The wake symmetry properties of a flapping-foil system are closely associated with its propulsive performance. In the present work, the effect of the foil flexibility on the wake symmetry properties of a self-propelled plunging foil is studied numerically. We compare the wakes of a flexible foil and a rigid foil at a low flapping Reynolds number of 200. The two foils are of the same dimensions, flapping frequency, leading-edge amplitude and cruising velocity but different bending rigidities. The results indicate that flexibility can either inhibit or trigger the symmetry breaking of the wake. We find that there exists a threshold value of vortex circulation above which symmetry breaking occurs. The modification of vortex circulation is found to be the pivotal factor in the influence of the foil flexibility on the wake symmetry properties. An increase in flexibility can result in a reduction in the vorticity production at the leading edge because ofthe decrease in the effective angle of attack, but it also enhances vorticity production at the trailing edge because of the increase in the trailing-edge flapping velocity. The competition between these two opposing effects eventually determines the strength of vortex circulation, which, in turn, governs the wake symmetry properties. Further investigation indicates that the former effect is related to the streamlined shape of the deformed foil while the latter effect is associated with structural resonance. The results of this work provide new insights into the functional role of passive flexibility inflapping-based biolocomotion.
[http://dx.doi.org/10.1017/jfm.2014.310]
This work was supported by the National Natural Science Foundation of China (grant numbers 11021262, 11023001, 11232011 and 11372331) and the National Basic Research Program of China (grant number 2013CB834100). We are grateful to Professor Z. C. Zheng for valuable discussions regarding the computation of vortex circulation.
The article was published as:
Zhu XJ,He GW,Zhang X. How Flexibility Affects The Wake Symmetry Properties Of A Self-propelled Plunging Foil[J].Journal Of Fluid Mechanics,2014,751:164-183.