Flow separation on flapping and rotating profiles with spanwise gradients

Abstract

The growth of leading-edge vortices (LEV) on analogous flapping and rotating profiles has been investigated experimentally. Three time-varying cases were considered: a two-dimensional reference case with a spanwise-uniform angle-of-attack variation α; a case with increasing α towards the profile tip (similar to flapping flyers); and a case with increasing α towards the profile root (similar to rotor blades experiencing an axial gust). It has been shown that the time-varying spanwise angle-of-attack gradient produces a vorticity gradient, which, in combination with spanwise flow, results in a redistribution of circulation along the profile. Specifically, when replicating the angle-of-attack gradient characteristic of a rotor experiencing an axial gust, the spanwise-vorticity gradient is aligned such that circulation increases within the measurement domain. This in turn increases the local LEV growth rate, which is suggestive of force augmentation on the blade. Reversing the relative alignment of the spanwise-vorticity gradient and spanwise flow, thereby replicating that arrangement found in a flapping flyer, was found to reduce local circulation. From this, we can conclude that spanwise flow can be arranged to vary LEV growth to prolong lift augmentation and reduce the unsteadiness of cyclic loads.

Near-wake structure of full-scale vertical-axis wind turbines

“…A yawed or swept blade with respect to the incoming flow exhibits strongly three-dimensional vortex shedding in dynamic stall (Visbal & Garmann 2019), due in part to a net transport of vorticity along the span of the blade (Smith & Jones 2019). A linearly increasing spanwise angle-of-attack profile in dynamic stall similarly induces a spanwise transport of vorticity that affects the stability of the leading-edge vortex and thus the character of the vortex shedding from the blade (Wong, laBastide & Rival 2017). Given these observations, we conclude that the spanwise non-uniformity of the flow over helical VAWT blades makes the vortex lines shed by the dynamic-stall mechanism inherently nonlinear and three-dimensional.…”

Transient leading-edge vortex development on a wing rolling in uniform flow

“…Similarly, Wong, Kriegseis & Rival (2013) also observed little effect of spanwise flow on large-aspect-ratio swept wings (in which spanwise gradients were not present). However, Wong, laBastide & Rival (2017) conducted a comparison of rotor-blade gust vs flapping-wing kinematics to show that spanwise flow on the wing could either strengthen or weaken the vortex, depending on the sign of the spanwise vorticity gradient. Chen, Wu & Cheng (2019) investigated the starting rotation of an aspect-ratio 4 wing at , and observed that spanwise convection of vorticity was only significant relatively early in the rotation, and suggested that it may not be important for steadily revolving wings.…”

Effects of aspect ratio on rolling and twisting foils

“…Another way to measure the spanwise change in 2D kinematics along the foil, used recently by [5], is the sectional angle-of-attack α…”

“…Spanwise flow promotes the fluid dynamic interaction between sections of the wing which can result in high lift generation [2][3][4]. The exact mechanism of this high lift on bird or insect wings is still highly debated between the spanwise flow [3,5,6] and downwash-induced flow caused by the tip [7][8][9]. In addition, the tip of a finite foil definitely produces non-planar wakes which have been shown to influence stall on an impulsively translated flat plates [10], pitching flat plates [11], revolving wings [12][13][14][15] and a finite foil undergoing 2D kinematics [16].…”