The flow over macroscopic patterned/structured surfaces was investigated in a subsonic wind tunnel over Reynolds numbers ranging from 3.14 x 104 to 2.77 x 105 for cylinders and from 5.34 x 105 to 11.27 x 106 for plates. The investigations were accomplished by measuring local and global drag, velocity profiles and by visualization of the flow above the surface. Flow visualization was carried out by using oil-film technique and velocity profile measurements to elucidate the observed effect, and hence present the mechanism responsible for theobserved drag reduction. Investigations on structured plates were performed with the help of hot wire anemometry and oil film interferometry.
The main concern of the experiments on structured plates was to examine the effect of hexagonal structures on local and global drag of a structured plate. It was accomplished by determining and analyzing the boundary layer quantities like shear stress velocities, shear stress coefficients and momentum thicknesses over a selected Reynolds number range and various locations in streamwise direction. Finally, a number of configurations of a wind turbine made of smooth and structured blades were investigated to compare their efficiencies at various flow velocities.
Table of Contents
1 INTRODUCTION
1.1 Motivation
1.2 Aims and Approaches
2 THEORETICAL BACKGROUND
2.1 Pressure drag
2.2 Skin Drag
2.3 Fundamentals of Boundary layer
3 STATE OF THE ART
3.1 Drag reduction of bluff bodies
3.2 Drag reduction of streamlined bodies
4 EXPERIMENTAL SETUP
4.1 Bluff body investigation
4.2 Streamlined body investigation
4.3 Investigations on experimental wind turbine
4.4 Invested Configurations
5 MEASUREMENT TECHNIQUES
5.1 Drag measurement
5.2 Flow visualization
5.2.1 Smoke flow visualization
5.2.2 Surface Oil flow visualization
5.2.3 Oil film interferometric Visualization
5.3 Velocity measurements
5.4 Shear stress measurements
5.4.1 Clauser chart method
5.4.2 Oil film interferometric method
5.5 Pressure & Temperature measurements
5.6 Braking moment and angular speed measurement
6 RESULTS AND DISCUSSION
6.1 Structured cylinders
6.1.1 Drag Variation
6.1.2 Velocity profiles in the wake region
6.1.3 Location of flow separation
6.1.4 Flow visualization on the surfaces of cylinders
6.1.5 Streamwise velocity measurements
6.1.6 Energy spectra of the flow over cylinders
6.1.7 Vortex shedding
6.2 Structured plates
6.2.1 Non dimensional velocity profiles
6.2.2 Boundary layer quantities
6.2.3 Flow over individual hexagonal structure
6.2.3.1 Optimization of the measurement techniques
6.2.3.2 Shear Stress Measurements
6.2.3.3 Boundary layer measurements
6.2.3.4 Flow structures within the hexagonal depressions and bumps
6.2.3.5 Power spectrum
6.2.3.6 Visualization of flow over the surface
6.2.3.7 Pressure distribution
6.3 Experimental Wind Turbine
7 CONCLUSIONS & FUTURE WORK
7.1 Conclusions
7.2 Future work
Research Objectives and Themes
This dissertation investigates the aerodynamic influence of macroscopic hexagonal structured surfaces on pressure drag, skin drag, vortex shedding, and boundary layer characteristics across various test objects including cylinders, plates, and wind turbine blades.
- Analysis of fundamental aerodynamic effects of hexagonal surface patterns.
- Evaluation of pressure and skin drag reduction potential.
- Visualization and behavior of flow within and around hexagonal structures.
- Validation of scaling laws in turbulent boundary layers.
- Optimization of experimental measurement techniques for structured surfaces.
Excerpt from the Book
6.1.4 Flow visualization on the surfaces of cylinders
Fig. 36 shows surface oil flow patterns for all the investigated configurations of k/D = 1.98x10-2 for a Reynolds number of 2.3x105. The flow direction is from right to left. It is commonly known that the powdered titanium dioxide leaves its traces wherever a separation or transition of flow takes place enabling the observer to visualize the flow on the surface. The areas with a low concentration of titanium dioxide are generally interpreted as a high shear force area, whereas a higher concentration or accumulation of titanium dioxide indicates a separation or transition of the flow.
