Experimental Complications Inherent to Low Reynolds Number Wind Tunnel Testing

Abstract

Recent initiatives in high accuracy wind tunnel testing of Micro Aerial Vehicles (MAVs) at the University of Colorado have illustrated some of the difficulties and complications involved with obtaining reliable experimental results at Reynolds numbers of less than 100,000. The low speeds and characteristic lengths inherently associated with MAV flow regimes result in small scale forces and moments which require a custom manufactured force balance to acquire data at a high enough resolution to provide accurate values. In addition, the quality of the flow itself has a significant effect on the test measurements as low Reynolds number flows are highly sensitive to the freestream turbulence intensity. Minor variations in turbulence (less than 1% deviation) can alter the separation point on the top surface of a test model and, thus, significantly change the recorded stall behavior. Complications also arise from the resolution of available commercial data acquisition hardware which are generally not designed for the small voltage outputs (on the order of 100 μV ) required for precise low Reynolds number experimentation. Finally, for any novel experimental setup such as the in-tunnel dynamic maneuvering capability being developed at CU, minimal literature exists to assist in the design or validation of the system. This paper focuses on the most significant of the experimental challenges which manifest themself at these low Reynolds number wind tunnel tests, describes several mitigation techniques used by the authors to minimize error, and presents system validation and additional test results.