Effect of a mesh on boundary layer transitions induced by free-stream turbulence and an isolated roughness element

Abstract

Streamwise streaks, their lift-up and streak instability are integral to the bypass transition process. An experimental study has been carried out to find the effect of a mesh placed normal to the flow and at different wall-normal locations in the late stages of two transitional flows induced by free-stream turbulence (FST) and an isolated roughness element. The mesh causes an approximately 30 % reduction in the free-stream velocity, and mild acceleration, 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. This high-intensity disturbance can possibly enhance transition of the accelerated flow far downstream, although a reduction in streamwise turbulence intensity occurs over a short distance downstream of the mesh. However, the shape of the large-scale streamwise structure in the wall-normal plane is found to be more or less the same as that without the mesh.

Characteristics of the turbulent non-turbulent interface in a spatially evolving turbulent mixing layer

“…Here, the solidities for mesh 1 and mesh 2 were found to be 0.41 and 0.43, respectively. In their experimental study on boundary layer transition, Phani Kumar, Mandal & Dey (2015) used a similar mesh to suppress the boundary layer transition caused by enhanced free stream turbulence and roughness elements. They also reported the presence of a plane mixing layer at the bottom edge of the mesh while the mesh was kept at the outer edge of the boundary layer (see their figure 26).…”

“…These particles were generated using a SAFEX fog generator (Dantec Dynamics, Denmark) placed at the tunnel entrance. The fog was distributed uniformly across the tunnel area with help of a fan, similar to the previous works (Mandal et al 2010;Phani Kumar et al 2015;Balamurugan & Mandal 2017). The cameras were mounted adjacent to each other to capture the entire region of interest.…”

Delaying transition induced by a strip of distributed roughness using additional fine grit roughness

“…Wall-normal (x-y) plane measurements are conducted along the centerline downstream of the roughness and spanwise (x-z) plane measurements are taken at a height of y∼1mm from the wall. For more details on the PIV setup and data processing, see [24].…”