Experiments and Computations on Abstractions of Perching

Abstract

The flight maneuver of perching is abstracted as a linear pitch ramp, with and without a deceleration in the free-stream direction. We consider, first, experimental-computational comparison for flowfield and aerodynamic force coefficients for an SD7003 airfoil pitching from α = 0º to 45º; and second, an experimental survey of reduced frequency and pivot point for a range of flat plate pitching cases from 0º to 90º. The computational approach is 3D Large Eddy Simulation, and the experimental approach is by three degree of freedom electric motion-rig in a water tunnel. Accurate flowfield resolution in deep-stall is seen to require a large spanwise extent of computational domain. Meanwhile, experiment can be plagued with blockage, and dynamic blockage was seen to behave differently than static blockage. Even very low reduced frequencies of motion give lift overshoot beyond static stall, but comparatively large frequencies are necessary before the lift curve slope changes, either due to rate effects or acceleration effects. Moving the pitch pivot point further aft tends to attenuate both lift and drag production, and pitching about the three-quarter chord point cancels the rate-effect, in agreement with quasi-steady linear airfoil theory. Surprisingly, the aerodynamic coefficient history differs little between cases with and without streamwise deceleration, except towards the very end of the motion. The implication is that perching-type of ground tests or computations can be adequately conducted in a steady free-stream.

Theoretical Analysis of Perching and Hovering Maneuvers

“…Perching maneuvers may be approximated using the classical pitch-ramp motion, 5 where the angle of attack varies rapidly over a large amplitude. The key difference between classical ramp motions and perching is that the latter also involves a deceleration to zero freestream velocity.…”

Experiments to investigate lift production mechanisms on pitching flat plates

“…Another kinematic case that has received attention in the literature is that of a pitching wing in a constant freestream velocity (Ol 2009;Granlund et al 2010;Wang and Eldredge 2012;Kang et al 2013;Hartloper et al 2013). Brunton and Rowley (2009) demonstrated that for attached flows, a simple model such as that of Theodorsen (1934), which includes both non-circulatory and circulatory effects can predict lift for a pure pitch motion reasonably well.…”

Numerical study of leading edge vortex circulation development on finite aspect ratio perching wings.

“…It has been widely concluded [18,16,21,22,23,20,8,24,25] that increasing pitch rate, and peak angle of attack increases the strength of the vortex, thus increasing lift for both motion sets. Both motions have been thoroughly tested by the referenced authors at moderate rates and angle amplitudes.…”

“…[29,32] For a dynamic stall problem, high Reynolds number (Re > 10 6 ) 3D experiments have been conducted by Piziali [33] and Tang & Dowell [34] for helicopter applications, but low Reynolds number (Re ≈ 10 3 −10 4 ) 3D experiments and simulations are sparse. [35,36,37,8,30,38] Since MAVs employ low aspect ratio wings, in order for the dynamic stall research to be applicable to MAVs, dynamic stall must be investigated as a 3D problem at low…”

“…[31,16,19]. Granlund et al [8] experimentally surveyed several pitching rates of a nominally 2D airfoil undergoing a single pitch-up maneuver at a Reynolds number of 20,000. They found that even at a low pitch rate, lift would be higher than its corresponding static stall value.…”