Effect of the Model-Sidewall Connection for a Dynamic Stall Airfoil Experiment

Abstract

The performance assessment of helicopter rotor blade airfoils no longer relies only on wind-tunnel testing for performance data in the linear region. For many flow cases, the computational fluid dynamics (CFD) is currently mature enough to give a reliable prediction of the airfoil performance. However, around maximum lift (and for dynamic stall performance), CFD can be more expensive and less reliable. In the past [1], the effect of the sidewall interaction was noted to vary widely between wind tunnels. As noted in Ref. [2], the sidewall connection geometry of the interface between the airfoil and the wind-tunnel wall can have a significant effect on the local corner flow. However at the airfoil midline, the difference between a gap and a solid connection was not detectable for a static flow at moderate airfoil lift. For Ref. [2], the sidewall effect required an additional angle-of-attack correction, as seen in the literature [3-5], which caused a decrease in the gradient of the lift polar for this type of solid-wall wind tunnel [6-8], but the total effect was relatively minor. Similarly, Ref. [9] showed that the difference between twodimensional (2-D) computations and the wind-tunnel data could be almost fully accounted for by considering the gap between the airfoil and the wind-tunnel wall. However, other authors [10,11] have noted that, particularly for dynamic stall test cases, wall interferences are propagated into the airfoil midline, causing significant differences in the dynamic stall lift and pitching-moment peaks, which characterize the flow. Ericson and Reding [12] noted that the main effect of the wind tunnel is to reduce the height of dynamic stall peaks, but more modern efforts [10] note that it is difficult to separate a sidewall correction from a generation of strongly three-dimensional (3-D) flow, which breaks the 2-D assumption of airfoil testing.