Effect of sudden change in free stream velocity on the wind turbine airfoil performance based on boundary element method

Abstract

In this study, the aerodynamic effects of sudden change in magnitude of incident velocity are investigated in detail for wind turbine airfoils. For this purpose, the unsteady panel method which is based on two-dimensional singularity approach has been developed. The model is first validated via using both the analytical functions and experimental data. The jump in free-stream velocity has been supposed to be applied in two different cases. The first is the airfoil at initially steady state and the second is the airfoil experiencing sinusoidal free-stream velocity. For both cases, the parametric study has been performed consequently investigating the effects of velocity difference magnitude, phase angle, airfoil characteristic and relative velocity oscillation amplitude. The response pattern of lift coefficient is shown to be a combination of three distinct behaviors: an initial jump, a secondary fall and a damped like envelope. The results showed that when the sudden velocity change is being applied on steady case, the time constant of recovery is finite and limited, while for the oscillatory case, the recovery time scale is considerably increased. Moreover, the local change in the normalized lift coefficient seems to be less than 0.3 for the steady case, while it can rise to 0.9 in dynamic oscillatory case. The magnitude of overshoot and undershoot has meaningful correlation with both relative velocity change and the initial phase in which the change occurs.