Compressibility-induced variation in drag and lift coefficients of NACA0012 airfoil*

The goal of this investigation is to elucidate the effect of compressibility on the temporal oscillation of lift and drag coefficients of an airfoil section. To this end, we perform two-dimensional direct numerical simulations (DNS) of a NACA0012 airfoil at an angle of attack (AoA) of 5° under both compressible (Mach 0.4) and incompressible conditions. The Reynolds number (Re) based on the chord length and freestream velocity is kept constant at  5×10⁴. Both cases exhibit periodic oscillations in lift and drag coefficients due to the vortex shedding associated with the shear layer roll-up at the leading edge of the airfoil, typical of the Re and AoA. While the incompressible case shows a nearly constant amplitude in the oscillation of the force coefficients, the compressible case exhibits intermittent excursions in the magnitude of these oscillations. The underlying physics of these transient excursions is investigated by creating an ensemble of such events and analyzing them using a recently developed conditional proper orthogonal decomposition (CPOD) framework. A deeper understanding of the underlying reasons for this amplitude variation can lead to significant improvements in aerodynamic performance of lifting surfaces.