A STUDY OF NONPARALLEL AND NONLINEAR EFFECTS ON THE LOCALIZED RECEPTIVITY OF BOUNDARY-LAYERS

The acoustic receptivity due to localized surface suction in a two-dim ensional boundary layer is studied using a finite-Reynolds-number theo ry and direct numerical simulation of the Navier-Stokes equations. Det ailed comparisons between the two methods are used to determine the bo unds for application of the theory. Results show a 4% difference betwe en the methods for receptivity in the neighbourhood of branch I with l ow suction levels, low acoustic levels, and a moderate frequency; we a ttribute this difference to non-parallel effects, not included in the theory. The difference is larger for receptivity upstream of branch I, and smaller for receptivity downstream of branch I. As the peak sucti on level is increased to 1% of the free-stream velocity, the simulatio ns show a nonlinear deviation from the theory. Suction levels as small as 0.1% are shown to have a significant effect on the instability gro wth between branch I and branch II. Increasing the acoustic amplitude to 1% of the steady free-stream velocity produces no significant nonli near effect.