with experimental results. Plot the results with respect to position along x – axis against velocity magnitude (m/s) from which velocity at different locations can be known and can be verified experimentally. Velocity streamline pattern can also be observed in the contours for proper flow pattern of water in the model.
6.1.1 Grid Generation
In computational solutions of computational fluid dynamics, meshing is an important part for analysing the flow over the models. In general, we can say meshing is applied for showing the proper working area to computer in the meshing software. There are various types of meshing options which are available in the software. Each has its own strength and advantage. The quality of mesh also plays an important role in meshing, so it must be maintained for better results.
Structured grids are known by their regular connectivity. The possible element choices are quadrilateral in 2d and hexagonal in 3D. In structured mesh, grids are of same shape. The advantages of structured grid over unstructured are better convergence and better resolution which results in efficient analysis result. That’s why the structured mesh was preferred on all the models for better results. The following image shows the typical structured mesh. Aspect Ratio must be less than 100 and the Equiangle Skew must be less than 1.[28],[29]
Meshing on 2D models was performed, which are explained below:
6.1.1.1 Cylinder
Cylinder was used as model and test section as a domain for analysis purpose and the mesh was obtained in between model and domain. Dense mesh is applied around the cylinder to observe the behaviour of fluid flow around it. First of all create cylinder and domain geometry and then split it into required no. of faces to achieve good possible ratio between faces Define the proper meshing nodes on every edge of every face, where boundary layer is needed to be captured. The aspect ratio of 5.8898 and equiangle skew of 0.493626 and total number of elements were 47300. The Fig.6.1 shows meshing over cylinder.
6.1.1.2 Eppler E854
Eppler E854 as model and test section as domain. To observe the proper flow pattern around the airfoil divide the geometry into required number of faces to achieve possible ratio between faces. Before creating the faces cut the trailing edge to eliminate sharp point and to observe proper flow behaviour at trailing edge. Define proper meshing nodes on every edge of every face and first length ratio of 0.001 around the airfoil to capture boundary layer. The aspect ratio of 95 and equiangle skew of 0.8. The total number of elements was 446200. . The Fig.6.2 shows meshing over Eppler E854.
6.1.1.3 NACA 2412
NACA 2412 airfoil was used as a model and the test section as domain. Define proper meshing nodes on every edge of every face and first length ratio of 0.001 around the airfoil to capture boundary layer. The aspect ratio of 100.84 and equiangle skew of 0.76337. The total number of elements was 130050. . The Fig.6.3 shows meshing over NACA 2412.
6.1.1.4 Delta
Delta model, the meshing was performed in parts by dividing the domain into number of faces. The domain has the same dimensions as that of the water tunnel's test section. The flow around the delta model needed to be observed. Create delta and domain geometry and then divide it into no. of faces to achieve possible good ratio between faces. Define the proper meshing nodes on every edge of every face. The aspect ratio of 377 and equiangle skew of 0.34 and total no. of elements were 190810. Fig.6.4 shows meshing over delta.
6.1.1.5 Nozzle Section
In case of nozzle there is no need of domain because analysis of the flow inside the model was been performed. Split the geometry in required no. of faces. The aspect ratio of 2.15764 and equiangle of 0.1525 and total no. of elements were 12000. Fig.6.5 shows meshing inside nozzle.
6.1.1.6 Diffuser Section
In case of diffuser there is no need of domain because analysis of the flow inside the model was been performed. Split the geometry in required no. of faces. The aspect ratio of 2.03 and equiangle of 0.1428 and total no. of elements were 11000. Fig.6.6 shows meshing inside diffuser.
6.1.1.7 C – D Section
In case of C – D section there is no need of domain because analysis of the flow inside the model was been performed. Split the geometry in required no. of faces. The aspect ratio of 6.7254 and equiangle of 0.1476 and total no. of elements were 22000. Fig.6.7 shows meshing inside C – D section.
6.1.2 Computational Results of 2D Models
6.1.2.1 Cylinder
A) Velocity
As seen from the Fig. 6.8 the velocity magnitude varies as per the contour represented by the analysis result. The velocity magnitude is 5.98e-002m/s in the inlet which reduces at the stagnation point going down up to zero while it remains unaffected in rest of the portion at the same cross section wherever there is not contact with the surface of the model. Beyond the stagnation point, the velocity magnitude increases varying from 7.69e-05 m/s to 1.11e-01 m/s. As the flow separation start drastic drop in velocity magnitude can be seen as eddies formations start to appear. The magnitude can be seen to have increased at some areas in the test section shown in orange and red.
