Quantitative study of localized mechanisms of excitation of cross-flow instability modes in a swept-wing boundary layer

The paper is devoted to some results of a EC project RECEPT (FP7). The experiments were carried out in a three-dimensional boundary layer developing on an experimental model of a long-laminar-run airfoil (sweep angle of 35°, chord of 0.8 m). The model was mounted in a test section of a low-turbulence wind tunnel MTL (KTH, Stockholm) at an angle of attack of -5 degrees. The cross-flow instability modes (CF) were the most amplified ones, while the Tollmien–Schlichting instability modes were suppressed by a favorable pressure gradient. The main measurements were performed by means of a single-wire hot-wire probe at conditions of excitation of fully controlled, unsteady surface and flow perturbations. These perturbations were excited by special sources: (i) a surface membrane and (ii) a vibrating wire. The two sources oscillated at frequencies fs and fv, respectively. The membrane had a diameter of 6 mm. It was mounted into the model surface and located at a chordwise coordinate of 120 mm downstream the leading edge. The vibrating wire was mounted at a tension upstream the leading edge and parallel to it and adjusted in a way to provide location of one of the amplitude maxima of the excited quasi-2D vortex street near the boundary layer edge. The incident flow velocity was of about 10 m/s and varied in the chordwise direction. A rather efficient excitation of packets of three-dimensional CF-modes was found at frequency fs (due to action of a mechanism of the boundary-layer receptivity to surface vibrations) as well as at two combinational frequencies fsv+ = fs + fv and fsv- = fs - fv (due to action of a receptivity mechanism associated with scattering of the controlled freestream vortices on the controlled surface vibrations). The amplification curves (amplitudes and phases) of CF-modes were obtained in a broad range of problem parameters (spanwise wavenumbers and frequencies fs and fv), as well as the amplitudes and phases of the corresponding receptivity coefficients of 'vibrational' and 'vortex-vibration' receptivity mechanisms. Comparison with calculations carried out by means of linear (locally-parallel) stability theory (LST) was performed. A very good, quantitative agreement between results of measurements and calculations was found at surface frequency fs. It is found that the CF-modes excited at the combinational frequencies fsv evolve downstream in a way that is different from the LST. A thorough analysis of disturbance spanwise profiles of amplitudes and phases, their spectra, and the disturbance amplification curves showed that the features of downstream evolution of the CF-modes observed at frequency fsv can be explained by action of a distributed receptivity mechanism. This mechanism is associated with scattering of the controlled freestream vortices with frequency fv on the CF-instability modes excited by surface vibrations at frequency fs. Nowadays such distributed receptivity mechanism is remained completely unexplored. Therefore, the obtained experimental data seems to be of great basic and practical importance