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The effects of Reynolds number, Mach number, and gas species (air and CO 2 ) on aerodynamic characteristics of a thin flat plate and a NACA 0012-34 airfoil were investigated under low-Reynolds-number (Re 0:43 × 10 4 to 4.1 × 10 4 ) and high-subsonic-flow (M 0.1 to 0.6) conditions. In addition to lift and drag measurements by a twocomponent 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.

Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.

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