Aerodynamic Characteristics of Dragonfly Wing Sections Compared with Technical Aerofoils

Abstract

During gliding, dragonfly wings can be interpreted as acting as ultra-light aerofoils which, for static reasons, have a well-defined cross-sectional corrugation. This corrugation forms profile valleys in which rotating vortices develop. The cross-sectional configuration varies greatly along the longitudinal axis of the wing. This produces different local aerodynamic characteristics. Analyses of the C(L)/C(D) characteristics, where C(L) and C(D) are the lift and drag coefficients, respectively (at Reynolds numbers Re of 7880 and 10 000), using a force balance system, have shown that all cross-sectional geometries have very low drag coefficients (C(D, min)<0.06) closely resembling those of flat plates. However, the wing profiles, depending upon their position along the span length, attain much higher lift values than flat plates. The orientation of the leading edge does not play an important role. The detectable lift forces can be compared with those of technical wing profiles for low Re numbers. Pressure measurements (at Re=9300) show that, because of rotating vortices along the chord length, not only is the effective profile form changed, but the pressure relationship on the profile is also changed. Irrespective of the side of the profile, negative pressure is produced in the profile valleys, and net negative pressure on the upper side of the profile is reached only at angles of attack greater than 0 degrees. These results demonstrate the importance of careful geometrical synchronisation as an answer to the static and aerodynamic demands placed upon the ultra-light aerofoils of a dragonfly.

2D numerical investigations derived from a 3D dragonfly wing captured with a high-resolution micro-CT

“…The highest lift coefficient, as well as the highest lift-to-drag ratio, resulted there. Comparing the results for lift and drag coefficients with the experimental results of Kesel and the numerical results of Kim et al, shows good agreement at comparable wing positions [9,11]. Figure 4 shows the corresponding pressure coefficient.…”

“…Higher lift coefficients occurred considering the downward pointing leading edge compared to the upward pointing one [9]. Kesel could not confirm this [9]. In this study, especially considering the steady analyses the lift coefficient and the L/D -ratio are higher after the nodus at 5…”

“…Okamoto et al states, that the orientation change of the leading edge influences the aerodynamic performance [13]. Higher lift coefficients occurred considering the downward pointing leading edge compared to the upward pointing one [9]. Kesel could not confirm this [9].…”

“…Therefore, the aerodynamic performance of the pleated dragonfly wing was compared to streamlined airfoils [11,12]. 5/11/2021; 14:51 File: thc-1-thc219010.tex; BOKCTP/ljl p. 2 2 V. Stelzer and L. Krenkel / 2D numerical investigations derived from a 3D dragonfly wing captured Furthermore, it was additionally compared to flat plates [9,10]. Lift and drag coefficients were calculated under different flow conditions [9][10][11][12].…”

“…5/11/2021; 14:51 File: thc-1-thc219010.tex; BOKCTP/ljl p. 2 2 V. Stelzer and L. Krenkel / 2D numerical investigations derived from a 3D dragonfly wing captured Furthermore, it was additionally compared to flat plates [9,10]. Lift and drag coefficients were calculated under different flow conditions [9][10][11][12]. Okamoto et al compared two-and three-dimensional (2D and 3D) models during gliding flight in a wind tunnel [13].…”