Experimental Investigation of Geometric and Virtual Camber on Cross-Flow Turbine Performance and Loading*

Cross-flow turbines show promise for renewable energy generation from wind and tidal sources. Their rotating reference frame introduces curved streamlines resulting in a virtual camber, where symmetrical profiles in a rotating flow produce lift similar to cambered blades in straight flows. Adding geometric camber is therefore likely to alter performance and loading, however such geometries have seen limited exploration, with little indication of which camber direction might be most favorable. This study compares positive and negative 2% cambered blades (NACA 2418) with symmetrical NACA 0018 foils for a turbine with a 0.49 chord-to-radius ratio. Experimental performance and PIV measurements across a range of tip-speeds are explored. Contrary to expectations, blades with concave sides facing away from rotation, which enhance virtual camber and lift in the power stroke, exhibited suboptimal performance. Conversely, oppositely cambered blades slightly improved on symmetrical blade performance by enhancing downstream flow reattachment, compensating for reduced upstream power extraction. Moreover, these blades reduced peak loading, which may prove critical in future designs, especially at high tip-speed ratios. The current work suggests that flow recovery in the downstream region is critical to overall performance and that reducing total camber (geometric plus virtual) leads to overall gains.