Wu Jianghao

LARGE AERODYNAMIC FORCES ON A SWEEPING WING AT LOW REYNOLDS NUMBER

The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically. After an initial start from rest, the wing is made to execute an azimuthal rotation (sweeping) at a large angle of attack and constant angular velocity. The Reynolds number (Re) considered in the present note is 480 (Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root). During the constant-speed sweeping motion, the stall is absent and large and approximately constant lift and drag coefficients can be maintained. The mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows. Soon after the initial start, a vortex ring, which consists of the leading-edge vortex (LEV), the starting vortex, and the two wing-tip vortices, is formed in the wake of the wing. During the subsequent motion of the wing, a base-to-tip spanwise flow converts the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength. This prevents the LEV from shedding. As a result, the size of the vortex ring increases approximately linearly with time, resulting in an approximately constant time rate of the first moment of vorticity, or approximately constant lift and drag coefficients. The variation of the relative velocity along the wing span causes a pressure gradient along the wingspan. The base-to-tip spanwise flow is mainly maintained by the pressure-gradient force.

Aerodynamic Power Efficiency Comparison of Various Micro-Air-Vehicle Layouts in Hovering Flight

Flapping rotary wing, rotating wing, and flapping wing are feasible wing layouts applicable to micro-air-vehicles capable of hovering flight. A numerical study in this paper presents which wing layout can be more efficient in terms of aerodynamic power for the given kinematic and geometric parameters with or without the constraint of vertical force. In the cases under typical conditions, rotating wing layout is the most efficient one when a small vertical force is needed. However, if a much larger vertical force is required, flapping rotary wing is the only wing layout that fulfills the requirements of the two aspects due to its coupling effect. At relatively high Reynolds number (Re>2000), flapping amplitude (>70°), and aspect ratio (=6), comparative relationships from the cases under typical conditions among three wing layouts in terms of vertical force and aerodynamic power efficiency are kept unchanged. Nevertheless, at relatively low Re(<500)), flapping amplitude (<40°), or aspect ratio (=3), horizon...

Aerodynamics on flapping rotary wing in low Reynolds number

Unsteady aerodynamics of a flapping rotary wing with aspect ratio of 5 is studied in low Reynolds number. The effect of five different dimensionless parameters is discussed. It is shown a positive or negative leading-edge vortex is formed corresponding to the negative or positive lift during upstroke or downstroke. The leading-edge vortex attach on the wing surface until they moves to trailing-edge despite the second vortex can be observed. Based on the analysis of different Reynolds number, it is indicated that relative high Reynolds number can obtain high lift and rotational moment due to the strengthened leading-edge vortex. The decrease of flapping amplitude and the increase of the ratio of flapping period by rotation period both lead to a higher rotation rate, which increase lift and decrease the rotational moment. With the increase of mean angle of attack, a stronger leading-edge vortex occurs during upstroke, which increases lift and decrease the rotational moment. The variation of pitching amplitude changes the mean angle of attack during upstroke or downstroke, which results in the increase of the lift and rational moment.