Transonic-Shock-Boundary-Layer Interaction: A Case for Phenomenology

Viscous interactions play a significant role in transonic flows over airfoils, greatly affecting the forces and moments on the airfoil. Of the interactions, that between the shock wave and the boundary layer plays the greatest role. In Fig. 1 the complexities of such interactions are sketched. The resulting flow is unsteady, due to the interaction of the coherent large-scale turbulence with the shock. Schlieren pictures, for example, show the foot of the shock fluctuating in reaction to the oncoming large-scale eddies. The shock wave penetrates into the boundary layer, weakens, and vanishes as it approaches and reaches the sonic line. The subsonic layer beneath the shock further serves to cushion the airfoil from the shock, attenuating the shock pressure rise.