B) Dynamic Pressure
The Fig. 6.9 above represents the contours of Dynamic Pressure in the test section of a flow over a cylinder. There is constant pressure of 7.28e-01 Pa till the flow touches the body surface. The stagnation point can be clearly seen in dark blue and the static pressure decreases as seen in the ocean green and blue zones. The dynamic pressure is minimum in the dark blue zone due to stagnant behavior of fluid as it hits the cylinder and the dynamic pressure reaches to zero. This can be observed near surroundings of eddies as well.
6.1.2.2 Eppler E854
A) Velocity
The Fig 6.10 shows the velocity contours of the Eppler E854 airfoil which is analyzed in software use for the industrial purpose. The maximum velocity of the flowing fluid is observed at the maximum camber on the upper surface and the relative lower velocity on the lower surface of the airfoil.
B) Dynamic Pressure
Figure 6.11 shows the variation of dynamic pressure when a moving fluid passes over the body. Dynamic pressure was observed to be maximum over a region where the fluid achieves maximum velocity due to the camber of an airfoil.
6.1.2.3NACA 2412
A) Velocity
The Fig. 6.12 shows the variation of velocity at different regimes of an airfoil. Since the fluid travels the curve path on the body the velocity increases as it is depicted in the above figure on the upper surface of the body; whereas there is a very minute change in velocity on the bottom surface.
B) Dynamic Pressure
Fig. 6.13 shows the variation of dynamic pressure on the body. The profile for dynamic pressure is almost the same for the velocity profile since the dynamic pressure changes with the change in the velocity. The dynamic pressure varies as the square of the velocity.
6.1.2.4 Delta
A) Velocity
As shown in Fig. 6.14 the velocity magnitude is less when the flow enters the water tunnel and remains almost the same until it reaches near the model. Just near the tip of the model the velocity magnitude is seen to be increasing slightly. As the flow passes over the model the velocity magnitude increases that is represented by the green and yellow shades. The velocity reaches the maximum value at the outer edge of the model. Just behind the model the velocity can be seen to be decreasing rapidly, representing the flow separation. Whereas the velocity seems to high at the boundaries of the water tunnel. Thus again the analytical results seems to be coincides with the practical results.
B) Dynamic Pressure
The dynamic pressure shown in Fig. 6.15 is almost constant when it enters the domain. But at the tip of the model the minimum value of dynamic pressure appears which means that the flow is stagnant at that point and the dynamic pressure is almost zero. Which means the total pressure at the tip is only the static pressure. Now as the flow moves over the body the dynamic pressure goes on increasing till the outer edge. Which means the static pressure is decreasing over the body surface. After the outer edge the dynamic pressure increases rapidly. But behind body wall the pressure seems to be decreased, representing the flow separation. Thus the practical results of dynamic pressure coincide with the results of simulation.
6.1.2.5 Nozzle
A) Velocity
Fig. 6.16 shows Velocity at the inlet of the nozzle is very less i.e. 4.6e -002 m/s. This is indicated by the blue streamlines in the figure. The velocity goes on increasing along the length of the nozzle and reaches the maximum value at the outlet area. This is represented by the red color streamlines. The maximum value at the exit area is approximately 1.680e-001 m/s which larger than the inlet velocity. Thus the result matches with the theoretical calculations.
B) Dynamic pressure
Fig. 6.17 shows the variation of dynamic pressure at different regimes of the nozzle. As it is seen from the above figure the variation can be easily seen from the inlet (right side) to the outlet (left side) of the figure. It can be observed that the velocity is very low at the inlet hence the dynamic pressure and increases in the converging section i.e. at the throat section.
6.1.2.6 Diffuser
A) Velocity
Fig. 6.18 shows the streamlines of the flow. They represent the velocity of flowing particles inside the diffuser. Inlet velocity was given to be 0.05m/s. As analyzed, the velocity keeps on decreasing with increment in the cross sectional area of the diffuser. At about 0.1 m distance from inlet, the flow separation can be observed as there is an eddy formation near the exit. Till then, the flow appears to be laminar. Eddies formation was observed in practical experimentation as well. The velocity at the exit ranges from about 0.0125m/s to 0.00003 m/s.
