Control of Acoustic Reflections in a Closed-Circuit Wind Tunnel

Closed return wind tunnels, such as the Klebanoff--Saric Wind Tunnel (KSWT) at Texas A\&M University, can provide low-disturbance flows that are required to study boundary layer receptivity. Receptivity is the process through which environmental disturbances become the initial condition for boundary layer instabilities. One instability mechanism, Tollmien--Schlicting waves, is especially receptive to freestream sound. The receptivity of these waves is studied by introducing downstream-traveling planar sound waves that interact with the leading edge of a flat plate; however, changes in wind tunnel area create reflected waves that complicate the experiment. Reflections are mitigated using a secondary speaker located downstream of the test section that eliminates upstream- traveling reflections. The secondary speaker is controlled using a finite impulse response (FIR) filter. Microphone measurements are used to document the wave cancellation at multiple locations in the test section.