A physical model for bypass transition

A new method of predicting boundary layer transition is presented which mod els the near wall velocity fluctuations induced in the laminar layer throug h pressure fluctuations associated with the freestream turbulence. These ne ar wall velocity fluctuations are then assumed to develop into turbulent sp ots when their amplitude exceeds a threshold value. A relationship for the near wall velocity frequency spectra is also established, which indicates a n increasing bias towards low frequencies as the skin friction coefficient for the boundary layer decreases, This result suggests that the dependence of transition on the turbulent length scale is greatest at low freestream t urbulence levels. This transition model is incorporated in a conventional b oundary layer integral technique and is used to predict eight of the ERCOFT AC test cases and measurements of Gostelow and co-workers. The model is dem onstrated to predict the development of the boundary layer through transiti on reasonably accurate for all the test cases. The sensitivity of start of transition to the turbulent length scale at low freestream turbulence level s is also demonstrated. The model is also able to predict the evolution of measured intermittency more accurately than the Narasimha empirical correla tion. (C) 1999 Elsevier Science Inc. All rights reserved.