Aerodynamic force generation and power requirements in forward flight in a fruit fly with modeled wing motion

Abstract

It has been shown that quasi-steady analysis cannot predict the aerodynamic forces and power requirements of insects in hovering (Ellington, 1984b,c) or forward flight (Dudley and Ellington, 1990b;Willmott and Ellington, 1997b). Researchers have been working to shed light on the unsteady mechanisms of aerodynamic force generation and predict satisfactorily the power requirements in such cases.Dickinson and Götz (1993) measured the aerodynamic forces on an aerofoil started impulsively at a high angle of attack in the Reynolds number (Re) range of a fruit fly wing and showed that lift was enhanced by the presence of a dynamic stall vortex, or leading-edge vortex (LEV). But lift enhancement was limited to only 2-3 chord lengths of travel because of the shedding of the LEV. For most insects, a wing section at a distance of 0.75R (where R is wing length) from the wing base travels approximately 5.3 chord lengths during an up-or downstroke in hovering flight (Ellington, 1984b); in forward flight, the section would travel an even larger distance during a downstroke. Aerodynamic force generation and power requirements in forward flight in a fruit fly with modeled wing motion were studied using the method of computational fluid dynamics. The Navier-Stokes equations were solved numerically. The solution provided the flow velocity and pressure fields, from which the vorticity wake structure and the unsteady aerodynamic forces and torques were obtained (the inertial torques due to the acceleration of the wing-mass were computed analytically). From the flowstructure and force information, insights were gained into the unsteady aerodynamic force generation. On the basis of the aerodynamic and inertial torques, the mechanical power was obtained, and its properties were investigated.The unsteady force mechanisms revealed previously for hovering (i.e. delayed stall, rapid acceleration at the beginning of the strokes and fast pitching-up rotation at the end of the strokes) apply to forward flight. Even at high advance ratios, e.g. J=0.53-0.66 (J is the advance ratio), the leading edge vortex does not shed (at such advance ratios, the wing travels approximately 6.5 chord lengths during the downstroke).At low speeds (J≈0.13), the lift (vertical force) for weight support is produced during both the down-and upstrokes (the downstroke producing approximately 80% and the upstroke producing approximately 20% of the mean lift), and the lift is contributed mainly by the wing lift; the thrust that overcomes the body drag is produced during the upstroke, and it is contributed mainly by the wing drag. At medium speeds (J≈0.27), the lift is mainly produced during the downstroke and the thrust mainly during the upstroke; both of them are contributed almost equally by the wing lift and wing drag. At high speeds (J≈0.53), the lift is mainly produced during the downstroke and is mainly contributed by the wing drag; the thrust is produced during both the down-and upstrokes, and in the downstroke, is contributed by the wing lift and in the upstrok...

Aerodynamic effects of corrugation in flapping insect wings in hovering flight

“…It was tested by measured unsteady aerodynamic forces on a flapping model fruit fly wing (Sun and Wu, 2003), on revolving model wings (Wu and Sun, 2004) and on a pair of wing in 'fling motion' (Sun and Yu, 2006). These tests showed that the unsteady aerodynamic forces computed by the present CFD code agreed well with the experimental measurements.…”

Unsteady aerodynamic forces of a flapping wing

“…It was tested by measured unsteady aerodynamic forces on a flapping model fruit fly wing (Sun and Tang, 2002b;Sun and Wu, 2003). The calculated drag coefficient agreed well with the measured value [see fig.·2A,C of Sun and Wu (2003)].…”

“…It was tested by measured unsteady aerodynamic forces on a flapping model fruit fly wing (Sun and Tang, 2002b;Sun and Wu, 2003). The calculated drag coefficient agreed well with the measured value [see fig.·2A,C of Sun and Wu (2003)]. For the lift coefficient, in the translation phase during the middle, and in the rotation phase at the end, of each half-stroke, the computed value agreed well with the measured value, whereas in the beginning of the stroke, the computed peak value was much smaller than the measured value [see fig.·2B,D of Sun and Wu (2003) and fig.·4 of Sun and Tang (2002b)].…”

Three-dimensional flow structures and evolution of the leading-edge vortices on a flapping wing

“…In fast forward flight, the structure of the LEV system could also be changed: the flows would become more attached to the wing as the increase of the flight speed (or advanced ratio) (Sun and Wu, 2003;Wang and Sun, 2005).…”