Stability of Low-Reynolds-Number Separated Flow Around an Airfoil Near a Wavy Ground

In this numerical-theoretical study, a linear BiGlobal stability analysis of the steady massively separated flow around a NACA 4415 airfoil was performed at a low Reynolds number (Re = 200) and a high angle of attack (alpha = 18 deg) close to a wavy ground, with a focus on the effect of three different types of stationary roughness: 1) a perfectly flat ground, 2) a wavy ground with small-amplitude undulations, and 3) a wavy ground with large-amplitude undulations. On increasing the undulation amplitude h(0) of the ground but keeping the mean ground clearance constant, it was found that the lift coefficient increased owing to an increase in the static pressure under the airfoil, which is reminiscent of the conventional ground effect over a flat surface. However, it was also found that the leading flow perturbation was the three-dimensional stationary global mode and not the two-dimensional traveling Kelvin-Helmholtz mode, contrary to the results of previous analogous studies of linear global instability of massively separated flow away from the ground. This study provides new insight into the stability of airfoil-ground flow systems at a low Reynolds number and a high angle of attack, contributing to a better understanding of the ground-effect aerodynamics of small insects and micro air vehicles flying over rough waters or complex terrain.