Influence of flexibility on the aerodynamic performance of a hovering wing

M. VANELLA; T. FITZGERALD; S. PREIDIKMAN; E. BALARAS; B. BALACHANDRAN

JOURNAL OF EXPERIMENTAL BIOLOGY

COMPANY OF BIOLOGISTS LTD

Año: 2009 vol. 212 p. 95 - 105

0022-0949

In the present work, a computational investigation is carried out to understand the influence of flexibility on the aerodynamic performance of a hovering wing. A flexible, two-dimensional, two-link model moving within a viscous fluid is considered. The Navier-Stokes equations governing the fluid dynamics are solved together with the equations governing the structural dynamics by using a strongly coupled fluid-structure interaction scheme.  Harmonic kinematics is used to prescribe the motions of one of the links, thus effectively reducing the wing to a single degree-of-freedom oscillator. The wing?s flexibility is characterized by the ratio of the flapping frequency to the natural frequency of the structure. Four different values of this frequency ratio are considered at the Reynolds numbers of 75, 250, and 1000. It is found that flexibility can enhance the aerodynamic performance and that the best performance is realized when the wing is excited by a nonlinear resonance at one-third of the natural frequency.   Specifically, at the Reynolds numbers of 75, 250 and 1000, the aerodynamic performance that is characterized by the ratio of lift coefficient to drag coefficient is respectively increased by 28%, 23%, and 21% when compared with the corresponding ratios of a rigid wing driven with the same kinematics.  For all Reynolds numbers, the lift generated per unit driving power is also enhanced in a similar manner. The wake capture mechanism is enhanced, due to a stronger flow around the wing at stroke reversal, resulting from a stronger end of stroke vortex at the trailing edge. The present study provides some clues about how flexibility affects the aerodynamic performance in low Reynolds number flapping flight. In addition, it points to the importance of considering nonlinear resonances for enhancing aerodynamic performance.