Transient growth of stationary disturbances in a flat plate boundary layer

Theoretical and direct numerical simulation models of transient algebraic growth in boundary layers have advanced significantly without an adequate, parallel experimental effort. Experiments that feature disturbances excited by high levels of freestream turbulence or distributed surface roughness show behavior consistent with optimal-disturbance theories but cannot address the theories’ key predictions concerning the growth and decay of disturbances at specific spanwise wavenumbers. The present experiment seeks to provide such data for a flat plate boundary layer using a spanwise roughness array to excite controlled stationary disturbances. The results show that although general trends and qualitative behaviors are correctly captured by optimal-disturbance theories, significant quantitative differences exist between the theories’ predictions and the current experimental measurements. Discrepancies include the location of the wall-normal disturbance profiles’ maxima and the streamwise location of the maximum energy growth. While these discrepancies do not argue against the validity of transient-growth theory in general, they do indicate that correct modeling of receptivity to realistic disturbances is critical and that realistic stationary disturbances can exhibit strongly nonoptimal behavior.