QUANTITATIVE STUDY OF EXCITATION OF UNSTEADY GÖRTLER INSTABILITY MODES IN CONCAVE-WALL BOUNDARY LAYER BY FREE-STREAM TURBULENCE

The paper is devoted to the first experimental study of distributed excitation of Görtler instability modes due the distributed mechanism of receptivity of a concave-wall boundary layer to streamwise freestream vortices. Experiments are carried out in the following range of problem parameters: Görtler numbers G* = 7,4÷21,3, frequencies f = 15, 20, and 26 Hz (nondimensional frequency parameters are F = 17,04; 22,72 and 29,54), and a broad range of spanwise scales of disturbances λ_z = 8÷24 мм (nondimensional scales are Λ = 149÷774). It is found that this receptivity mechanism is quite efficient and can lead to amplification of unsteady Görtler vortices even in regimes where the boundary layer is linearly stable to these boundary-layer disturbances. Estimations of quantitative characteristics of the investigated physical phenomenon: the complex values of the distributed vortex receptivity coefficients are obtained in the present study for the first time. It is found that examined receptivity mechanism is especially effective for vortices with spanwise wavelengths close to the most dangerous in terms of the linear stability theory. The amplitudes of the receptivity coefficients are found to decrease with the streamwise coordinate.

Keywords:
boundary layer, concave wall, laminar-turbulent transition, unsteady Görtler instability, distributed vortex receptivity
УДК 532.526

QUANTITATIVE STUDY OF EXCITATION OF UNSTEADY GÖRTLER INSTABILITY MODES IN CONCAVE-WALL BOUNDARY LAYER BY FREE-STREAM TURBULENCE