The bright line in Fig. 36 (S) indicates the laminar separation of flow over smooth cylinder. This separation line lies at about 85°. A clear trace of titanium dioxide and hence the separation could be visualized because of the smoothness of the surface. On the other hand, the patterns create extreme difficulties for oil film in marking a clear separation or transition of the flow especially for the cylinders with patterns pressed inwards (I90 and I0). A recurring series of bright lines nearly at the middle of the hexagon at about 75° on O90 represent a local separation. This local separation triggers a shear layer instability causing the flow to reattach to the surface where the hexagon ends.
Summary of Chapters
1 INTRODUCTION: Outlines the motivation for using structured surfaces in aerospace and automotive industries and defines the research aims regarding drag reduction.
2 THEORETICAL BACKGROUND: Details the fundamental concepts of pressure drag, skin drag, and boundary layer theory relevant to the investigation.
3 STATE OF THE ART: Reviews existing literature on passive and active flow control methods, including dimples, riblets, and other structured surface applications.
4 EXPERIMENTAL SETUP: Describes the wind tunnel facilities, test object configurations (cylinders, plates, wind turbine), and specific experimental setups used.
5 MEASUREMENT TECHNIQUES: Explains the methodologies employed, including drag measurements, flow visualization (smoke, surface oil), and hot-wire anemometry.
6 RESULTS AND DISCUSSION: Analyzes the experimental data gathered from structured cylinders, structured plates, and the experimental wind turbine.
7 CONCLUSIONS & FUTURE WORK: Summarizes key findings regarding drag reduction mechanisms and proposes areas for further research.
Key Words
Aerodynamics, Hexagonal structures, Drag reduction, Boundary layer, Wind tunnel, Hot wire Anemometry, Oil film Interferometry, Turbulent flow, Pressure drag, Skin drag, Vortex shedding, Flow visualization, Reynolds number, Surface patterns, Turbine efficiency
Frequently Asked Questions
What is the core subject of this doctoral thesis?
The thesis focuses on the experimental investigation of aerodynamic flow over macroscopic hexagonal structured surfaces to determine their influence on drag and flow behavior.
Which aerodynamic quantities are primarily investigated?
The work examines pressure drag, skin drag, vortex shedding, and boundary layer characteristics, specifically looking at how hexagonal surface patterns alter these quantities.
What is the primary goal of the research?
The goal is to determine if hexagonal structures can effectively reduce aerodynamic drag on bluff and streamlined bodies and to understand the mechanisms responsible for any observed alterations in flow.
Which scientific methods are employed?
The research uses wind tunnel testing, hot-wire anemometry, surface oil flow visualization, and oil film interferometry to gather data on velocity profiles, shear stress, and pressure distribution.
What topics are covered in the main section?
The main part covers the experimental setup, measurement of drag on cylinders and plates with various hexagonal configurations, analysis of boundary layer behavior, and efficiency testing of wind turbine blades.
Which keywords define this work?
The work is defined by terms such as Aerodynamics, Hexagonal structures, Drag reduction, Boundary layer, Hot wire Anemometry, and Oil film Interferometry.
How do outwardly curved structures (bumps) affect flow separation compared to smooth cylinders?
Outwardly curved structures induce early partial separation, which increases turbulence intensity, allowing the flow to reattach with higher momentum and significantly delay the final separation compared to a smooth cylinder.
Why did the author conclude that these hexagonal structures are not simple roughness?
Unlike rough surfaces that show a dramatic increase in drag at higher Reynolds numbers, the hexagonal structures demonstrated stable or reduced drag performance over a wide range of Reynolds numbers.
- Arbeit zitieren
- Mohammad Usman Butt (Autor:in), 2014, Flow over macroscopic hexagonal structured surfaces. Experimental Investigation, München, GRIN Verlag, https://www.grin.com/document/1184440
