Vortex formation in a cavity with oscillating walls

The vortex formation in a two-dimensional Cartesian cavity, which their vertical walls move simultaneously with an oscillatory velocity and the horizontal walls are fixed pistons, is studied numerically. The governing equations were solved with a finite element method combined with an operator splitting scheme. We analyzed the behavior of vortical structures occurring inside a cavity with an aspect ratio of height-to-width of 1.5 for three different displacement amplitudes of the vertical oscillatory walls (amplitude/width=Y=0.2, 0.4, and 0.8) and Reynolds numbers based on the cavity width of 50, 500, and 1000. Two vortex formation mechanisms are identified: (a) the shear, oscillatory motion of the moving boundaries coupled with the fixed walls that provide a translational symmetry-breaking effect and (b) the sharp changes in the flow motion when the flow meets the corners of the cavity. The vortex cores were identified using the Jeong–Hussain criterion and it is found that the area occupied by the cores ...