A physical model for bypass transition

Abstract

Abstract A new method of predicting boundary layer transition is presented which models the near wall velocity fluctuations induced in the laminar layer through pressure fluctuations associated with the freestream turbulence. These near wall velocity fluctuations are then assumed to develop into turbulent spots when their amplitude exceeds a threshold value. A relationship for the near wall velocity frequency spectra is also established, which indicates an 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 turbulence levels. This transition model is incorporated in a conventional boundary layer integral technique and is used to predict eight of the ERCOFTAC test cases and measurements of Gostelow and co-workers. The model is demonstrated to predict the development of the boundary layer through transition reasonably accurate for all the test cases. The sensitivity of start of transition to the turbulent length scale at low freestream turbulence levels is also demonstrated. The model is also able to predict the evolution of measured intermittency more accurately than the Narasimha empirical correlation.