Transition induced by free stream turbulence

Authors: Fransson, J. H. M., Matsubara, M., Alfredsson, P.H.
Document Type: Article
Pubstate: Published
Journal: J. Fluid Mech.
Volume: 527   1-25
Year: 2005


Free stream turbulence is maybe the most important source to force by-pass transition in boundary layer flows. The present study aims at describing the initial energy growth of streamwise oriented disturbances in the boundary layer originating from the presence of turbulence intensities between 1.4% and 6.7%, but also to lay the ground for a prediction model for free stream turbulence induced transition. For this study three passive and one active turbulence generating grids were used. The active grid was built in order to vary the turbulence intensity (Tu) with only a small change of the characteristic turbulence scales. It is shown that the initial energy growth in the boundary layer is linear in the downstream distance and proportional to Tu^2, whereas the transitional Reynolds number is shown to be inversely proportional to Tu^2. The intermittency in the transitional zone was determined and it was shown that by scaling the intermittency function with the length of the transition zone a universal function could be obtained. The length of the transition zone was found to increase linearly with the transition Reynolds number, however it was also noted that the length has a minimum value. With these results we were able to formulate an expression for the spot production rate which has a better physical base than previous models.