# Wind Tunnels. A detailed approach to analysing and comparing wind tunnel theory

Excerpt

Introduction

Task 1 – wind tunnel types
Subsonic wind tunnel
Transonic wind tunnel
Open return wind tunnel
Closed return wind tunnel
Open return wind tunnel
Closed return wind tunnel
Supersonic wind tunnel
1. Introduction
1.1 Experiment 1 – Wind tunnel calibration
1.2 Experiment 2 – NACA 2412 aerofoil with variable flap
2. Objectives
2.1 Experiment 1 – Wind tunnel calibration
2.2 Experiment 2 – NACA 2412 aerofoil with variable flap
3. Theory
3.1 Experiment 1
Measuring velocity using a pitot probe
Measuring pressure distribution using a manometer
Boundary layer theory
Force coefficients from distribution of pressure
3.2 Experiment 2
Aerofoil theory - forces on an aerofoil
Drag force equation
Lift force equation
Pitching moment equation
High lift devices
4. Instrumentation used on the wind tunnel
5. Precautions and procedure
5.1 Precautions
5.2 Procedure
5.2.1 Experiment 1 – reference velocity
Experiment 1 - Velocity profile
5.2.2 Experiment 2 – NACA 2412 variable flap aerofoil
Starting the wind tunnel
Inserting the aerofoil model
Wind tunnel test procedure
6. Results and discussions
6.1 Experiment 1
6.1.1 Reference velocity
6.1.2 Velocity profile
6.2 Experiment 2
7. Sources of error
Conclusion
Bibliography

## Introduction

Wind tunnels are a great way of obtaining precise and accurate data on an aerofoil or model aircraft. They are crucial because they help engineers save a lot on resources; rather than having to make a full prototype by “winging it”, as they say, with a wind tunnel, it is possible to test a collection of low-cost models prior to the prototyping stage in order to find the best configuration for a design.

As such, this assignment will cover the operation of wind tunnels from multiple points of view, looking at the various uses of a wind tunnel both from theoretical and practical points of view.

## Task 1 – wind tunnel types

The type of wind tunnel used depends on the experimental application. The common types are subsonic, transonic, supersonic, hypersonic, and hypervelocity.

“Wind tunnel research has a wide range of applications, from normal aircraft testing to basic study on the boundary layer. At multiple locations on the model, measurements of air pressure and other variables provide information about how the entire wind load is distributed. Aerodynamic research in wind tunnels have proved extremely lucrative instruments for solving design difficulties in autos, boats, railways, bridges, and building structures, in addition to aviation and spacecraft.” (Encyclopaedia Britannica, 2018)

Subsonic and transonic wind tunnels fall into the category of low speed wind tunnels. With a 100 horsepower variable frequency motor, “the low speed wind tunnel is a closed-circuit, continuous flow type. The flow area of the test portion is 0.6 x 0.9 metres (2 x 3 feet). A maximum flow velocity of 50 metres per second is possible in the tunnel. A 6-component force balance, a multiplexed data collecting system, and smoke visualisation equipment are also included in the tunnel.” A high-pressure air supply is also offered as an option. (arc.uta.edu, 2014)

### Subsonic wind tunnel

An aerodynamically optimised effusor (cone) draws air into the tunnel and accelerates it in a linear fashion. It then passes via a grille, a diffuser, and lastly a variable-speed axial fan to reach the working component. The grille protects the fan from stray objects. The air leaves the fan, passes through a silencer, and then returns to the outside world. A separate control and instrumentation unit, which also supplies electrical power to other instruments, controls the air velocity in the working component.

The tunnel's working space is a square with a transparent floor, sides, and roof. The sides can be taken off for access into the testing space. Wind tunnel models are supported by a particular region on the floor and each side panel. A protractor and a model holder are included with the wind tunnel to support and precisely modify the angle of any model installed.

The wind tunnel comes with two Pitot-static tubes. One is placed near the working section's intake and monitors the air flow rate. The second Pitot-static tube is coupled to a two-axis traverse that permits measurements to be taken both horizontally (fore and aft) and vertically across the working area.

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Figure 1 Subsonic wind tunnel cross-section diagram (TecQuipment LTD., 2021)

The air stream is upheld by a metal casing. For simple compactness, the edge has lockable castors. The coordinated hardware has electronic sensors that might be associated with TecQuipment's "Flexible Data Acquisition System" ("VDAS®", which is given). On an appropriate PC, "VDAS®" offers precise constant data gathering, observing, display, calculation, and outlining of every relevant boundary.

