THE PHYSICS OF SHOCK WAVE/BOUNDARY LAYER INTERACTION CONTROL: LAST LESSONS LEARNED

In high speed flow, the existence of shock waves most often entails either drag increase or efficiency losses. A major cause of performance degradation is the interaction of the shock with a boundary layer. Then complex phenomena occur which contributes to increase friction losses, especially if the shock is strong enough to separate the boundary layer. To a separated flow are associated typical wave patterns resulting from the shocks induced by the separation and reattachment processes and which play a major role in the production of entropy by the flow. Since shocks cannot be avoided in most situations, control techniques have been proposed to limit their negative effects. The mode of operation of these techniques can be well understood by a clear identification of shock wave/boundary layer interaction properties. The control actions can be performed by a proper manipulation of the boundary layer upstream of the interaction domain in order to increase its resistance to the shock action (by blowing or lowering the wall temperature, or using vortex generators) or by a local action in the shock foot region. Then active, passive or hybrid control which combines the two previous actions can be applied. Other methods can be envisaged, like the installation of a bump in the shock foot region to adapt the surface contour in order to weaken the shock. None of these techniques brings the ideal answer to the problem of shock wave/boundary layer interaction control. Thus, the definition of a solution closely depends on the objective of the control. In addition, the appropriateness of implementing a control device highly depends on economical issues in terms of weight penalty, manufacturing and maintenance cost and energy consumption.