Aerodynamic Characteristics of Dragonfly Wing Corrugation in Hovering Flight

In this study, the aerodynamic characteristics of dragonfly wing corrugation in hovering flight are investigated using the computational fluid dynamics (CFD) methods. Three corrugated cross sections of the right forewing of a dragonfly (Aeshna cyanea) are considered. The effects of wing corrugation are identified by comparing the aerodynamic differences between the corrugated and the corresponding smoothed wing sections, and the underlying flow mechanisms of the corrugation effects are revealed by analyzing the flow fields and surface pressure distributions of the wings. The results show that the wing corrugation has little effects on aerodynamic performance of wings. The time histories of aerodynamic forces of the corrugated wings are very close to those of the smoothed wings. The flow information indicates that the corrugated and smoothed wings have approximately the same vorticity around the wings. Introduction Recently, the aerodynamic performance of flying insects is gaining more attention because of the possible applications in Micro Aerial Vehicles (MAVs). Much work has been done on aerodynamic characteristics of insect flapping wings using experimental and computational methods and considerable understandings of the aerodynamic mechanisms have been achieved [1-4]. It is well known that most of insect wings are corrugated. In aforementioned studies, ellipse sections, flat plates and traditional airfoils were employed as model wings. The authors have implicitly assumed that the corrugation has minor effects on aerodynamic performance of the wings. It is of great interest to test this assumption, and to know whether the wing corrugation significantly influences the aerodynamic performance of flying insects. To investigate the unsteady aerodynamic forces of corrugated wings in flapping flights, Premanchandran [5] analyzed the aerodynamic characteristic of corrugated wings at low Reynolds number in hovering flight. Their results showed that the lift force generated by the corrugated airfoil was a little larger than that of the NACA0012 airfoil. Meng et al. [6,7] compared the aerodynamic characteristic of corrugated airfoils with the flat plate under the forward flight and hovering flight conditions. Their results showed that the corrugation decreased the aerodynamic forces slightly, and using a flat plate wing to model the corrugated wing was a good approximation. Considering that the wing models in Meng et al. [6,7] have extremely different contour thickness, which may have significant effects on aerodynamic performance of flapping wings, the effects of wing corrugation on its flight performance is still not fully understood. In this paper, a numerical study on aerodynamic characteristics of dragonfly wing corrugation in hovering flight is carried out. Three corrugated cross sections of the right forewing of a dragonfly (Aeshna cyanea) are considered. The effects of wing corrugation are identified by comparing the aerodynamic differences between the corrugated and smoothed wing sections. The corrugation effects on aerodynamic forces, the energy consumption and flow field of the wings are examined in detail Problem Descriptions To clarify the effects of varied corrugated sections along the longitudinal axis of dragonfly wings, three corrugated cross sections at 0.3Lref, 0.5 Lref and 0.7 Lref of the right forewing of a dragonfly 51 Advances in Engineering Research (AER), volume 105 3rd Annual International Conference on Mechanics and Mechanical Engineering (MME 2016) Copyright © 2017, the Authors. Published by Atlantis Press. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/). (Aeshna cyanea) are digitally extracted from the experiments of Kesel [8] and shown in Figure 1, where Lref is the relative span length. Three pairs of wing profiles are compared in present studies, where the pleats of each corrugated airfoil are filled with straight lines, forming the enveloped airfoil. The thickness of corrugated airfoils are fixed at 1%c, where c is the chord length. The chord lengths of each airfoil are equal. The wing undergoes the combined translational and rotational motions. The schematic of wing motion is drawn in Figure 2, where a thick line with a circle at one end is used to represent the wing section and the circle indicates the leading edge of the wing. The dashed line shows the stroke plane which has an inclined angle of β. Fig. 1. Schematic of corrugated and enveloped airfoils. Fig. 2. Schematic of the flapping motion. The flapping wing motions can be described as: ( ) cos(2π ), m A t A ft  (1) 0 ( ) sin(2π ). m t ft      (2) Where A(t) and α(t) are the translational displacement and rotational angle, respectively. Am and αm are amplitudes of the translational and rotational motion, respectively. α0 is the initial rotational angle and f is the flapping frequency. Numerical Methods Governing Equation of Fluid Flow Considering the flight characteristics of insects in nature, the flow around the wing is assumed to be laminar and incompressible. The two-dimensional Navier-Stokes equations are applied in the present study. The equations can be written as the following form: 0,   u (3) b) Corrugated-03L