Shock Induced Flow Oscillations in a Laval Nozzle
The physical system consisting of a shock wave and the downstream separated flow causes oscillatory instabilities in a Laval nozzle flow. The shock-boundary layer interaction creates or displaces a separation bubble, thereby changing the flow field downstream of the shock root. This process usually strengthens the shock wave by increasing the back pressure. The shock wave then becomes unsteady, moving the separation point with it in an upstream direction, so that a self-preserving instability occurs, thus reducing the flow velocity in the whole transonic field. By means of the later reattachment of the boundary layer, the entire flow is accelerated and tends to return to the initial condition, i.e., the process is cyclic. The length of the oscillation periods can be estimated. Using a one-dimensional model for the Laval nozzle flow, it can actually be calculated. Consideration of the gasdynamics of the process leads to limits in terms of pressure or Mach number for the existence of oscillations, these limits being in good agreement with experimental results.
KeywordsShock Wave Mach Number Back Pressure Separation Point Separation Bubble
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