Simulations of bypass transition

Bypass transition in an initially laminar boundary layer beneath free-strea m turbulence is simulated numerically. New perspectives on this phenomenon are obtained from the numerical flow fields. Transition precursors consist of long backward jets contained in the fluctuating u-velocity field; they f low backwards relative to the local mean velocity, The jets extend into the upper portion of the boundary layer, where they interact with free-stream eddies. In some locations a free-stream perturbation to the jet shear layer develops into a patch of irregular motion - a sort of turbulent spot. The spot spreads longitudinally and laterally, and ultimately merges into the d ownstream turbulent boundary layer. Merging spots maintain the upstream edg e of the turbulent region. The jets, themselves, are produced by low-freque ncy components of the free-stream turbulence that penetrate into the lamina r boundary layer. Backward jets are a component of laminar region streaks. A method to construct turbulent inflow from Orr-Sommerfeld continuous modes is described. The free-stream turbulent intensity was chosen to correspond with the experiment by Roach & Brierly (1990). Ensemble-averaged numerical data are shown to be in good agreement with laboratory measurements.