Boundary Layer Receptivity to Localized Disturbances in Freestream Caused by a Vortex Ring Collision

The receptivity of a smooth flat plate to localized disturbances in freestream is investigated experimentally and numerically. The disturbances are generated outside a nominally-zero-pressure-gradient laminar boundary layer by a collision of two identical vortex rings with opposite signs. The vortex rings are generated by intermittent ejections of short duration jets from nozzles facing each other in the spanwise direction. A pair of rolled up vortex rings is given as the initial disturbances in the direct numerical simulation, and the growth of a boundary layer is simulated for a range of the Reynolds number based on the displacement thickness of boundary layer, 704 ≦ Re * ≦ 844. In the experimental results, high- and low-speed regions aligned in the streamwise direction are observed in the boundary layer before the external disturbances in the freestream reach the outer-edge of the boundary layer. Although velocity fluctuations inside both regions become stronger downstream, a transition to turbulence takes place only in the highspeed region at approximately Re * = 844. In the numerical results, vortical fluctuations similar to the experiment appear near the wall immediately after the vortex-ring-type disturbances are added in the freestream, but it is found that the vortical fluctuations do not directly grow into strong vortical structures. On the contrary, the development of strong vortical structures that leads to transition is triggered by the external disturbances directly intruding the boundary layer.