UWE Bristol Engineering showcase 2015
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Drag Coefficient (Cd)<br />
Tristan Ingham<br />
Beng - Mechanical <strong>Engineering</strong><br />
Project Supervisor<br />
Dr Mike Ackerman<br />
Wind Tunnel Development<br />
The University of the West of England’s wind tunnel has not been previously used commercially to test objects with complex profiles<br />
(such as vehicles) due to uncertainties in its performance.<br />
The data acquisition software currently used is 20 years old, limiting the testing capabilities of the wind tunnel as a result. With<br />
computing power advancements new software is available that can perform a wide range of data analysis tasks in a short time period.<br />
Research into a suitable software package for development would be highly beneficial and could vastly improve the capabilities of the<br />
wind tunnel.<br />
Certain equipment had not been calibrated in recent years such as the force balancing system. This is another source of uncertainty in<br />
the results produced by the wind tunnel, so testing had to be carried out to investigate the sensitivity of the mass balance system. This<br />
would allow the operators of the wind tunnel to know what level of accuracy is acceptable with their results.<br />
Dead Weight Testing<br />
Dead weight testing was the primary method used to investigate the<br />
accuracy of the force balance readings. Several tests were run to investigate<br />
the Lift, Drag, Lateral forces and the Roll, Yaw, Pitch moments. A range of test<br />
apparatus had to be constructed with the use of pulleys to induce pure<br />
moments and lateral forces. The deviation of the readings with the expected<br />
results were calculated to give an indication to the results an operator of the<br />
<strong>UWE</strong> wind tunnel could expect.<br />
Measuring the Tunnel Turbulence Intensity<br />
Turbulence can have an influential impact on results. It was therefore<br />
necessary to examine the tunnel turbulence intensity. This was achieved<br />
through two testing methods.<br />
One method used was with hot wire anemometry. This involves a very fine<br />
wire with a current passing through it. Fluctuations in the velocity are picked<br />
up by this hot wire probe and can be used to calculate the turbulence<br />
intensity.<br />
The first method used was with turbulence spheres. The spheres detect<br />
turbulence when the drag coefficient drops to a value of 0.3. From this the<br />
effective Reynolds number can be found using the graph plotted below. The<br />
turbulence factor can then be derived which in turn allows the turbulence<br />
intensity to be found.<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
Turbulence Sphere Test Results<br />
0<br />
10000 100000 1000000<br />
Reynolds Number<br />
Small Turbulence Sphere Cd<br />
Large Turbulence Sphere Cd<br />
Small Turbulence Sphere Cd<br />
(2005 Data)<br />
Aerofoil Design Results<br />
Comparison<br />
Another method of testing was to<br />
use data carried out by the<br />
National Advisory Committee for<br />
Aeronautics (NACA ). The NACA<br />
0012 aerofoil design was chosen<br />
and manufacture with an aspect<br />
ratio of 6 as can be seen in the<br />
image to the right with the model<br />
assembled in the wind tunnel.<br />
Strut Drag Force Analysis<br />
The force balance struts that are<br />
used to secure models in the wind<br />
tunnel and transfer the forces<br />
exerted by the model exert their<br />
own drag force.<br />
A series of tests were run to<br />
investigate the magnitude of the<br />
drag force. ANSYS CFX was used to<br />
provide a comparison which<br />
provided a clear illustration of the<br />
vortices produced by the force<br />
balance struts (pictured right)<br />
Project summary<br />
The accuracy and repeatability of the<br />
University of The West of England wind<br />
tunnel has not been validated, to the degree<br />
that the wind tunnel is not currently<br />
commercially available.<br />
The aim of the project is to investigate the<br />
uncertainty in the results produced by the<br />
wind tunnel and the other components such<br />
as the force balancing system.<br />
Project Objectives<br />
The main project objective is to measure the<br />
repeatability and precision of the wind tunnel<br />
to determine the result accuracy of the tunnel<br />
force balance system.<br />
Another aim of the project was to conduct<br />
research into suitable new data acquisition<br />
software to add new testing capabilities of<br />
the wind tunnel.<br />
Understanding where the turbulence hot<br />
spots occur in the wind tunnel at certain wind<br />
speeds is another task along with research<br />
into applicable methods to mitigate them.<br />
Project Conclusion<br />
From research taken LabView would be the most<br />
suitable software package for use with the <strong>UWE</strong> wind<br />
tunnel.<br />
The deviation in the force balance readings were<br />
found as a guide for future operators of the <strong>UWE</strong><br />
wind tunnel as to what level of deviation is<br />
acceptable.<br />
The table to the left contains the deviation values<br />
gathered.
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