Effect of a Mesh on Boundary Layer Transition Induced by Free-stream Turbulence and an Isolated Roughness Element

A high level of free-stream turbulence and surface roughness are known to cause breakdown of an otherwise stable laminar flow. In transition induced by free-stream turbulence, streaks are formed due to the lift-up effect and low-speed streaks with high shear breakdown to turbulence. Streaks are also present in transition caused by a roughness element and they may breakdown via sinuous or varicose instability. In general, streamwise streaks, their lift-up and streak instability are integral to the bypass transition process. If the lift-up of a high-shear layer or its breakdown is manipulated by some external means, then the downstream flow is expected to change. An experimental study was carried out to understand the effect of flow modification caused by a mesh placed normal to the flow and at different wall-normal locations in the late stage of bypass transitions induced separately by an isolated cylindrical roughness element and a high level of free-stream turbulence. The measurements were made on a flat plate boundary layer in a low-speed wind tunnel using the particle image velocimetry technique. The mesh causes an approximately 30% reduction in the free-stream velocity, and mild acceleration in the boundary layer, irrespective of its wall-normal location. Interestingly, when located near the wall, the mesh suppresses several transitional events leading to transition delay over a large downstream distance. The transition delay is found to be mainly caused by suppression of the lift-up of the high-shear layer and its distortion, along with modification of the spanwise streaky structure to an orderly one. However, with the mesh well away from the wall, the lifted-up shear layer remains largely unaffected, and the downstream boundary layer velocity profile develops an overshoot which is found to follow a plane mixing layer type profile up to the free stream. Reynolds stresses, and the size and strength of vortices increase in this mixing layer region. The high-intensity disturbance in this region can possibly enhance the transition of accelerated flow far downstream, although a reduction in streamwise turbulence intensity occurs over a short distance downstream of the mesh. However, the shape of large-scale streamwise structure in the wall-normal plane is found to be more or less the same as that without the mesh