DNS Study for Mechanism of Formation of Lambda Rotational Core and Ring-like Vortex in Late Boundary Layer Transition

摘要

A-vortex and ring-like vortex play a critical role in the boundary layer transition process. In this paper, the mechanisms of the formation of A-shaped rotational core and ring-like vortex are studied by DNS. The enforced unstable 2D T-S wave at inflow helps lift up the spanwise vorticity. The 3D TS wave is enlarged spanwisely during the linear growth process and it is considered as the mechanism of alternating spanwise vortex to A-shaped rotation centers. However, it is proved that the linear T-S waves cannot form the vortex structures by themselves. The role of the linear unstable modes is only pushing wall vorticity up ( vorticiy roll up) and then form an inflection point of the velocity profile, where the shear transfers to rotation as a result of shear instability. Therefore, the analytic linear solution is very different from DNS at the very beginning. It is also found that the high shear layer formed by the A-vortex is the mechanism of ring-like vortex generation. The generation and evolution process of the first ring-like vortex structure is studied in detail.