B) Dynamic Pressure
Fig. 6.19 shows the contour of the dynamic pressure at various positions inside the diffuser respectively. It can be observed that the dynamic pressure keeps decreasing with increase in distance from the inlet. The max dynamic pressure obtained is 1.24e00 Pa minimum is obtained once divergence becomes prominent the lowest being about 4.01e-005 Pa.
6.1.2.7 C-D Section
A) Velocity
Figure 6.20 shows increase in velocity inside the converging section increased till the throat area. After that it started decreasing in the diverging section. It is clearly seen in the velocity profile taken out from the analytical results. As both of our converging and diverging sections were of same dimensions, the velocity at both end of converging diverging section was found out to be same. The velocity of the flow near the walls of the duct was found out to be much lower. This is due to formation of boundary layer over the walls of the duct and resistance offered by the wall to the flow.
C) Dynamic Pressure
From Fig. 6.21 it can be seen that as the velocity of the fluid increases inside the CD section as the area of cross section decrease upto the throat area. As the fluid reaches to the minimum are section the velocity increases and also the dynamic pressure
11.2 3-D Model Computational Analysis
Computational Analysis of low speed water tunnel was performed in commercial software’s to observe the change in parameters in computational and experimental conditions. Further the changes in pressure and flow patterns were observed starting from inlet plenum to outlet module. Shear stresses along the walls of model were computed. Contours were display to show the results of different parameters and graphs were plotted for the same. Computational analysis was performed to finalize the design of low speed water tunnel.
The basic step in numerical analysis of 3-D model is to create geometry in commercial CAD software and save it in .IGS file format. In case of our geometry slight modifications are required. First is the geometry must be closed i.e. it must be filled from the top surface so that the boundary conditions could be easily be defined and analysis will be much easier to be performed. To generate the proper meshing at the inlet and outlet section circular section of the required height was constructed in CAD which will be beneficial to generate proper results. (Refer Fig. 11.2.1 and 11.2.2)
After completion of required modification in CAD, import the geometry in commercial pre-processing meshing software. After importing the geometry, check for multiple edges and faces available in geometry if any of them are exist connect them. Split the geometry in easily map able volumes. After splitting the geometry create faces by connecting all the required edges for forming the faces. Next stitch the require faces for creation of volume. Precautions must be taken while select the faces as one gets confuse while creating the volume. (Refer Fig. 11.2.3)
Now form mesh by selecting each and every volume at once. Two types of mesh can be created viz, structured mesh and unstructured mesh. Structured mesh is not appropriate for 3D CFD simulation as elements of high aspect ratio are created. More number of elements are created for capturing the flow pattern and it led to lot of time to perform the iterations. Such elements are not capable of capturing the flow patterns efficiently. Unstructured mesh is most widely used for 3D CFD simulation. Elements created in unstructured mesh are of Tet/Hybrid scheme and their type is of TGrid type. Less number of elements is created for lower aspect ratio and Equiangle skew as compared to structured mesh. Default interval size is been selected. For dense meshing in a particular volume, the interval size can be alter accordingly. (Refer Fig. 11.2.4)
Worst Equiangle skew found is 0.907494 and aspect ratio of 6.7753 both of which lie within the safe limits for the total elements of 2079924.The safe limits for Equiangle skew is between 0 – 1 and for aspect ratio it is between 1- 100.
The boundary conditions for the mesh model were defined as follows:
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As the mass flow rate of water is know from the specifications provided by the manufacturer of motor the boundary type of mass flow inlet was chosen. As no conditions at the outlet face were known so outflow condition was define to it. And walls were defined with wall conditions. After defining the boundary conditions export the file to the commercial processing software and save the file with extension as msh.
In processing software various conditions are define which are essential for performing correct iterations. The steps that were followed before processing for iterations were as follows:
1. Read the mesh file which was saved during pre-processing.
2. Check the quality of the mesh created.
3. Scale the model if required in desire units
4. Define viscous model
- For turbulent flow the Reynolds number in case of internal flow must be 2300.
a. Select k-epsilon (2-eqn) method
b. Under k- epsilon select realizable model (RKE model)
1. RKE model stratifies certain mathematical constraints on the Reynolds stresses consistent with physics of turbulent flows.
2. It can predict the spreading rate more accurately in both planar and round jets.
3. It performance superiorly for the flows involving rotation, boundary layer under strong adverse pressure gradients, separation and recirculation.
c. Select Enhanced wall treatment.
