Experimental Investigation of Force Transients during Gust Encounters

Abstract

The small size and low speed of Micro Air Vehicles make them vulnerable to wind gusts such that sustaining controlled flight becomes a challenge in the unsteady environments. A better understanding of the gust flow is crucial to develop models capable of predicting unsteady forcing. Therefore, this study aims to improve understanding of fundamental flow physics behind the unsteady force production during gust encounters. The bulk of the results presented here were obtained experimentally for a flat plate passing through a transverse gust created in a water towing tank and found to have a sine-squared velocity profile. The effects of 4 different parameters, namely angle of attack, gust ratio, effective angle of attack, and aspect ratio, were explored. A wide investigation range was used for each parameter. The forces increased significantly from their steady-state values during the encounter. 2D flowfields showed the formation and shedding of vortices from the leading and trailing edges of the wing. The flow was found to stay nominally two-dimensional until the forces peak. Only thereafter, spanwise variations were observed in the 3D flowfields. The accuracy and limitations of Kussner's model were evaluated by comparing the sine-squared and top-hat transverse gusts (the latter experiments performed by collaborators). The gradients in the transverse velocity profile were found to significantly affect the force response such that sharper gradients cause higher nonlinearities. Kussner's model was found to provide accurate predictions for the sine-squared gust even when the flow is highly nonlinear, whereas it failed to do so for the top-hat gust. A momentum flux-based normalization was proposed and found to scale the responses of different velocity profiles as long as the response can be predicted by Kussner's model. The effect of gust type on the unsteady forcing was examined by comparing the sine-squared transverse and vortex gusts (the latter experiments performed by collaborators). The results showed that both gust encounters result in large transients in the lift. The increase in the lift force and the leading-edge vortex strength for the transverse gust was found to be steeper than the vortex gust. A flowfield-based force prediction method was proposed and found to be effective for low-to-moderate effective angles of attack.