ACTIVE VORTICITY CONTROL IN A SHEAR-FLOW USING A FLAPPING FOIL

It is shown experimentally that free shear flows can be substantially altered through direct control of the large coherent vortices present in the flow. First, flow-visualization experiments are conducted in Ka lliroscope fluid at Reynolds number 550. A foil is placed in the wake of a D-section cylinder, sufficiently far behind the cylinder so that it does not interfere with the vortex formation process. The foil perf orms a heaving and pitching oscillation at a frequency close to the St rouhal frequency of the cylinder, while cylinder and foil also move fo rward at constant speed. By varying the phase of the foil oscillation, three basic interaction modes are identified. (i) Formation of a stre et of pairs of counter-rotating vortices, each pair consisting of one vortex from the initial street of the cylinder and one vortex shed by the foil. The width of the wake is then substantially increased. (ii) Formation of a street of vortices with reduced or even reverse circula tion compared to that of oncoming cylinder vortices, through repositio ning of cylinder vortices by the foil and interaction with vorticity o f the opposite sign shed from the trailing edge of the foil. (iii) For mation of a street of vortices with circulation increased through merg ing of cylinder vortices with vortices of the same sign shed by the fo il. In modes (ii) and (iii) considerable repositioning of the cylinder vortices takes place immediately behind the foil, resulting in a regu lar or reverse Karman street. The formation of these three interaction patterns is achieved only for specific parametric values; for differe nt values of the parameters no dominant stable pattern emerges. Subseq uently, the experiments are repeated in a different facility at larger scale, resulting in Reynolds number 20000, in order to obtain force a nd torque measurements. The purpose of the second set of experiments i s to assess the impact of flow control on the efficiency of the oscill ating foil, and hence investigate the possibility of energy extraction . It is found that the efficiency of the foil depends strongly on the phase difference between the oscillation of the foil and the arrival o f cylinder vortices. Peaks in foil efficiency are associated with the formation of a street of weakened vortices and energy extraction by th e foil from the vortices of the vortex street.