1. They are suitable for low Reynolds number flow.
5. Define material water – liquid (H2O) and its density and viscosity for 30 ͦC.
6. Define the cell zone conditions and select the fluid as water – liquid.
7. In operating conditions define the operating pressure at the height of 14 cm in addition to atmospheric pressure. (102696 Pa)
8. Define the boundary conditions which were earlier define during pre – processing software
a. Walls: Must be defined with specified shear in case of internal flow.
b. Inlet: In mass flow inlet mass flow rate of water must be define which lies between 0.11 to 0.25 Kg/sec in case of 0.5 Hp pump.
1. Select turbulence Specification method as Intensity and Hydraulic Diameter as its is ideally suited for internal flow through duct or pipe work
2. At upstream conditions turbulence intensity must define less than 20 % and Hydraulic Diameter as inlet diameter from where the water is entering the model.
c. Outflow: As no parameters were practically calculated at the outlet face.
9. Define the Reference Values
a. Compute all the calculation from Inlet
b. Define area of model = 1.2 Sq. Mtr
c. Take the default values of Density, Enthalpy, Velocity and Viscosity.
d. Define the actual length of model 195 cm
e. Define the operating Pressure in Pa
f. Define the temperature of fluid.
g. Define the ratio of specific Heats as 1.3 for water.
h. Select reference zone as fluid.
10. Define Solution Methods
1. Pressure Velocity Coupling
a. Select the scheme for pressure – velocity coupling as SIMPLEC (Semi – Implicit Method for Pressure Linked Equations – Consistent) because it allows faster convergence in case of simple problems
b. Take the default values for skewness corrections as zero.
2. Spatial Discretization
a. Gradient: Select Least Squares Cell Based as it is recommended for unstructured mesh and it is known for its more accuracy and computationally less intensive.
b. Pressure: Select PRESTO! As it is use for highly Swirling flows and it is strongly used in case of curved domain.
c. Momentum, Turbulent Kinetic Energy and Turbulent Dissipation rate: Select Second Order Upwind as it is essential in case of Tri/Tet mesh. It makes the convergence slower but more accurate.
11. Define the Solution Controls to its default value.
12. Edit Residuals monitors option by unchecking the convergence option or increase its absolute criteria so that it will be helpful for performing more iteration.
13. Initialize the solution by using standard initialization method and compute all parameters from Inlet face. Let all values set to be as default values.
14. Auto save every iterations at the interval of 250 or depending upon the configuration PC.
15. Check the case, Make the corrections if recommended and define number of iterations that need to be performed and then calculate.
16. Stop the iterations when any of the residuals is converge.
17. Save the results as case and data.
Now the last step of 3D – Computational analysis is post processing. In post processing the computational results are verified with experimental results. Plot the results with respect to position along x – axis against velocity magnitude (m/s) from which velocity at different locations can be known and can be verified experimentally. Velocity streamline pattern can also be observed in the contours for proper flow pattern of water in the model. (Refer Fig.11.2.5 and 11.2.6) [28],[29]
Baffle and Honeycomb structure were not consider during the numerical analysis as they are difficult for meshing and hence proper streamlines are not achieved at the inlet of the test section.
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Figure 6.1 Mesh over Cylinder
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Figure 6.2 Mesh over Eppler E854
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Figure 6.3 Mesh over NACA 2412
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Figure 6.4 Mesh Over Delta
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Figure 6.5 Mesh Inside the Nozzle
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Figure 6.6 Mesh Inside the Diffuser
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Figure 6.7 Mesh Inside the C – D Section
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Figure 6.8 Velocity Streamline for Cylinder
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Figure 6.9 Dynamic Pressure Contour Of Cylinder
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Figure 6.10 Velocity Streamline for Eppler E854
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Figure 6.11 Dynamic Pressure Contour for Eppler E854
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Figure 6.12 Velocity Streamline for NACA 2412
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Figure 6.13 Dynamic Pressure Contour for NACA 2412
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Figure 6.14 Velocity streamline for Delta
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Figure 6.15 Dynamic Pressure Contour of Delta
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Figure 6.16 Velocity Streamline in Nozzle
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Figure 6.17 Dynamic Pressure Contour of Nozzle
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Figure 6.18 Velocity Streamline in Diffuser
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Figure 6.19 Dynamic Pressure Contour of Diffuser
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Figure 6.20 Velocity Streamline in C-D Section
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Figure 6.21 Dynamic Pressure Contour of C – D Section
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Figure 6.22 Shows the open surface model for analysis
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Figure 6.23 Shows the closed surface model modified for analysis
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Figure 6.24 Volumes Created for Meshing
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Figure 6.25 Shows the Unstructured Mesh
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Figure 6.26 Shows Velocity Magnitude vs. Position Graph
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Figure 6.27 Shows Velocity Streamline Pattern
CHAPTER 7 Comparison of Experimental and Computational Results
Graph shows the comparison between experimental results and computational results. Results obtain from commercial software it is clear that the drag coefficient which is found from computational is higher than experimental results. It is due to in commercial software we consider cell boundary conditions and some other parameter. Additionally other error includes improper meshing whereas in experimental method all calculations were performed with the help of standard formulas i.e. for ideal conditions.
