Boundary Layer Receptivity to the External Disturbances

The influence of external and local disturbances in the main flow to the transition of a flat-plate boundary layer is investigated by a wind-tunnel experiment and a numerical simulation. In the experiment, the intermittent collisions of pairs of vortex rings are used as the external disturbance. The high- and low-speed regions are observed in the boundary layer before the external disturbance in the freestream reaches the outer-edge of the boundary layer. They grow downstream and the transition occurs in the high-speed region. The intensity of random component became lager in the high-speed region than that in the low-speed region. In the computation, the external localized disturbances are generated using random numbers and they are added to the cubic area in the freestream outside the laminar boundary layer. As a result, the vorticity fluctuations, similar to the experiment, are observed near the wall inside the boundary layer immediately after the external disturbances are added, and develop into a strong vortical structure. It is found that the vortical structures that appear inside the boundary layer directly below the outer disturbances are strong.