Statistical description of laminar-turbulent transition in a boundary layer at high freestream turbulence degree

The statistical approach is applied to calculate the intermittence coefficient in boundarylayer laminar-turbulent transition due to external turbulence. It is assumed that turbulent regions in the boundary layer are associated with the appearance of turbulent spots generated by the secondary instability of streaky structures that have reached a threshold amplitude. An universal dependence of the intermittence coefficient in the transition region on the velocity fluctuation amplitude and the Reynolds number is obtained. It describes well the experimental results.