Jaime G. Wong

On the Simulation of Complex Gusts at Low Reynolds Numbers

Two specific gust forms were simulated using Computational Fluid Dynamics. Simultaneously a flat-plate model was manipulated in a water-tunnel facility to generate equivalent effective incidence and velocity variations at the leading edge. By comparing the two results, the potential of simulating a gust with equivalent model motions was thus undertaken. A first-order analysis using classical unsteady theory predicted that a gust was equivalent to a moving model experiencing the same velocities so long as the reduced frequency was low. However, direct force measurements were found to contradict this result from classical analysis showing large deviations in the total measured forces. Further measurements using Particle Image Velocimetry revealed that leading-edge vortex (LEV) growth on a moving model was equivalent to LEV growth from a gust for an equivalent effective leading-edge incidence and velocity. Despite this similarity in the LEV growth, both the added mass effects of an accelerating model, as well as the convective speed of the gust, were found to contribute large discrepancies between the gust simulation techniques.

Flow separation on flapping and rotating profiles with spanwise gradients

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.

A Model of Fluid-Structure-Interaction for Impulsively Started Spanwise-Flexible Wings

The effect of spanwise flexibility on the development of leading-edge vortices for impulsively-started flat plates at low Reynolds numbers has been investigated. A theoretical model is proposed, based on the Euler-Bernoulli beam theory, coupled with a vortex growth model based on vorticity flux through a leading-edge shear layer. The model was validated for rigid and flexible flat plates undergoing a towing motion at an angle-of-attack of 45◦. It is shown that a phase-delay in lift and drag generation occurs between rigid and flexible cases. The model indicates decreasing vorticity along the span as the wing is accelerated and begins to bend. Particle Image Velocimetry measurements conducted at three different spanwise planes showed a delay in vortex growth along the span, despite a uniform spanwise circulation.

Exploring Vortex Stability on Two-Dimensional Rotating Plates with Varying Sweepback

A method for predicting the relative stability of leading-edge vortices (LEVs) on flapping profiles is proposed and validated experimentally with time-resolved Particle Image Velocimetry and three-dimensional Particle Tracking Velocimetry. LEV stability is predicted by estimating the growth of an LEV from the relative strength of vorticity feeding, vorticity convection, and vortex stretching for a given limiting vortex size. The proposed method accurately predicts relative LEV stability. In particular, more stable LEVs are observed at higher reduced frequencies, which represent the ratio between the limiting vortex size and the rate of vorticity feeding. The introduction of profile sweep increased both LEV stability and spanwise vorticity transport. It is thought that spanwise vorticity transport improved LEV stability by acting as a sink for vorticity generated in the leading-edge shear layer.

The Effect of Rotation on the Spanwise Transport of Vorticity in the Absence of Tip Effects

The vortex growth around plunging and flapping profiles of varying sweep angle was studied with three-dimensional particle tracking velocimetry and direct-force measurements. For plunging kinematics all sweep angles tested resulted in the same force coefficient history and circulation history. Therefore, it was concluded that nominally two-dimensional spanwise flow has no effect on vortex growth or force history. However, when flapping kinematics were introduced, vortex growth was reduced due to vorticity convection. Vorticity convection for flapping cases could be modulated with the nominally two-dimensional spanwise flow. Reduced circulation for rotating cases corresponded to reduced force coefficient histories. No relationship between vortex stretching and vortex strength was observed at the Rossby number considered in this study. Through an analysis of the vorticity transport equation it was concluded that spanwise flow must be accompanied by gradients in vorticity magnitude in order to limit vortex growth.