# Leading-Edge Vortex Characteristics of Low-Aspect-Ratio Sweptback Plates at Low Reynolds Number
## 1 Introduction  
State-of-the-art aerial vehicles such as personal air vehicles, flying cars, drones, and micro/nano air vehicles do have their unique shapes creating a fully three-dimensional flow. The exceptional maneuverability required for the vehicles, e.g., vertical takeoff/landing, stable hovering, and smooth transition to forward flight, also provokes separated flows with a complicated interaction in shed vortices at low Reynolds number. These severely affect the aerodynamic performance overall; a thorough investigation of the separated flows is a crucial step for the appropriate development. A separated flow has often been studied using low-aspect-ratio (LAR) wings at high angles of attack where interaction in shed vortices is pretty intense, thus, conventional aerodynamic descriptions are hard to cover. [PERSON] and [PERSON] [1], for example, measured the aerodynamic lift and drag of various LAR wings and showed that a classical description--[PERSON]'s lifting line theory--is not appropriate for the LAR wings due to nonlinear characteristics in the aerodynamic coefficients. They rather presented that the leading-edge suction analogy, which had been originally developed for a vortex lift force caused by a leading-edge vortex (LEV) attachment on delta wings [2], provides better estimations than the lifting line theory. This was in line with the study of [PERSON] and [PERSON] [3], which revealed the drastic decrement in drag coefficient as a result of tip vortex (TiV) detachment. [PERSON] et al. [4] also investigated the aerodynamic characteristics of LAR wings, and obtained [PERSON]'s efficiency factors, approximately 0.39, 0.34, and 0.22 for the aspect ratios (ARs) of 2, 3, and 6, which are much lower than that of a typical higher AR. Recent studies focused more on the underlying physics of the flow such as the formation, behavior, and interaction of the shed wakes and vortices. [PERSON] and [PERSON] [5] analyzed aerodynamic characteristics of impulsively translated flat plates at low Reynolds number via direct numerical simulations (DNS). They found periodic/aperiodic sheddings depending on an AR, angle of attack, and Reynolds number, which were mainly due to a strong engagement of TiVs into the midspan region. [PERSON] and [PERSON] [6] investigated much lower aspect ratio (AR) plates including one and below. They revealed that a massive downwash induced by the TiVs of the LAR wings is the key to augmenting the lift production at a high angle of attack; the downwash depressed the vortex shedding at the trailing edge, thereby sustaining the Kutta condition at the midspan, where a periodic vortex shedding such as a Karman vortex street emerges at a typical higher AR. [PERSON] et al. [7] analyzed the formation of vortices over the wings of ARs of 1 to 6. They found that the effect of TiVs, which evokes a strong downwash and additional lift, can be extended to the midspan where an LEV exists at a low AR, resulting in full three-dimensionality of the shed vortices. This suggests that an LEV would be one solution for separated flows of the state-of-the-art aerial vehicles. In terms of flapping-wing aerodynamics, an LEV has been seriously investigated as biological flyers employ an LEV to stabilize a flow around the wing, thereby boosting a lift force [8]. Comprehensive studies such as establishing appropriate Navier-Stokes equations, extracting dimensionless numbers [9; 10], and several parametric studies on the dimensionless numbers (refer to [PERSON] et al. [11] for an aspect ratio; [PERSON] et al. [12] and [PERSON] et al. [13] for an advance ratio; [PERSON] [14] for dimensionless numbers), have successfully revealed the stabilization mechanism of the LEV-the spanwise flow running to the wingtip in the core of the LEV, which is driven by the radial pressure gradient along the span with appropriate centripetal and Coriolis forces, stabilizes the LEV. Here, the AR of the wings of biological flyers has been seriously dealt with as it governs the Coriolis force and consequent stabilization mechanism of the LEV.