Experimental Evaluation of a Naca 0022 Airfoil Under Steady and Dynamic Pitch Oscillations at Low Reynolds Numbers

Abstract

The initial design phase of wind turbines utilizes low-fidelity models to predict turbine performance. These models require accurate and reliable airfoil polars to estimate the performance. Medium-fidelity solvers also model the dynamic behavior of the airfoil to predict its dynamic stall characteristics. Dynamic behavior is vital for Vertical Axis Wind Turbines (VAWTs), since the blades continuously experience cyclic stall conditions. Recently, several studies, regarding VAWTs, have used NACA 0022 airfoil section. However, experimental polars of NACA 0022 airfoil at steady and dynamic conditions are missing in the literature. This study presents the coefficients of static lift, drag, and moment variations of the NACA 0022 section across the Angles of Attacks (AOAs) ranging from 00 to 1800. These polars are generated using an open-type low-speed wind tunnel at Reynolds numbers of 75k, 100k, and 150k, based on airfoil chord. The dynamic wind tunnel testing is carried out at three reduced frequencies of 0.1, 0.05, and 0.025. Significant hysteresis in the aerodynamic coefficient curves is observed due to hysteresis in the separation-reattachment flow phenomenon, indicating unsteady flow conditions. It is observed that the onset of the stall is delayed during the up-stroke. In contrast, the flow is reattached earlier during the down stroke in dynamic conditions compared to static and quasi-steady conditions. The hysteresis loop shape, in the vicinity of the stall region, is found to be dependent on the reduced frequency.

Keywords: vertical axis wind turbine, Oscillating Airfoil, Wind Tunnel Testing, Unsteady Aerodynamics