Comparison of cicada hindwings with hindwing-less drosophila for flapping motion at low Reynolds number

Abstract

The drosophila wing has a simple shape and has been used as a model in many studies. In contrast, the cicada wing has a hindwing that is relatively small compared to a forewing. Because of its hindwing, it seems that the cicada has two trailing edges, This distinction can induce differences in vortex structures around the wings. In the present work, we have conducted the immersed boundary - lattice Boltzmann method (IB-LBM) simulations to compare the vortex structures around the two wings. We considered a simple flapping motion for the wing kinematics. From the numerical results, this study shows two major differences in terms of the vortex structures around the wings. Firstly, in the case of, two trailing edge vortices(TEVs) are formed due to the wing shape. The TEVs and leading edge vortex(LEV) cause a reduction in the spanwise velocity. The low spanwise flow results in a reduced spanwise vorticity flux. In the case of the drosophila, the edge vortices, the TEV and the LEV, are balanced. The edge vortices induce a strong spanwise flow. Secondly, in the case of the cicada wing, a smaller-scaled vortex structure emanates from the unbalanced edge vortices, i.e. the TEVs and LEV. The smaller vortex contributes to an enhancement of the spanwise flow, but it has a very small magnitude. In addition, we tested two wing shapes which are simplified for the drosophila and cicada wings. This test showed that two trailing edges form the TEV1 and TEV2 at low Reynolds numbers. The simplified planforms also presented the vortex loop which is similar to that of the drosophila and cicada wings, respectively. As a result, we demonstrate that the hindwing induces the different vortex structures. (C) 2019 Elsevier Ltd. All rights reserved.