Experimental and numerical investigation of the three-dimensional transition in plane wakes

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Abstract
The three-dimensional structure of a moderate-Reynolds-number (≈ 100) plane wake behind a flat plate subjected to periodic spanwise perturbations has been studied both experimentally and numerically. Comparisons between experimental interface visualizations and numerical calculations demonstrate that important features of the development of the three-dimensional evolution can be reproduced by numerical inviscid vortex dynamics. It is shown that the redistribution, reorientation and stretching of vorticity leads to the formation of counter-rotating pairs of streamwise vortices which superimpose onto the spanwise Kármán-like vortices. These streamwise vortices exhibit lambdashaped structures and are located in the braids connecting consecutive Kármán vortices of opposite sign. The interaction of the evolving streamwise structure with the spanwise Kármán vortices results in the formation of closed vortex loops. Depending on the orientation of the initial perturbation, the three-dimensional vorticity field of the wake acquires either a symmetric or a non-symmetric configuration. Under the effect of a periodic vertical perturbation, the wake develops a non-symmetric vorticity mode exhibiting a staggered array of closed vortex loops of alternating sign. In contrast, under the effect of a periodic horizontal perturbation, the wake acquires a symmetric vorticity mode with the closed vortex loops of alternating sign aligned in the flow direction.