1THE INFLUENCE OF FIN RIGIDITY AND GUSTS ON THE FORCE PRODUCTION IN FISHES AND INSECTS: A COMPUTATIONAL STUDY

The three-dimensional unsteady computations of fish swimming with oscillating and deforming fins of varying rigidity were carried out. The objective of these variable rigidity computations was to investigate the importance of fin deformation on the fluid dynamics of force production. An unstructured grid-based unsteady Navier-Stokes solver with automatic adaptive remeshing was used to compute the flow about the wrasse through several complete cycles of pectoral fin oscillation for each of the fins studied. The computations show that when the fin is made rigid by specifying the motion with just the leading edge of the fin tip, the thrust produced during the upstroke is less than half of the peak thrust produced by the flexible cases. During the downstroke, the rigid fin and the fin with the motion prescribed with only the leading and trailing edges produced no positive thrust, while all the flexible cases considered reproduced the thrust production of the fully deformable fin. In the case of the rigid fin, there is a substantial penalty in lift during the upstroke. We have also computed the unsteady flow computations over the Drosophila wing with the flight conditions ranging from hovering to a downward gust velocity nearly equal to the mean wing tip velocity. We showed that the wake capture mechanism which is responsible for a peak in thrust production just after stroke reversal diminished with increasing downward velocity and is entirely absent when this velocity reaches the mean wing tip velocity