Distributed Vortex–Roughness Receptivity of a Swept-Wing Boundary Layer: 2. Receptivity Characteristics

The distributed receptivity of a swept-wing laminar boundary layer to unsteady free-stream vortices with streamwise vorticity in the presence of spanwise-uniform surface roughness has been investigated experimentally. The experiments were performed on a 25-degree swept-wing model under fully controlled disturbance conditions. It has been found that unsteady streamwise flow vortices can lead to very efficient streamwise-distributed excitation of unsteady crossflow instability modes at combination spanwise wavenumbers due to vortex scattering on surface roughness. Part 1 of this study (published in the previous issue of the journal) is devoted to describing the experimental approach and its theoretical justification; the experimental setup; the base-flow structure; the excitation method and characteristics of free-stream and surface disturbances; experimental evidence of the high efficiency of the receptivity mechanism under study; and experimental confirmation of the important role of streamwise wavenumber resonance in the excitation of the most amplified crossflow instability modes. Part 2 of this study (this article) is devoted to the experimental evaluation of the amplitudes and phases of distributed vortex–roughness receptivity coefficients as a function of disturbance frequency and spanwise wavenumber. In addition, the receptivity coefficients responsible for the excitation of crossflow waves on a smooth surface are obtained, and the relative efficiencies of the distributed vortex receptivity and vortex–roughness receptivity mechanisms are compared. The results are also compared with those obtained previously for distributed vortex receptivity on a smooth surface.