High frequency dynamics of force coefficients in vawt blades under dynamic stall condition

Blades of lift driven Vertical Axis Wind Turbines can experience dynamic stall especially at low tip speed ratios. Dynamic stall has significant consequences in terms of performance, vibration, noise and structural integrity of the blades. For this reason, it is worth to investigate this complex phenomenon. In particular, detailed CFD analyses have been carried out on a pitching NACA 0015 airfoil performing several cycles using a RANS approach and implementing the Transition SST turbulence model in order to take into account the laminar-to-turbulent boundary layer transition. A good agreement has been achieved in terms of phase-averaged force coefficients versus angle of attack when comparing these numerical results with the experimental data obtained at Glasgow University. However, looking at the instantaneous time-dependent force coefficients over several cycles, it appears that, in particular during the blade down-stroke, the hysteresis cycles are quite different one from the other and all from the phase-averaged one. Moreover, each hysteresis cycle shows oscillations at frequencies higher than that of the pitching motion. The investigation of such a behavior can be important in order to avoid the occurrence of dangerous resonance conditions at the blade natural frequency.