Flow physics and control of separation for low aspect ratio swept and tapered wings

Neal, Jacob, Martin
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Other Contributors
Theofilis, Vassilis
Taira, Kunihiko
Hicken, Jason
Kopsaftopoulos, Fotios
Amitay, Michael
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Aeronautical engineering
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Flow separation is the last thing you want to happen over an aircraft wing: in most cases attached flow over the wing is the only thing keeping the plane in the air. The formation of 3D separated flows over wings and other control surfaces is an under-explored topic due to difficulties measuring or computing these phenomena. Despite this dearth in the literature, aircraft wings routinely encounter high-angle-of-attack scenarios either from transient gusts or intentional maneuvers. Unmanned aerial vehicles tend to operate at high angles of attack and low Reynolds numbers. The present experimental investigation seeks to provide a basis with which 3D separation can be characterized and controlled. Wind tunnel and water tunnel experiments were performed to show the effects of planform shape on the large scale flow structures over finite wings at high angles of attack. First, geometrically simple unswept planforms were considered. Wind tunnel experiments explored unswept planforms at the moderate Reynolds number $Re_{\bar{c}} = 247,500$, showing that these wings experience the owl-eye stall cell pattern at high angles of attack. The stall angle of attack increases with decreasing aspect ratio, as the end effects of the tip vortex and the horseshoe vortex at the wall get moved closer together and encourage attached flow. The surface foci of the stall cell are connected by an arch vortex which grows into the wake according to the spanwise distribution of the reversed flow region. At the midspan, the wake has some periodicity through the quasi-2D shedding of spanwise vortices into the wake. Next, the geometric complexity was systematically ramped up with a series of wind tunnel experiments on swept and tapered planforms at the moderate Reynolds number $Re_{\bar{c}} = 247,500$. For the untapered unswept wings, the stall cell pattern forms; for the swept back untapered wings the root-to-tip ram's horn forms, for the wings with forward swept trailing edge the inverted (tip-to-root) ram's horn forms and for swept back and tapered wings the ram's horn type surface spiral would form, though the 3D flow field shows some similarity to the arch type separation. When the leading edge is swept back, the reversed flow region and peak unsteadiness is shifted towards the wing tip, while when the trailing edge is swept forward, the reversed flow region and peak unsteadiness is shifted towards the root. Nine model wings were explored in water tunnel experiments at the very low Reynolds number $Re_{\bar{c}} = 600$. Some analogous topological patterns were identified between the wind tunnel and water tunnel tests in time-averaged volumetric velocity fields. Namely, the area of the reverse flow region as a function of the span showed very similar distributions at $Re_{\bar{c}}=600$ and $Re_{\bar{c}}=247,500$ as a function of sweep angle and taper ratio. The global flow structures like reverse flow region and spiraling 3D streamlines identified a ram's horn vortex for the swept back wing at both Reynolds numbers and an inverted (tip-to-root) ram's horn for the forward swept trailing edge wing at both Reynolds numbers. Finally, flow control using leading edge steady blowing with seven equally spaced rectangular jets was applied to two of the tapered wings in the wind tunnel at $Re_{\bar{c}}=247,500$. Both wings had the same taper ratio and aspect ratio, but one had a forward swept trailing edge and the other had a swept back leading edge. The flow control for the trailing edge swept forward model was very effective, with beneficial aerodynamic augmentation being achieved when blowing is applied near the midspan. The flow control for the leading edge swept back model was less effective, as the leading-edge-normal blowing for the swept back leading edge angled the jet in a way that would increase the drag and fail to reattach the flow.
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