Experimental and Numerical Investigation of the Effect of Vertical Axis Wind Turbine Blade Cascading on the Dynamic Stall Phenomenon

Abstract

The study is began with a wide numerical investigation on the deep dynamic stall phenomenon of unsteady flow around oscillating NACA 0012 airfoil at low Reynolds number. The numerical work uses two experimental cases for validating the predicted hysteresis loops and flow structure. The investigation is performed by solving the Navier-Stokes equations with three turbulence models Spalart-Allmaras, kɛ-Realizable and kɷ-SST. The results show that the kɷ-SST is a good model for predicting the dynamic stall stages especially the detecting of leading edge vortex (LEV) and post stall shedding comparing to the experimental data. Comparison between static and dynamic situation is performed. Effects of reduced frequency and oscillation amplitude, effects of airfoil thickness and effects of wing end plates on the dynamic stall phenomenon are studied. Fifteen cascading configuration are studied under the same operating conditions including upper cascading, upper forward cascading, and lower forward cascading during oscillation motion. Comparison between the overall generated torque with azimuthal angle resulting from the CFD and DMST of VAWT models in Darrieus motion is performed including forward and backward auxiliary airfoils with specific offset distances effects. Three cases of vertical axis wind turbine are tested experimentally in static situation. The first configuration is the conventional VAWT with three straight blades. The second configuration is the same turbine in the first configuration but adding auxiliary blade parallel to the main blade at each arm with offset radial distance 0.27c with no forward or backward shifting distance. The last configuration is adding auxiliary blade parallel to the main blade at each arm with offset radial distance 0.27c and 0.5c backward offset distance. The results show that the last configuration enhances the self-starting capability by increasing the static torque four times more than the conventional configuration. In dynamic situation, the tangential force increases by increasing the inlet wind speed. The increasing in applied load cause the decreasing in the tangential faces produced by each blade. In case of the back ward cascading, significant value of the positive pitching moment is produced over the main blade and increases with increases the turbine rotation. It causes a negative load cells reading however the turbine rotate in clockwise.