The paper is devoted to an experimental and numerical investigation of the problem of excitation of three-dimensional Tollmien–Schlichting (TS) waves in a boundary layer on an airfoil owing to scattering of an acoustic wave on localized microscopic surface non-uniformities. The experiments were performed at controlled disturbance conditions on a symmetric airfoil section at zero angle of attack. In each set of measurements, the acoustic wave had a fixed frequency fac, in the range of unstable TS-waves. The three-dimensional surface non-uniformity was positioned close to the neutral stability point at branch I for the two-dimensional perturbations. To avoid experimental difficulties in the distinction of the hot-wire signals measured at the same (acoustic) frequency but having a different physical nature, the surface roughness was simulated by a quasi-stationary surface non-uniformity (a vibrator) oscillating with a low frequency fv. This led to the generation of TS-wavetrains at combination frequencies f1,2=fac ∓ fv. The spatial behaviour of these wavetrains has been studied in detail for three different values of the acoustic frequency. The disturbances were decomposed into normal oblique TS-modes. The initial amplitudes and phases of these modes (i.e. at the position of the vibrator) were determined by means of an upstream extrapolation of the experimental data. The shape of the vibrator oscillations was measured by means of a laser triangulation device and mapped onto the Fourier space.

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The influence of humps and steps on the stability characteristics of a 2D laminar boundary layer is investigated by means of Direct Numerical Simulations (DNS). The localized surface irregularity is hereby modeled within the cartesian grid by assigning body forces over surfaces that need not coincide with grid lines. Compared to the use of a body fitted coordinate system this method saves memory and computation time. The method is validated by grid refinement tests as well as by a comparison with water channel experiments. The DNS results for the steady flow over a rectangular hump as well as for an instability wave traveling over a hump show a good agreement with the experimental ones. Simulation results show that a localized hump destabilizes the laminar boundary layer, whereas a forward facing step stabilizes it. The destabilization is stronger when the height or the width of the localized hump are increased. A rounded shape of the hump is less destabilizing than a rectangular shape with sharp corners. The parameter which shows the strongest influence on the stability characteristics of the boundary layer is clearly the height of the localized hump.