Acoustic receptivity due to weak surface inhomogeneities in adverse pressure gradient boundary layers
Abstract
The boundary layer receptivity to freestream acoustic waves in the presence of localized surface disturbances is studied for the case of incompressible FalknerSkan flows with adverse pressure gradients. These boundary layers are unstable to both viscous and inviscid (i.e., inflectional) modes, and the finite Reynolds number extension of the GoldsteinRuban theory provides a convenient method to compare the efficiency of the localized receptivity processes in these two cases. The value of the efficiency function related to the receptivity caused by localized distortions in surface geometry is relatively insensitive to the type of instability mechanism, provided that the same reference length scale is used to normalize the efficiency function for each type of instability. In contrast, when the receptivity is induced by variations in wall suction velocity or in wall admittance distribution, the magnitudes of the related efficiency functions, as well as the resulting coupling coefficients, are smaller for inflectional (i.e., Rayleigh) modes than for the viscous TollmienSchlichting waves. The reduced levels of receptivity can be attributed mainly to the shorter wavelengths and higher frequencies of the inflectional modes. Because the most critical band of frequencies shifts toward higher values, the overall efficiency of the wall suction and the wall admittanceinduced receptivity decreases with an increase in the adverse pressure gradient.
 Publication:

NASA STI/Recon Technical Report N
 Pub Date:
 February 1995
 Bibcode:
 1995STIN...9521258C
 Keywords:

 Boundary Layer Transition;
 FalknerSkan Equation;
 Laminar Boundary Layer;
 Pressure Gradients;
 Reynolds Number;
 Sound Waves;
 Transition Flow;
 Aircraft Design;
 Critical Frequencies;
 Incompressible Flow;
 TollmienSchlichting Waves;
 Fluid Mechanics and Heat Transfer