Wall-Resolved LES Evaluation of Mach Number and Angle of Attack Effects on the Flow Field around NACA Airfoils at Low Reynolds Numbers under Compressible Conditions

We performed wall-resolved large-eddy simulations (LES) to investigate the effects of Mach number, angle of attack, and airfoil thickness on the flow field and aerodynamic characteristics of NACA 4-digit airfoils. The Reynolds number was set to 2.3 × 10^4, and the angle of attack was varied from 0 to 9 degrees in increments of 1 degree. The Mach number was set to 0.2, 0.6, and 0.8. At higher Mach numbers, the formation position of the vortex structure and the separation point are located closer to the leading edge under the condition of α=3°, while for α=6° and α=9°, they are located closer to the trailing edge. The size of the vortex structure increases as the Mach number increases. The low-pressure region caused by the separation bubble extends further downstream as the Mach number increases, and for thinner airfoils, pressure recovery occurs closer to the leading edge. This indicates that thinner airfoils are more resistant to flow separation, as evidenced by the sign change point (i.e., the reattachment point) in the skin friction coefficient distribution being located closer to the leading edge for thinner airfoils. Consequently, while the lift-to-drag ratio decreases significantly with increasing Mach number for the thicker airfoil, it remains almost unchanged for the thinner airfoil.