Aerodynamic characterization of low-aspect-ratio swept wings at $Re=400$

Abstract

The low-Reynolds-number aerodynamics have been extensively studied over the past few decades due the interests in designing small-scale air vehicles and understanding biological flights. At these scales, flows over wings exhibit complex flow physics comprised of unsteady separation, vortex formation, and wake interaction that are different from the high-Reynolds-number counterparts. In fact, the aerodynamic characteristics of low-Reynolds-number flows exhibit strong nonlinearity arising from the rich vortex dynamics. Liu et al. (2012) and Kurtulus (2015) numerically investigated two-dimensional unsteady flows over a NACA 0012 airfoil at a Reynolds number of 1000. Both studies highlighted the nonlinear behavior of the aerodynamic characteristics. Rossi et al. (2018) assessed the Reynolds number effects (Re = 100 − 3000) on the two-dimensional flows over a NACA 0010 airfoil and an ellipse at a fixed angle of attack of 30 • . The presence of multiple bifurcations in the flow behavior and aerodynamic characteristics was reported. More recently, Menon & Mittal (2019) conducted a comprehensive study on the effects of twodimensional airfoil shapes and Reynolds number (Re = 500 − 2000) on the aerodynamic characteristics. Their results showed sizeable and rapid changes in the aerodynamic quantities with angle of attack, due to the presence of distinct flow phenomenon such as the Kármán vortex shedding and the formation of the leading-edge vortex.Low-Reynolds-number flows around wings are further enriched by the end effects for finite-aspect-ratio wings. The nonlinear interactions among the tip vortices and the leading-/trailing-edge vortices lead to three-dimensional and aperiodic wakes (Winkelman & Barlow 1980;Freymuth et al. 1987;Taira & Colonius 2009;Zhang et al. 2020b). Such vastly different flow physics from the analogous two-dimensional flows suggests that the understanding of fully three-dimensional analysis is necessary for practical wing designs at low Reynolds numbers. In our previous study, the wake dynamics of a NACA 0015 finite-aspect-ratio wings has been examined for a range of aspect ratios and angles of attack at Re = 400 (Zhang et al. 2020b). The aerodynamic force coefficients of the finite-aspect-ratio wings were observed to be sigificantly lower than those of the twodimensional airfoils even for wings with large aspect ratios. As we recently studied the flow over finite-aspect-ratio swept wings (Zhang et al. 2020a), we observed that the sweepinduced midspan effects add another source of three dimensionality to the wake dynamics. However, the aerodynamic characteristics of the swept wings were not systematically reported in that study.In this Technical Note, we present a database of aerodynamic force coefficients for finite-aspect-ratio swept wings at a low Reynolds number of 400. The aerodynamic data are obtained from three-dimensional unsteady direct numerical simulations over a range