Distributed vortex-roughness receptivity of a swept-wing boundary layer 1: Resonant excitation of crossflow instability modes

We experimentally investigate the distributed receptivity of a laminar swept-wing boundary layer to unsteady freestream vortices with streamwise-aligned vorticity in the presence of spanwise-uniform surface waviness. Experiments were performed on a 25$^{\circ}$ swept-wing model in a well-controlled disturbance environment. We demonstrate that unsteady longitudinal vortices can very efficiently excite-in a distributed manner along the streamwise direction-unsteady crossflow instability modes at specific combination transverse wavenumbers. This excitation results from the scattering of the vortices by the surface inhomogeneities. The present paper (Part 1 of this study) is devoted to describing the experimental approach and its theoretical background, the mean flow structure, the experimental evidence of the high efficiency of this receptivity mechanism, and the experimental verification and critical role of the streamwise wavenumber resonance. Part 2 of this investigation focuses on the experimental determination of the amplitude and phase of the distributed vortex-roughness receptivity coefficients as functions of disturbance frequency and transverse wavenumber. In Part 2, we also determine the receptivity coefficients responsible for exciting crossflow instability waves on a smooth surface and present a comparative assessment of the relative efficiencies of these two distinct mechanisms.