Stretching and retracting wingspan has been widely observed in the flight ofbirds and bats, and its effects on the aerodynamic performance particularlylift generation are intriguing. The rectangular flat-plate flapping wing with asinusoidally stretching and retracting wingspan is proposed as a simple modelof biologically-inspired dynamic morphing wings. Direct numerical simulationsof the low-Reynolds-number flows around the flapping morphing wing in aparametric space are conducted by using immersed boundary method. It is foundthat the instantaneous and time-averaged lift coefficients of the wing can besignificantly enhanced by dynamically changing wingspan in a flapping cycle.The lift enhancement is caused not only by changing the lifting surface area,but also manipulating the flow structures that are responsible to thegeneration of the vortex lift. The physical mechanisms behind the liftenhancement are explored by examining the three-dimensional flow structuresaround the flapping wing.
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