Experimental study of the wing-rotation mechanism using a mechanical flapper operating at high Reynolds numbers (10$^{5}\sim $10$^{6})$

This work aims to investigate whether the `wing-rotation' mechanism remains effective for flapping wings operating at Reynolds numbers ranging between 10$^{5}$ and 10$^{6}$. We also address the feasibility of evaluating transient lift forces based on a modified vortex-ring model. The experiments were conducted with a biomimetic mechanical flapper that partially emulates the dominant motions of flapping wings of hovering \textit{Zosterops japonicus} and \textit{Erythrura gouldiae}. Lift forces produced by the flapping wing of the mechanical flapper were directly measured via a load cell, and indirectly evaluated using a modified vortex-ring model, according to the wing wake flow-fields quantified by DPIV. Lift forces evaluated through the vortex-ring model were found to be of values approximately 15{\%} less than those measured by the load cell. It was also found that an increase in the pitching frequency of the flapping wing is capable of enhancing the lift production. This finding suggests that for a flapping wing, the `wing-rotation' mechanism remains effective for high Reynolds numbers in a range 10$^{5}\sim $10$^{6}$.