COMPUTATIONAL INVESTIGATION OF CAMBERED BLADES AND VIRTUAL CAMBER IN CROSS-FLOW TURBINES

Abstract

This research computationally investigates how geometric camber interacts with the virtual camber induced in cross-flow turbine blades. Most cross-flow turbines use uncambered airfoils, likely due to the symmetries in the change of angle of attack over a complete rotation. Unlike axial-flow turbines, flow is curvilinear relative to the rotating airfoil, so an equivalent airfoil in rectilinear flow has virtual camber that affects the flow dynamics and generated forces. In this study, single-bladed turbines with camber of up to ±3% are simulated using unsteady Reynolds-averaged Navier-Stokes and large eddy simulation models for a range of tip speed ratios and chord-to-radius ratios. Results are validated against experimental performance and particle image velocimetry data. It is shown across all methods that geometric camber affects the turbine performance at different parts of the cycle, depending on the direction of concavity of the camber line. This is due to camber augmenting both the lift produced by the blade and the timing of leading-edge vortex formation. It is also shown how both geometric and virtual camber affect blade pitching moments. All of these effects are seen to be dependent on tip speed ratio and chord-to-radius ratio, since these are the main parameters that influence the virtual camber effect.