Experimental investigation on dynamic stall of cross-flow turbine airfoils in harmonic motion

Abstract

Dynamic stall governs the unsteady aerodynamic performance and loading of cross-flow turbine blades undergoing periodic angle-of-attack variations. This work presents an experimental investigation of static and dynamic stall on a NACA0021 airfoil in harmonic pitching motion at chord-based Reynolds numbers up to 6 × 105 and reduced frequencies up to k = 0.1, which are representative of vertical-axis wind turbine operating conditions. Experiments were conducted in the open-section wind tunnel at the University of Pisa. A distinctive feature of the experimental apparatus is the simultaneous acquisition of the surface pressure distribution and the global aerodynamic loads acting on the extruded airfoil. Pressure measurements are performed using two sets of high-frequency pressure sensors: the first set is housed within the pitching airfoil to minimize tubing length, while the second is located externally to avoid dynamic effects associated with airfoil motion. Aerodynamic loads are measured by means of a six-component strain-gauge balance. The blade endplates are not connected to the balance. Static polars exhibit strong agreement between pressure-integrated and balance-derived coefficients, as well as with published data. Dynamic tests under both light and deep-stall conditions reveal hysteresis in the lift, drag, and moment polars. Notably, cycle-to-cycle measurement dispersion is reduced by approximately 50% compared to prior studies. The resulting dataset provides a validated benchmark for reduced-order dynamic stall models applicable to cross-flow turbine blade sections.