Optimal disturbances and bypass transition in boundary layers

Streamwise streaks are ubiquitous in transitional boundary layers, particul arly when subjected to high levels of free-stream turbulence. Using the ste ady boundary-layer approximation, the upstream disturbances experiencing ma ximum spatial energy growth are numerically calculated. The calculations us e techniques commonly employed when solving optimal-control problems for di stributed parameter systems. The calculated optimal disturbances consist of streamwise vortices developing into streamwise streaks. The maximum spatia l energy,growth was found to scale linearly with the distance from the lead ing edge. Based on these results, a simple model for prediction of transiti on location is proposed. Available experiments have been used to correlate the single constant appearing in the model. (C) 1999 American Institute of Physics. [S1070-6631(99)01601-3].