Effect of free stream turbulence on turbulent boundary layers in incompressible flow

The influence of a free-stream turbulence on a turbulent boundary layer subjected to a positive pressure gradient was studied. A high level of turbulence was generated by a grid of rods. Measurements of the mean velocity and of the fluctuations in the boundary layer are presented. The longitudinal velocity fluctuation is characterized by its mean quadratic value and by its spectrum. The influence of the free-stream turbulence seems to be restricted to the external region of the boundary layer. The most important effects are obtained around the separation point. Calculations of the boundary layer by means of a set of transport equations are compared with the present experiments and other available data. An integral method is also proposed, in which an adapted mixing length scheme is used. Both calculation methods give a good level of agreement with experiments.