Atypical Aerodynamics of Large Chord/Radius Vertical Axis Wind Turbines

An elementary kinematic analysis of large chord-radius ratio (c/R) vertical axis wind turbines was performed and a small two-bladed model turbine (0.4c/R0.6) was built and tested. The analysis shows that these machines operate at low blade-speed/wind-speed ratios and hence the blades experience large changes in angle of attack, that produce dynamic stall, together with large dynamic pressure variations, that are phase-shifted by approximately 90°. This raises the possibility of exploiting dynamic lift overshoot effects (associated with formation of a dynamic stall vortex) to drive the turbine when the relative dynamic pressure is high, while allowing lift-stall to occur (shedding of the vortex) when the relative dynamic pressure is low. Blades also experience variable virtual camber (or “virtual morphing”) effects and favorable pressure gradients, where the former produces a virtual dynamic leading-edge droop that reduces the leading-edge angle of attack. Open jet wind tunnel tests of the model turbine revealed maximum power coefficients of 16% that were independent of c/R. Flow visualization confirmed the conjecture that dynamic stall, with the associated dynamic stall vortex, is the mechanism that drives the turbine to maximum power. Due to the lack of reliable design tools for these machines, either empiricism and dimensional analysis or high-fidelity CFD are proposed for optimization and analysis.

Jaar van publicatie:2020

Toegankelijkheid:Closed