Unsteady Compressible Flow on a NACA 0021 Airfoil

Abstract

,This paper details a modification to Ohio State’s 6” × 22” Transonic Wind Tunnel to enable modulation of the freestream Mach number and application of this oscillatory flow on a NACA 0021 airfoil. The test section Mach number in this blowdown facility is set dynamically by varying the choke area in a harmonic fashion. Flow properties such as wave propagation speed were evaluated using 1-D theory and experiments in order to understand the fundamental mechanisms of the Mach number oscillation process. The current configuration of the facility can produce a Mach number oscillating between 0.44 and 0.65 for a Reynolds number range of 17 – 43 million per meter at sufficiently low oscillation frequencies. The control frequency can be varied up to 25Hz, though the wind tunnel emulates a low-pass filter with a -3dB cutoff frequency of approximately 8Hz. Unsteady measurements in this oscillating freestream flow were then made on a NACA 0021 airfoil. Unsteady surface pressure measurements were made using a fast-acting Pressure-Sensitive Paint with a single-shot lifetime-based technique. Oscillatory standing shocks and variations in surface pressure coefficients due to compressibility effects are observed on the airfoil.