Abstract
Flapping wings have attracted significant interest for use in miniature unmanned flying vehicles. Although numerous studies have investigated the performance of flapping wings under quiescent conditions, effects of freestream disturbances on their performance remain under-explored. In this study, we experimentally investigated the effects of uniform vertical inflows on flapping wings using a Reynolds-scaled apparatus operating in water at Reynolds number ≈ 3600. The overall lift and drag produced by a flapping wing were measured by varying the magnitude of inflow perturbation from JVert = −1 (downward inflow) to JVert = 1 (upward inflow), where JVert is the ratio of the inflow velocity to the wing's velocity. The interaction between flapping wing and downward-oriented inflows resulted in a steady linear reduction in mean lift and drag coefficients, and , with increasing inflow magnitude. While a steady linear increase in and was noted for upward-oriented inflows between 0 < JVert < 0.3 and JVert > 0.7, a significant unsteady wing–wake interaction occurred when 0.3 ≤ JVert < 0.7, which caused large variations in instantaneous forces over the wing and led to a reduction in mean performance. These findings highlight asymmetrical effects of vertically oriented perturbations on the performance of flapping wings and pave the way for development of suitable control strategies.
Highlights
Remarkable agility and manoeuvrability of insects have led to an increasing interest in insect-inspired flight mechanisms for small-scaled flying vehicles (Reynolds number, Re < 10 000) [1,2,3]
While Chirarattananon et al [14] experimentally showed that the effects of frontal inflows were more pronounced compared with oriented laterally, Jones & Yamaleev [9] revealed that flapping wings can alleviate the effect of moderate or even strong frontal inflows whose mean velocity is comparable with the wing tip velocity
The effects of downward-oriented inflows on lift and drag forces were explored numerically by Jones & Yamaleev [9]. They revealed that a downward inflow reduced the time-averaged lift force compared with the case with quiescent flow due to reduction in the effective angle of attack which led to a smaller leading-edge vortex (LEV) to form over the wing compared with quiescent flow
Summary
The experiment consisted of a dynamically scaled, single flapping wing operated in a 900 × 900 × 600 mm water tank (figure 1a). The flapping rig itself consisted of two servo motors (RoboStar SBRS-5314HTG 280°, Digital Gear High Voltage Robot Servo) for wing kinematic control. A six-axis force/torque sensor (ATI Nano 17-IP68) was mounted between the main shaft of the flapper and the root of the wing. The flapping rig was attached to a vertical linear actuator (multi-axis ball screw linear motion stage, FUYU Motion) consisting of a stepper-motor (NEMA 23, STEPPERONLINE) controlled leadscrew which allowed for 400 mm of vertical motion to 0.05 mm position accuracy. There was an offset from the flapping axis of rotation to the root of the wing of 20 mm to accommodate flapping rig attachment mechanisms (figure 1b)
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