Numerical Investigation of Flow Dynamics Over a Multi-Element Airfoil in Ground Proximity

Abstract

Low Reynolds number (O(10⁴)) flows play a crucial role in aviation, particularly for unmanned aerial vehicles (UAVs), micro air vehicles (MAVs), and low-altitude flights, where aerodynamic efficiency and stability are critical. When operating near the ground, these flows become more complex due to changes in pressure distribution and wake interactions. Understanding these effects on multi-element airfoils is essential for optimizing performance during takeoff, landing, and low-altitude flight. In this study, we employ Direct Numerical Simulation (DNS) with a high-order spectral element method code, Nek5000, to investigate the flow dynamics around a 30P30N multi-element airfoil in ground proximity, at a low Reynolds number of 1.27 x 10⁴ and an angle of attack (AoA) of 4°. Two-dimensional (2D) simulations are conducted for the varying ground clearance ratios, height from the ground (h) non-dimensionalized by stowed chord length (c), h/c =1, 0.8 and 0.6. The results are compared to previous work, which studied the same airfoil under free-stream conditions, to assess how ground proximity modifies the flow characteristics. The simulation reveals that the shear layer separates near the leading edge of the main element, forming a wave-like roll-up before periodically reattaching after mid-span. As the reattached flow moves further downstream toward the flap, we observe a periodic slot jet flow between the trailing edge of the main element and the flap. The separation bubble in the slat cove region is significantly smaller in ground proximity than in the free-stream case from earlier work, indicating that ground effect modifies the slat wake evolution, potentially altering transition mechanisms and downstream flow structures. Aerodynamic performance metrics, including lift, drag, and pressure and skin-friction coefficients, are analyzed to provide a comprehensive assessment of the airfoil's aerodynamic behaviour. The presence of ground effect results in increased lift and a substantial reduction in drag. Consequently, the lift-to-drag ratio ( C_L/C_D) by 28% to 68% relative to previous free-stream studies, corresponding to more than a 1.7-fold enhancement in aerodynamic efficiency.