Acoustic Receptivity Measurements Using Modal Decomposition of a Modified Orr-Sommerfeld Equation
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Copyright 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Boundary-layer receptivity to acoustic disturbances plays a key role in transition from laminar to turbulent flow. Acoustic disturbances interact with strong streamwise gradients at the leading edge to create Tollmien-Schlichting waves in the boundary layer. Measurement of Tollmien-Schlichting receptivity to downstream-traveling sound is complicated by the presence of Stokes waves in the boundary layer and upstream-traveling acoustic reflections that also generate downstream-traveling Tollmien-Schlichting waves. Active noise control is used to cancel reflections and enables the measurement of boundary-layer receptivity to downstream-traveling sound. Tollmien-Schlichting wave amplitudes are extracted from hot-wire data using biorthogonal decomposition of modified Orr-Sommerfeld equations that include acoustic disturbance solutions. The new method is implemented to measure the acoustic receptivity of a 20:1 modified superellipse leading edge on a flat plate. The data yield acoustic receptivity coefficients consistent with the results of previous computational studies. For comparison, pulsed-sound measurements are also obtained, which are comparable to both biorthogonal decomposition and direct numerical simulation results. The new measurement technique provides for efficient decomposition of measurements and sets the stage for renewed experimental effort in this area.