Effects of airfoil thickness and Mach number on flowfield and aerodynamic characteristics of a NACA 4-digit symmetric airfoil were experimentally investigated. Schlieren visualization, pressure-sensitive paint measurements, and aerodynamic force measurements were conducted in a low-density wind tunnel. Symmetric airfoils with three airfoil thicknesses (NACA0006, NACA0012, and NACA0018) were investigated at Formula: see text for Formula: see text. The schlieren visualization showed that the Mach number affects the behavior of the separated shear layer and recirculation region. The change in the flow pattern is reflected in the pressure coefficient distribution, and the flowfield is categorized into four flow regimes based on the pressure profile. The flow separation easily occurs, and flow reattachment rarely occurs as the Mach number increases. The lift coefficient of NACA0006 is almost independent of the Mach number, but those of NACA0012 and NACA0018 are influenced by the Mach number at Formula: see text and Formula: see text, respectively. The trend of the drag curves is similar. The lift-to-drag ratio of the thin airfoil is much better than that of the thick airfoil, as observed in low-Reynolds-number incompressible flows.
Nagata et al. (2026) studied this question.