Typical uses of subsonic wind tunnels include:

- “Airflow past bluff and streamlined bodies with velocity and pressure observations in the wake”
- “Analysis of boundary layer propagation”
- “Aspect ratio’s effect on performance of aerofoils”
- “Distribution of pressure around a cylindrical object under sub-critical or super-critical flow conditions”
- “Study of model characteristics that involve the basic measurement of drag and lift forces”
- “Analysing the characteristics of 3D aerofoils including drag, lift, and pitching moment forces”
- “Analysis of pressure distribution around an aerofoil model for deriving the lift force, and comparing the value obtained with direct lift measurements”
- “Measuring the drag force on a bluff body normal to the airflow”
- “Visualisation of airflow” (TecQuipment LTD., 2021)

### Transonic wind tunnel

The “Transonic Wind Tunnel Göttingen” (TWG) is a closed return tunnel capable of airflow speeds ranging from subsonic to supersonic. The way it achieves this is due to its test section, which can be changed to fit one of three configurations as shown in the diagram below. The perforated test section is utilised for the transonic range of airflow.

Each of the three sections is 1x1x4.5 metres. The transonic test section has flexible lower and upper walls, which allows a 2D adaptation to the flow field. The wall interference is minimised compared to typical test sections, and/or bigger models in the range 0.3 M 0.9 may be employed. The adaptation is performed using wall pressure distribution data and a single-step technique based on Cauchy's integral formula for aerofoil (2D) tests and the Wedemeyer-Lamarche method for 3D models, respectively. Small residual wall interferences are estimated using Green's integral formula and utilised for final rectification in 3D model testing.

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Figure 2 TWG wind tunnel (DNW, 2021)

Typical uses of transonic wind tunnels include:

- High precision tuning of Mach no. in increments of 0.001
- Continuous sweep measurement of pitch
- Direct simulation of sideslip angle
- Suction and blowing of heated air through a model or tunnel walls
- Simulation of air intake
- Drag analysis by duct flow measurement
- Free and forced pitch/heave oscillations and flutter simulation of 2D profiles and half models (DNW, 2021)

### Advantages of low-speed wind tunnels

#### Open return wind tunnel

- Low construction cost.
- “Superior propulsion and smoke visualisation design. In an open tunnel, there is no build-up of exhaust products”. (grc.nasa.gov, 2021)

#### Closed return wind tunnel

- For a given speed, the power required is smaller.
- Within the circuit, particulate debris can be confined.
- The amount of noise is greatly reduced.
- Wind tunnel flow is unaffected by laboratory air movement (air vents, doors, windows, and so on).
- There is no laboratory dust in the air that enters the test portion.
- Model failure does not cause as much harm to fan blades. (AEROLAB.com, 2021)

### Disadvantages of low-speed wind tunnels

#### Open return wind tunnel

- “In the test portion, poor flow quality is possible. Extensive screens or flow straighteners may be required to turn the corner into the bell-mouth. The tunnel should also be situated away from any items in the room that cause asymmetries to the bell-mouth (walls, desks, people, etc.). Winds and weather have an impact on tunnels that are open to the atmosphere.”
- “High operating costs due to the fact that the fan must continuously accelerate flow through the tunnel”.
- “Generates high amounts of noise”. (grc.nasa.gov, 2021)

#### Closed return wind tunnel

- “For a particular test section size, the cost is usually three times higher”.
- “When dealing with combustion engines, the air supply is recycled, which might be a problem”.
- “The footprint is significantly greater, necessitating more total area”.
- “When used for a long time, the rising air temperature might become a problem”. (AEROLAB.com, 2021)

### Supersonic wind tunnel

A supersonic wind tunnel contains a “remote-control device that regulates a high-capacity vacuum pump is housed in an instrument frame (provided)”. The pump produces low pressure downstream of the working area to pull air into the wind tunnel. The operator can reduce the air flow through the working component by employing a bypass duct with a hand-operated valve without impacting the main air flow quality. This is necessary for subsonic testing as well as beginning and stopping.

The working component of the wind tunnel is a convergent-divergent nozzle with a removable top half ('liner'). The shape of the liner determines the maximum air velocity at the diverging zone of the working section. There are three different shaped liners provided.

In the instrument frame, a 'mimic' panel and multi-pressure display unit are linked to pressure tappings along the working portion. The display device shows the pressures at the tappings. The display includes pressure sensors calibrated to monitor pressures relative to the atmosphere. One of the models' forces are also displayed.

An analogue pressure gauge is used to measure and display the pump's suction (tunnel reference pressure). This pressure line also connects to the multi-pressure display for data collection.

