Generation of unsteady CF-instability modes by vibrational and vibration-vortex localized receptivity mechanisms

The paper is based on results obtained within an international project ‘RECEPT’ of the European Framework Program FP7. The experiments were carried out in a three-dimensional boundary layer developing on an experimental model of a long-laminar-run swept airfoil (sweep angle of 35°). The model was mounted in a test section of the lowturbulence wind-tunnel MTL (KTH, Stockholm) at an angle of attack of −5° and equipped with sidewalls provided satisfaction of infinite-span conditions. The cross-flow (CF) instability modes were predominant in this case, while the Tollmien–Schlichting (TS) waves were suppressed by a favorable pressure gradient. The main measurements were carried out by means of hot-wire anemometry at conditions of excitation of fully controlled, unsteady surface and flow perturbations. These perturbations were excited by special sources: (1) a row of oscillating membranes and (2) a vibrating wire, at frequencies of fs and fv, respectively. A very good, quantitative agreement between the measured and calculated (by linear stability theory based on PSE approach) amplification curves was found at surface frequency fs. However, the evolution of the CF-modes excited at difference combination frequency fsv– = fs – fv turned out to be very much different from the theoretical one. Thorough analysis of the obtained results has shown that the only explanation of these discrepancies can be associated with presence of a distributed receptivity mechanism due to scattering of freestream vortices on the CF-instability waves excited by surface vibrations. Another unusual and unexpected phenomenon found in the present experiments is associated with anomalous amplification of difference combination modes with the zero spanwise wavenumbers E′. This phenomenon was observed in the flow, which is stable with respect to both CF- and TSwaves having E′ = 0 for all frequencies. There is no explanation of this finding at present