Experimental evaluation of a NACA-0022 airfoil under steady and dynamic pitch oscillations at low Reynolds numbers for wind power applications

The initial design phase of wind turbines utilizes low-fidelity models to predict wind turbine performance. These models require accurate and reliable airfoil polars to estimate the aerodynamic performance. The dynamic behavior is vital for Vertical Axis Wind Turbines (VAWTs), since the blades continuously experience cyclic stall conditions. Recently, the NACA 0022 airfoil is utilized within Vertical Axis Wind Turbines (VAWTs) for its high lift-to-drag ratio and structural strength, which can significantly enhance performance and efficiency of VAWTs under diverse wind conditions. Nevertheless, a comprehensive set of experimental polar for the NACA 0022 airfoil, under both steady-state and dynamic conditions, remains challenging. Hence, the main objective of the study is to experimentally evaluate the aerodynamic performance of a NACA 0022 airfoil under both steady and dynamic pitch oscillations at low Reynolds numbers (75k, 100k, and 150k). An open-type low-speed wind tunnel is used for static and dynamic testing at three reduced frequencies of 0.1, 0.05, and 0.025. Moreover, the performance coefficients of static lift, drag, and moment of the NACA 0022 airfoil is thoroughly analyzed at different Angles of Attacks (AOAs) ranging from 0⁰ to 180⁰. The experiments show that significant hysteresis in the aerodynamic coefficient curves is observed due to hysteresis in the separation-reattachment flow phenomenon, indicating unsteady flow conditions. Additionally, the shape of the hysteresis loop near the stall region was found to be dependent on the reduced frequency of oscillation. Moreover, it is also 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 results reveal a stall angle shift from 11° at low to 12° at intermediate and 14° at high Reynolds numbers, as well as show an increase in maximum lift coefficient prior to stall, with fluctuations in aerodynamic forces and moment coefficients between 135° and 165. Overall, the proposed approach is applicable to the evaluation of such NACA-0022 airfoil variants that can be parameterized at a broader range of flow conditions.