Generation of 3D Tollmien-Schlichting waves in swept-wing boundary layer at scattering of free-stream vortices on surface non-uniformities

The paper is devoted to results of quantitative experimental study of a mechanism of excitation of three-dimensional Tollmien-Schlichting (TS) waves in a boundary layer of a 35-degree swept wing due to scattering of unsteady free-stream vortices on streamwise localized surface vibrations. The experiments are performed in the Royal Institute of Technology in Stockholm (Sweden) in the framework of studies ‘B’ of Working Package 1 of project ‘RECEPT’ of the Seventh European Framework Program. The measurements are carried out on a model of a real swept wing of an infinite span at conditions of controlled surface perturbations. Calculations and measurements have shown that at the selected angle of attack of +1.5°, the TS instability dominates in the boundary layer, while the cross-flow instability is absent due to very low cross-flow intensity at conditions of very weak streamwise pressure gradient. The base-flow parameters, as well as the parameters of the free-stream and surface perturbations (such as frequencies and range of spanwise wavenumbers) were selected based on special calculations, performed for the particular experimental conditions, as well as on preliminary experimental studies of the TS instability. The calculations have shown a very good agreement of the TS-stability characteristics with the measurements obtained in a broad range of parameters. Deep experimental data processing enables obtaining estimates of values of the coefficients of linear flow receptivity to freestream vortices (with vorticity vectors parallel to the wing leading edge) at their scattering on streamwise localized surface vibrations. This mechanism responsible for excitation of 3D TS waves has been investigated for various relations of the vortex and surface frequencies in a great range of spanwise wavenumbers and propagation angles of the excited TS instability modes