Home – Home – Jornals – Thermophysics and Aeromechanics 2004 number 3

In the present study, we examined the distributed (along the streamwise coordinate) excitation of 3D Tollmien ( Schlichting (TS) waves by weak non-stationary free-stream vortices propagating along the edge of a thickening laminar boundary layer obtained on a plate with 2D small-height surface roughness. We performed a theoretical analysis of the operating excitation mechanism and, based on this analysis, introduced functions (coefficients) of distributed receptivity for the flow and proposed an experimental procedure for determining these functions based on fitting experimental distributions with analytical solutions. Under conditions of controllable disturbances, we performed a quantitative experimental study of Blasius boundary-layer receptivity to non-stationary free-stream vortices with wall-normal vorticity vector in the presence of 2D streamwise-periodic surface roughness. A detailed thermoanemometric study of disturbances in the free stream and in the boundary layer was carried out, and the shape (waveform) of the surface roughness was measured. By the point-source technique, we experimentally examined the linear 3D stability characteristics of the flow under study necessary for obtaining the coefficients of distributed receptivity. The free-stream vortices were found to excite 3D TS waves via two receptivity mechanisms: on the smooth surface and through the interaction of free-stream vortices with surface roughness. Based on the new approach, we experimentally determined the amplitudes and phases of the coefficients of distributed vortical receptivity of both above-indicated types as functions of spanwise wavenumber. The flow is found to be most sensitive to quasi-2D vortical disturbances. At the same time, for the chosen streamwise wavenumber of surface roughness, it is 3D TS waves that are excited most efficiently. For these waves, a wavenumber resonance is possible, proved to play a fundamental role in the generation of instability waves. A large width of this resonance in the spectrum of spanwise wavenumbers is demonstrated.