7.1 Cylinder
The nature of the graph in Fig. 7.1 is exponentially decreasing .It shows relationship between the velocity and drag coefficient .The velocity is inversely proportional to drag coefficient. As the velocity increases the drag coefficient decreases exponentially. The error calculated was found to be 5.90%
7.2 Eppler E854
The graph in Fig. 7.2 shows the relationship between velocity and drag coefficient. The nature of the graph is exponentially decreasing. The error calculated was 10.99%.
7.3 NACA 2412
The graph in Fig. 7.3 shows the relationship between velocity and drag coefficient. The nature of the graph is exponentially decreasing. The error calculated was 9.2%.
7.4 Delta
The graph in Fig. 7.4 shows the relationship between velocity and drag coefficient. The nature of the graph is exponentially decreasing. The error calculated was 9.81%.
7.5 Nozzle Section
There is the slight variation between both the results and error of 13.86% is found between both the values shown in Fig. 7.5.
7.6 Diffuser Section
There is the slight variation between both the results and error of 13.86% is found between both the values shown in Fig. 7.6
7.7 C –D Section
It can be seen that as the cross section area decreases there is a increase in velocity. The total error of 13.58% is found between both the values shown in Fig. 7.7.
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Figure 7.1 Comparison of Cylinder
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Figure 7.2 Comparison of Eppler E854
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Figure 7.3 Comparison of NACA 2412
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Figure 7.4 Comparison of Delta
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Figure 7.5 Comparison of Nozzle Section
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Figure 7.6 Comparison of Diffuser Section
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Figure 12.7 Comparison of C - D Section
CHAPTER 8 Conclusion
- Design of water tunnel setup was successfully done.
- Study on the shape and size of various parts of the setup was done before finalizing the dimensional parameters.
- Use of composite material was beneficial as well.
- Suitable regulator was used to achieve the variable speed of water in the test section. Plumbing systems and cushions provided to the motor mount was effective in reducing the vibration.
- Flow visualization was done over various scaled models inside the test section.
- Dye injection system proved to be a successful method for the flow visualization.
- After the fabrication was done, the water tunnel was fully operational. No leakages were seen. Laminar flow within the desired specifications was achieved.
- Experimental as well as computational studies were carried out. Comparison was done between the results obtained from both studies. Errors in the results were successfully found out and were optimized later. Hence approximately similar results were obtained.
- Many commercial types of software were helpful to carry out computational studies and thus the project has been concluded.
8.1 Future Scope
- Modern particle tracking instruments can be use to calculate the velocity in the test section.
- Strain gauge can be use to find out various forces and moments on the model.
- Water can be made more clear by using advanced filtration techniques.
- Fluorescent dyes can be use for better flow visualization.
- Schileren method can be use for flow visualization eliminating the use of color dyes.
- Variable density can be achieved.
- Flow over marine bodies can be visualized and there hydrodynamic behavior can be study.
- Oil can be use instead of water for low Reynolds number flow.
- Cavitation can also be study in water tunnel.
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28. Fluent Inc, GAMBIT 2.4 modelling Guide, 2007.
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- Er Sanket Kalgutkar (Author)Abhay Meshram (Author)Chirag Sakhare (Author)Kishor U. Masurkar (Author)Mahendra Mhatre (Author)Sayalee Sawant (Author)Shruti Wasnik (Author)Shubham Khobragade (Author)Vivek Tople (Author), 2016, Low Speed Water Tunnels. Design, Fabrication and Analysis, Munich, GRIN Verlag, https://www.grin.com/document/340156
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