Turbulence Sphere Method: Evaluating Wind Tunnel Flow Quality

Overview

Source: Jose Roberto Moreto and Xiaofeng Liu, Department of Aerospace Engineering, San Diego State University, San Diego, CA

Wind tunnel tests are useful in the design of vehicles and structures that are subjected to airflow during their use. Wind tunnel data are generated by applying a controlled air flow to a model of the object being studied. The test model usually has a similar geometry but is a smaller scale compared to the full-sized object. To ensure accurate and useful data is collected during low speed wind tunnel tests, there must be a dynamic similarity of the Reynolds number between the tunnel flow field over the testing model and the actual flow field over the full-sized object.

In this demonstration, wind tunnel flow over a smooth sphere with well-defined flow characteristics will be analyzed. Because the sphere has well-defined flow characteristics, the turbulence factor for the wind tunnel, which correlates the effective Reynolds number to the test Reynolds number, can be determined, as well as the free-stream turbulence intensity of the wind tunnel.

Procedure

1. Preparation of turbulence sphere in the wind tunnel

Connect the wind tunnel pitot tube to port #1 on the pressure scanner, and connect the static pressure port to port #2 on the pressure scanner.
Lock external balance.
Fix the sphere strut to the balance support inside the wind tunnel.
Install the sphere with 6 in diameter.
Connect the leading-edge pressure tap to port #3 on the pressure scanner, and connect the four aft pressure taps to port #4 on t

Results

For each sphere, the stagnation pressure and the pressure at the aft ports were measured. The difference between these two values gives the pressure difference, ΔP. The total pressure, P_t, and static pressure, P_s, of the test section were also measured, which are used to determine the test dynamic pressure, q = P_t - P_s, and the normalized pressure

Application and Summary

Turbulence spheres are used to determine wind tunnel turbulence factor and estimate the turbulence intensity. This is a very useful method to evaluate a wind tunnel flow quality because it is simple and efficient. This method does not directly measure the air velocity and velocity fluctuations, such as hotwire anemometry or particle image velocimetry, and it cannot provide a complete survey of the flow quality of the wind tunnel. However, a complete survey is extremely cumbersome and expensive, so it is not suitable for

References

Barlow, Rae and Pope. Low speed wind tunnel testing, John Wiley & Sons, 1999.
Crawford T.L. and Dobosy R.J. Boundary-Layer Meteorol. 1992. 59; 257-78.
Eckman R.M., Dobosy R.J., Auble D.L., Strong T.W., and Crawford T.L. J. Atmos. Ocean. Technol. 2007; 24; 994-1007.