Hovering insects are divided into two categories: “normal” hoverers that moves the wing symmetrically in a horizontal stroke plane, and those with an inclined stroke plane. Normal hoverers have been suggested to support their weight during both down- and upstroke, shedding vortex rings each half stroke. Insects with an inclined stroke plane should, according to theory, produce flight forces only during downstroke, and only generate one set of vortices. The type of hovering is thus linked to the power required to hover. Previous efforts to characterize the wake of hovering insects have used low-resolution experimental techniques or simulated the flow using CFD, and so it remains to be determined if insect wakes can be represented by any of the suggested models. Here, we used tomographic PIV, with a horizontal measurement volume placed below the animals, to show that the wake shed by hovering hawkmoths are best be described as a series of bilateral, stacked vortex “rings”. While the upstroke is aerodynamically active, despite an inclined stroke plane, it produces weaker vortices than the downstroke. In addition, compared to the near wake, the far wake lacks structure and is less concentrated. Both near and far wakes are clearly affected by vortex interactions, suggesting caution is required when interpreting wake topologies. We also estimated induced power (Pind) from downwash velocities in the wake. Standard models predicted a Pind more than double that from our wake measurements. Our results thus question some model assumptions and we propose a reevaluation of the model parameters.
The drag-adjoint field of a circular cylinder wake at Reynolds numbers 20, 100 and 500
