Pan C.   Wang J.  

Lagrangian-based Visualization of Stagnation Vortex Pair

Reporter: Pan C.

It is well known that the stagnation flow in front of a bluff body can be significantly affected by free-stream vortical disturbance [1, 2], which has practical use in convective heat transfer enhancement. Recent experiment [3] has shown that the wake of an upstream-positioned thin interference wire can induce a pair of counter-rotating vortices in the stagnation region of a circular disk, which has a size one order larger than that of the wire. This vortex pair grows steadily around the centre of the disk till an elliptic instability occurs on the vortex pair, leading to a quick shedding from either direction. In [3], the dynamic process of vortex formation/growth/shedding was shown by means of vorticity contour plots obtained from 2D PIV measurement. From the sense of vortex identification, vorticity is not a good choice of vortex indicator because of two aspects: 1) the ambiguity between vortical structure and background shearing and 2) the manually selected threshold to define the vortex boundary. Therefore, here we use a Lagrangian-based vortex identification method, Finite Time Lyapunov Exponents (FTLE), to visualize the vortex dynamics from PIV-measured velocity field. This method is based on the concept that vortex boundary can be regarded as the hyperbolic material surface which distinguishes fluids inside the vortex from its surrounding background, and thus uses time-integrated divergence/convergence rate of neighbouring fluid particle trajectories as the indicator.

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