Excitation of unsteady cross-flow instability modes at scattering of free-stream vortices on surface vibrations

Kurzfassung

The results of quantitative experimental study of a mechanism of excitation of unsteady (in general) cross-flow instability modes in a boundary layer of a 35-degree swept wing due to scattering of unsteady free-stream vortices on streamwise localized surface vibrations are presented in this paper. The experiments are performed in the Royal Institute of Technology in Stockholm (Sweden) in the framework of studies ‘A’ 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 −5°, cross-flow instability dominates in the boundary layer. 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 calculations, performed for the particular experimental conditions, as well as on preliminary experimental studies of the cross-flow instability. The calculations have shown very good agreement of stability characteristics with the measurements obtained in a broad range of parameters. After deep processing of the experimental data, estimates of values of 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 are obtained for the receptivity mechanism responsible for excitation of the cross-flow instability modes. The coefficients are obtained for various relations of the vortex and surface frequencies in a great range of spanwise wavenumbers of the excited cross-flow instability modes.