Direct Numerical Simulation of Transonic Flows Induced Pitching of NACA Airfoil
A Direct Numerical Simulation (DNS) investigates transonic flow over a NACA0012 airfoil coupled with single-DOF structural dynamics. Analysis is performed for angle of attack (α) 8° with varying reduced velocity (U*) from 2 to 8, reduced mass (m*) 5.0, Reynolds number (Re_\infty) of 3 \times 10^6, and Mach number (M_\infty) 0.8. The speed of sound in air is calculated as a = sqrt(γRT_\infty). It is noted that the magnitude of the aerodynamic loads and moments increased with higher reduced velocity. The Fast Fourier Transform (FFT) of the moment coefficient (C_M) in the fixed-airfoil condition is carried out to extract the non-dimensional dominant frequency, referred to as the Strouhal number (St), along with the coupled oscillation frequency of the airfoil (Stc). The FFT analysis for the stationary case at α = 8° yields a value of St = 1.3161. The presence of high-frequency components indicates the onset of instability associated with shock-boundary layer interaction. The computed values of the coupled oscillation frequency (Stc ) are 0.0762 and 0.1716 for the cases with angle of attack (AoA) 8° and reduced velocities U* = 2.0 and 8.0, respectively. The corresponding natural aeroelastic frequencies (Stn) are taken as 1/U*, yielding values of 0.5 and 0.125. It is observed that for U* = 2, the coupled frequency (Stc) aligns with the subharmonic of shock-buffet frequency, indicating flow-dominated dynamics. In contrast, for U* = 8, Stc coincides with the natural pitching frequency, indicating a dominant influence of the structural mode. Pressure fluctuation contours are examined to analyze the characteristics of acoustic radiation. Vorticity and schlieren visualizations are employed to identify the nature of vortex structures, the strength of the shock, and its spatial location, respectively.