Direct numerical simulation of laminar boundary-layer separation and separation control on the suction side of an airfoil at low Reynolds number conditions

Laminar flow separation from lifting surfaces at low Reynolds number conditions can result in significant performance losses for aircraft with small geometric dimensions and is therefore a limiting factor in research areas involving unmanned flight technology and aircraft design. We investigated separation and transition of the flow on the suction side of a NACA 643 - 618 airfoil at a chord Reynolds number Re_c = 64, 200 and two angles of attack, α = 8.64° and α = 13.85°, respectively. For a simplified model geometry we conducted highly-resolved direct numerical simulations (DNS) using a high-order accurate numerical code that solves the incompressible Navier-Stokes equations for general orthogonal grids. Results for the uncontrolled, "natural" flow are presented and compared to available experimental data and numerical simulations of the entire airfoil. For the larger angle of attack, α = 13.85°, a shallow leading-edge separation bubble develops that exhibits clear differences to the more typically observed separation starting from the aft section of the airfoil at lower angles of attack. In a side-by-side comparison we will elaborate on these differences in an effort to understand the relevant physical mechanisms that govern the flow field in both scenarios. This understanding is necessary in order to design and improve flow control strategies aimed at preventing or delaying flow separation. We also present DNS results of separation control using high-amplitude, two-dimensional blowing and suction through a narrow spanwise slot. It is demonstrated that flow control can lead to a significant performance increase but can also cause performance degradation when the control parameters are chosen inappropriately.