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Figure 3 TecQuipment supersonic wind tunnel (TecQuipment, 2021)

Typical uses of supersonic wind tunnels include:

- “Pressure distribution with subsonic and supersonic air flows along a convergent/divergent (Laval) nozzle”
- “Real and theoretical area ratios of a nozzle at supersonic air speeds”
- “Comparison of theoretical and actual pressure distributions (Mach numbers)”
- “Pressures at different angles of incidence around a two-dimensional model under subsonic and supersonic flow circumstances”
- “Lift coefficients for supersonic flow aerodynamic models”
- “Shock waves and expansion patterns in supersonic flow around a two-dimensional model” (TecQuipment, 2021)

- “Mach capability is high. Tunnel "start-up" is simple and building and running expenses are low.”
- “Excellent propulsion and smoke visualisation design. In an open tunnel, there is no build-up of exhaust products.”
- “Faster starts result in smaller loads on the model at start-up.”

- Faster (and typically more costly) instrumentation is required for shorter test periods.
- Pressure regulator valves are required.
- It's a noisy process. (grc.nasa.gov, 2021)

### 1. Introduction

#### 1.1 Experiment 1 – Wind tunnel calibration

The aim of this experiment is to understand the operation of an open return subsonic wind tunnel by learning how to calibrate the instruments on the wind tunnel. The instrumentation calibrated included a pressure transducer (pitot probe) and manometer.

Two sets of readings were taken: reference velocity variation with manometer height and reference velocity variation with probe height.

#### 1.2 Experiment 2 – NACA 2412 aerofoil with variable flap

The second experiment involves the NACA 2412 3D aerofoil profile. The aim of this experiment is to “determine the effect that the flap deflection angle has on the aerofoil’s aerodynamic characteristics”.

### 2. Objectives

#### 2.1 Experiment 1 – Wind tunnel calibration

- “Investigate the velocity at the test section inlet”
- “Familiarise ourselves with wind tunnel operation”
- “Examine the velocity uniformity”
- “Check the boundary layer height”

#### 2.2 Experiment 2 – NACA 2412 aerofoil with variable flap

- “Determine the effect of flap deflection angle on the aerofoil’s aerodynamic characteristics”.

### 3.1 Experiment 1

#### Measuring velocity using a pitot probe

Visualising flow patterns and measuring pressure at a given place in the flow field and computing the accompanying air speed are two significant applications of wind tunnels. The equation connects the fluid's speed at a given position to its mass density as well as the pressures at that same location in the flow field. For an incompressible fluid with steady flow for which viscosity is negligible, the equation of flow velocity is given by:

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Where v = fluid speed, P0 = total (stagnation) pressure, and P = static pressure.

This can be rewritten as:

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Figure 4 Pitot static tube (vlab.amrita.edu, 2015)

Bernoulli's equation for the steady flow of an incompressible and inviscid fluid down a streamline is the source of this equation. Integrating Euler's equations along a streamline usually yields Bernoulli's equation. It should be remembered that “Euler's equations are a specific case of the Navier-Stokes equations in which the fluid's viscosity is ignored. When Newton's 2nd law is applied to a fluid whose shear deformation follows Newton's law of viscosity, the Navier-Stokes equations are produced”. (vlab.amrita.edu, 2015)

#### Measuring pressure distribution using a manometer

A manometer is used for measuring pressure differentials between 2 or more points. A simple manometer has 2 pressure ports. Each pressure port has a pressure exerted on it. The hydrostatic equation that links the pressure difference between the two ends is given by:

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Figure 5: Simple manometer (Dr. Hui Hu, 2021)

A wind tunnel has a multi-tube manometer such as the one shown in the diagram below.

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Figure 6: Multi tube manometer (Dr. Hui Hu, 2021)

Each subsequent tube paired with the tube after it can be considered as a separate manometer. Even though the fluid is shared throughout all the tubes, with a big enough reservoir, tube interference may be ignored. The vertical height difference between the fluid levels of the reference and the ith tubes are denoted by ∆hi. Therefore, using the hydrostatic equation, the following equation is derived:

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The absolute reference pressure is needed to get the absolute pressure at the ith tube. Because the pressure is usually ambient, or room pressure, a barometer can be used to find Pref. (Dr. Hui Hu, 2021)

[...]

Excerpt out of 36 pages

Details

Title
Wind Tunnels. A detailed approach to analysing and comparing wind tunnel theory
Course
Aerospace Technology II
A
Author
Year
2021
Pages
36
Catalog Number
V1264786
ISBN (Book)
9783346707925
Language
English
Keywords
wind tunnel, aerodynamics, aerospace, engineering, aeronautical
Quote paper
Abdusselam Šabić (Author), 2021, Wind Tunnels. A detailed approach to analysing and comparing wind tunnel theory, Munich, GRIN Verlag, https://www.grin.com/document/1264786