Effects of Mach Number and Specific Heat Ratio on Low-Reynolds-Number Airfoil Flows

The effects of Reynolds number, Mach number, and gas species (air and ) on aerodynamic characteristics of a thin flat plate and a NACA 0012-34 airfoil were investigated under low-Reynolds-number ( to ) and high-subsonic-flow ( to 0.6) conditions. In addition to lift and drag measurements by a two-component balance system, the pressure-sensitive paint technique was applied to measure pressure profiles on the model surface. For the flat plate, the Reynolds number moderately affects the lift and drag characteristics because of a simple behavior of the leading-edge separation bubble; the length of the separation bubble increases as the angle of attack increases. By contrast, the Mach number and specific heat ratio contribute little to the aerodynamic performance. For the NACA 0012-34 airfoil, the lift curves are highly dependent on the Reynolds number because of the formation, shift, and burst of the separation bubble, whereas the compressibility affects only the stall characteristics. The specific heat ratio has little effect on the aerodynamic performance. In common for both airfoils, it was observed that the Mach-number effect allows for the delay of a laminar–turbulent transition and reattachment of the separated shear layer.