Research on the hysteresis properties of a pitching NACA0012 airfoil under low transonic ground effect

This study proposes a layering criterion for transonic static ground effects based on investigating the pitch oscillation encountered in electromagnetic-assisted near-ground takeoff using the NACA0012 airfoil. The flow patterns and aerodynamic characteristics within each layer were examined first. Subsequently, the influences of ground effects and reduced frequencies on pitch hysteresis properties were investigated, with emphasis on the mechanisms governing variations in hysteresis loop morphology, steering direction, and fullness. The results demonstrate that the difference between the flow channel contraction ratio beneath the airfoil and the isentropic limit serves as a critical parameter for characterizing ground effects and determining flow regimes. Under angle-of-attack variations, transonic static ground effects can be categorized into three distinct layers: Layer 1 exhibiting unobstructed flow throughout, Layer 2 showing alternating unobstructed and choked states, and Layer 3 demonstrating fully choked flow. Correspondingly, near-ground pitch hysteresis characteristics exhibit layer-dependent behaviors. In Layer 1, hysteresis properties resemble those in free space due to weak ground effects. In Layer 2, intensified ground effects induce choking that causes the oscillating shock wave S2 to become stationary or shift toward the trailing edge, resulting in hysteresis loop contraction or translational displacement, respectively. In Layer 3, severe choking and overflow lead to significant differences between oscillating shock waves S1 and S2 at positive angles of attack but minimal differences at negative angles, transforming hysteresis loops from elliptical to "water droplet" shapes. Increasing the reduced frequency can revert this morphology to elliptical. Common characteristics persist across layers, including counterclockwise-rotating moment hysteresis loops, lift hysteresis loop steering synchronized with curve phase sign changes, quasi-steady behavior during low-frequency pitching, and pronounced hysteresis at high frequencies.