Two- and three-dimensional numerical simulations of the clap–fling–sweep of hovering insects

The importance of three-dimensional effects for flapping wings is addressed by means of numerical simulation. In particular, the clap–fling–sweep mechanism is examined. The flow at the beginning of the downstroke is shown to be in reasonable agreement with the two-dimensional approximation. After the wings move farther than one chord length apart, three-dimensional effects become essential. Two values of the Reynolds number are considered. At Re=128, the spanwise flow from the wing roots to the wing tips is driven by the centrifugal forces acting on the mass of the fluid trapped in the recirculation bubble behind the wings. It removes the excess of vorticity and delays the periodic vortex shedding. At Re=1400, vortex breakdown occurs past the outer portion of the wings, and multiple vortex filaments are shed into the wake.

► We study the three-dimensional clap–fling–sweep mechanism.
► At the beginning of the stroke, the two-dimensional approximation is justified.
► After the wings move apart, three-dimensional effects become essential.
► Centrifugal forces acting on the recirculation bubble generate a spanwise flow.
► Leading-vortex breakdown is observed at the Reynolds number Re=1400.