Computational and Experimental Investigation of Transition Control in BoundaryLayers. Receptivity of a Flat Plate Boundary Layer to Free-Stream Pressure Waves

摘要

A numerical simulation of a receptivity experiment by Kendall was performed,solving the 2-D, incompressible Navier-Stokes equations for a flat plate boundary layer. Emphasis was placed on the investigation of the mechanism that lead to the shift of energy from low frequencies to higher frequencies and to the different output/input ratios for different cylinder diameters in the experiment. The rotating cylinders that were used in the experiment to generate disturbances in the freestream were modeled by a numerical forcing function at the freestream boundary. It was found that the low frequency components of the induced disturbances, together with the steady flow, form a new, transient baseflow for disturbances with higher frequencies. It was demonstrated how the changing receptivity and stability characteristics of this baseflow lead to distinct peaks in the wavepackets that are generated by the freestream disturbance. Further investigation showed that the different output/input ratios can essentially be explained by the different amplitudes of the lowest frequencies in the forcing. Finally, a numerical experiment was performed with only two frequency components present in the forcing function. The validity of the proposed mechanism was confirmed by comparison of the results of this numerical experiment with those calculated from linear stability theory for the transient baseflow. (AN).