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

Abstract

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 (α 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.

Nonlinear Control Strategies for an Autonomous Wing-In-Ground-Effect Vehicle

“…After that, in [ 8 ] CFD simulation cycles where used to gain understanding on aerodynamics of small scale WIG ships, and flight test performed afterwards to validate those numerical results. In [ 9 ] authors present a thorough study of the aerodynamic behavior of a WIG craft by using CFD simulations, introducing some findings concerning the periodicity of the aerodynamic forces as the craft proceed on a wavy surface; in the same direction, in [ 10 ] authors generalize this study to three qualitative different surface undulation amplitudes to determine how they affect to the aerodynamic behaviour of the craft. Close to these works, in [ 11 ] authors explore the effects of varying the angle of attack when overflying a wavy ground.…”

Effect of airfoil distance to water surface on static stall

“…They observed that at low angles of attacks a steady shock on the lower surface of the airfoil, also at small airfoil distance from the surface, the small angle of attack and high Mach numbers exists a new coupling between the shock buffets on the lower and the upper surface of the airfoil [12]. Other related studies include the studies conducted by Wei He et al [13] studied a numerical-theoretical of the steady massively separated flow around a NACA 4415 airfoil at a low Reynolds number and a high angle of attack near the wavy ground, this research provided new insight into the stability of airfoil-ground flow systems at a low Reynolds number and a high angle of attack [13,14].…”

Linear modal instabilities around post-stall swept finite wings at low Reynolds numbers

“…At Re = 800 and 1000 these authors identified two oscillatory unstable modes corresponding to near-wake and far-wake instabilities, alongside a stationary unstable mode, whereas only one unstable mode was found at the lower Re = 400 and 600. Ground-proximity effects on the stability of separated flow over NACA 4415 at low Reynolds numbers were studied using two-dimensional global (BiGlobal) theory with consideration of both flat (He et al 2019c) and wavy ground surfaces (He et al 2019b). Finally, Rossi et al (2018) considered incompressible flow over a NACA 0010 airfoil and a narrow ellipse of the same thickness at a large α of 30 • (100 ≤ Re ≤ 3000) documenting multiple bifurcations.…”