Spatial numerical simulations and experiments on the breakdown of streamwise vortices in a blasius boundary layer

Hairpin vortices have been found to be one of the predominant coherent structures in both transitional and turbulent wall-bounded shear flows. Here, we study, both experimentally and numerically, the evolution of a hairpin vortex from its birth, through its growth as it interacts with the mean shear, to its breakdown. The vortices are introduced in a subcritical laminar boundary layer by fluid injection through a long and thin slot aligned with the free stream direction. With the Reynolds number and slot geometry fixed, the control parameter of this experiment is the ratio of the injection velocity to the free stream velocity. This investigation is relevant to both the study of near-wall turbulent coherent structures and by-pass transition.