Three-dimensional acoustic-roughness receptivity of a boundary layer on an airfoil: experiment and direct numerical simulations

Abstract

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.

Receptivity coefficients at excitation of cross-flow waves due to scattering of free-stream vortices on surface vibrations

“…After this recalculation, the initial phase of the vibrations becomes equal to zero. As was indicated by Würz et al (2003), phases of this kind are convenient for extrapolation of the receptivity characteristics to the zero frequency of vibrations and for comparison of these characteristics with those obtained at scattering of the free-stream disturbances on steady surface non-uniformities. In the present case, this point is important for comparison of the obtained receptivity phases with those obtained in experiments by Borodulin et al (2008bBorodulin et al ( , 2013.…”

On the disturbance evolution downstream of a cylindrical roughness element

“…In the following, this receptivity process was also addressed for three-dimensional roughness elements (Choudhari & Kerschen 1990;Tadjfar & Bodonyi 1992) interacting with different types of free-stream disturbances. Würz et al (2003) found an increased receptivity towards external, in that case acoustical, disturbances for non-zero spanwise wavenumbers of the roughness element in their experimental and numerical investigations. Further details on the receptivity process are reviewed by Kachanov (2000) and Saric, Reed & Kerschen (2002).…”

The Influence of a Cylindrical Roughness Element on a Perturbed Laminar Boundary Layer

“…A linear relationship inbetween roughness height and generated TS-wave amplitude is only present below a certain threshold value for the roughness height. Furthermore, experimental and numerical results by Wuerz et al 7 show an increased receptivity for non-zero spanwise wave numbers of the roughness element towards external, in that case acoustical, disturbances.…”