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Start-up vortex flow past an accelerated flat plate

Viscous flow past a finite flat plate moving in direction normal to itself is studied numerically.The plate moves with velocity $at^p$, where $p=0,0.5,1,2$. We present the evolution of vorticity profiles, streaklines and streamlines, and study the dependence on the acceleration parameter $p$. Four stages in the vortex evolution, as proposed by Luchini & Tognaccini (2002), are clearly identified. The initial stage, in which the vorticity consists solely of a Rayleigh boundary layer, is shown to last for a time-interval whose length shrinks to zero like $p^3$, as $p \to 0$. In the second stage, a center of rotation develops near the tip of the plate, well before a vorticity maximum within the vortex core develops. Once the vorticity maximum develops, its position oscillates and differs from the center of rotation. The difference between the two increases with increasing $p$, and decreases in time. In the third stage, the center of rotation and the shed circulation closely satisfy self-similar scaling laws for inviscid flow. Finally, in the fourth stage, the finite plate length becomes relevant and the flow begins to depart from the self-similar behaviour. While the core trajectory and circulation closely satisfy inviscid scaling laws, the vorticity maximum and the boundary layer thickness follow viscous scaling laws. The results are compared with experimental results of Pullin & Perry (1980), and Taneda & Honji (1971), where available.