Modification of Transonic Blowdown Wind Tunnel to Produce Oscillating Freestream Mach Number

This paper details the design, construction, and evaluation of a modification to Ohio State University's 6 x 22 in. transonic wind tunnel to enable modulation of the freestream Mach number. The test-section Mach number in this blowdown facility is dynamically set by varying the choke area in a harmonic fashion. Disturbances from the modulated throat area propagate upstream to vary the test-section Mach number. Flow properties, such as wave propagation speed, were evaluated using one-dimensional theory, two-dimensional computational fluid dynamics, and experiments in order to understand the fundamental mechanisms of the Mach number oscillation process and subsequent performance limitations of the facility. The current configuration of the facility can produce Mach number oscillations 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 25 Hz, although the wind tunnel emulates a low-pass filter with a -3 dB cutoff frequency of approximately 8 Hz.