A computational study of the aerodynamic forces and power requirements of dragonfly (Aeschna juncea) hovering

Abstract

Dragonflies are capable of long-time hovering, fast forward flight and quick manoeuvres. Scientists have always been fascinated by their flight. Kinematic data such as stroke amplitude, inclination of the stroke-planes, wing beat frequency and phase-relation between the fore-and hindwings were measured by taking high-speed pictures of dragonflies in free-flight (e.g. Norberg, 1975;Wakeling and Ellington, 1997b) and tethered dragonflies (e.g. Alexander, 1984). Using these data in quasi-steady analyses (not including the interaction effects between forewing and hindwing), it was shown that the lift coefficient required for flight was much greater than the steady-state values measured from dragonfly wings (Wakeling and Ellington, 1997a). This suggested that unsteady wing motion and/or flow interaction between the fore-and hindwings must play important roles in the flight of dragonflies (Norberg, 1975;Wakeling and Ellington, 1997c).Force measurement on a tethered dragonfly was conducted by Somps and Luttges (1985). It was shown that over some part of a stroke cycle, lift force was many times larger than that measured from dragonfly wings under steady-state conditions. This clearly showed that the effect of unsteady flow and/or wing interaction was important. Flow visualization studies on flapping model dragonfly wings were conducted by Luttges (1988, 1989), and it was shown that constructive or destructive wing/flow interactions might occur, depending on the kinematic parameters of the flapping motion. In these studies, only the total force of the fore-and hindwings was measured and, moreover, force measurements and flow visualizations were conducted in separated works.In order to further understand the dragonfly aerodynamics, it was desirable to determine the aerodynamic force and flow structure simultaneously and also to know the force on the individual forewing and hindwing during their flapping motions. Freymuth (1990) conducted force measurement and flow visualization on an airfoil in hover modes. One of the hover modes was for hovering dragonflies. Only mean vertical force was measured. It was shown that large mean vertical force coefficient could be obtained and the force was When the midstroke angles of attack in the downstroke and the upstroke are set to 52°and 8°, respectively (these values are close to those observed), the mean vertical force equals the insect weight, and the mean thrust is approximately zero. There are two large vertical force peaks in one flapping cycle. One is in the first half of the cycle, which is mainly due to the hindwings in their downstroke; the other is in the second half of the cycle, which is mainly due to the forewings in their downstroke. Hovering with a large stroke plane angle (52°), the dragonfly uses drag as a major source for its weightsupporting force (approximately 65% of the total vertical force is contributed by the drag and 35% by the lift of the wings).The vertical force coefficient of a wing is twice as large as the quasi-steady value. The interaction between t...

A computational study of the aerodynamics and forewing-hindwing interaction of a model dragonfly in forward flight

“…The model wings The model fore-and hindwings (Fig.·1) are the same as those used in Sun and Lan (2004). The thickness of the wings is 1% of c (where c is the mean chord length of the forewing).…”

“…Recently, Sun and Lan (2004) studied the aerodynamics and the forewing-hindwing interaction of the dragonfly Aeshna juncea in hover flight, using the method of computational fluid dynamics (CFD). Three-dimensional wings and wing kinematics data of free-flight were employed in the study.…”

“…The result of detrimental interaction is interesting. But Sun and Lan (2004) investigated only a specific case of flight in Aeshna juncea, i.e. hovering with 180°phase difference between the fore-and hindwings.…”

“…Alexander, 1986;Thomas et al, 2004), this phase difference is also included for reference. As in Sun and Lan (2004), the approach of solving the flow equations over moving overset grids is employed because of the unique feature of the motion, i.e. the fore-and hindwings move relative to each other.…”

Implicit Virtual Boundary Method for Moving Boundary Problems on Non-Staggered Cartesian Patch Grids

“…In this branch of numerical methods, the grid system should be re-meshed at each time step and thus large CPU time is generally needed [1]. To improve the computational efficiency, the method of moving overset grids [2][3][4] generates a local non-inertial boundary-fitted grid system for each moving object that is allowed to move on the fixed global Cartesian grid system. However, the solution interpolation and data transfer among the grid systems are very complex and time consuming.…”