Aspect ratio effects on wings at low Reynolds numbers

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Virginia Polytechnic Institute and State University

In this study the primary objective was to determine the effect of aspect ratio in particular and in general the effect of three dimensionality on the flow around wings at low Reynolds numbers. It was seen that the effects observed at high Reynolds number are also present in this Re range. There is the usual increase in lift slope and this increase can even be predicted with reasonable accuracy using Prandtl's lifting line theory. In addition to the change in lift slope the zero lift angle of attack was also influenced by the aspect ratio.

Through flow visualization it was ascertained that the wingtips have a rather restricted effect on the laminar separation bubble. The disappearance of the bubble extends only for a small distance inboard from the tips.

The size of the hysteresis loop and the Reynolds number at which hysteresis starts was found to be influenced by the aspect ratio.

The momentum deficit method was used to validate the data obtained by the strain gauge method and there was adequate agreement between the values found through the two methods.

From the measurements of pressure done around the airfoil contour one could determine both the location of the laminar separation bubble and the regions were flow is separated. The pressure taps themselves were found to influence measurements somewhat in certain regions of angle of attack and Reynolds number.

In the future it would be beneficial to continue strain gauge measurements on this airfoil with flaps and control surfaces to determine their effect on the formation of the laminar separation bubble. Also measurements on other shapes would give more insight into the phenomena occurring here. The effects of turbulence and noise will have to be investigated in detail to determine what performance to expect from an actual aircraft. Finally detailed measurements on boundary layer stability and its effect on the occurrence of reattachment should be studied in detail to gain insight into the reasons for the presence of a hysteresis loop in stall at these Reynolds numbers.