Unsteady Aerodynamics Around a Pitching Airfoil with Shock and Shock-Induced Boundary-Layer Separation

Abstract

This study numerically investigated the characteristics of unsteady aerodynamic forces around a pitching airfoil in the transonic flow regime. The unsteady Reynolds-averaged compressible Navier–Stokes (URANS) equations were solved using a Spalart–Allmaras (SA) turbulence model. The pitching NACA64A010 airfoil at mean angles of attack of 0, 2, 4, and 6 deg with the amplitude of 1 deg and reduced frequency of 0.2 was parametrically simulated. The study found that the trend of the unsteady aerodynamic characteristics around the pitching airfoil, particularly that of the unsteady lift force, significantly changes with the mean-angle-of-attack conditions. The shock-induced boundary-layer separation and reattachment during the pitching oscillation have a crucial role in determining the trend of the unsteady aerodynamic forces, such as the phase-delayed or phase-advanced features for the variation in the angle of attack. The boundary-layer separation during the oscillation, observed at a high mean angle of attack of 6 deg, produces the phase-advanced feature of the lift force. Because the phase-advanced feature is associated with a positive damping effect, the result indicates that the boundary-layer separation may serve as a stabilizing effect on the pitching airfoil motion. The boundary-layer reattachment from separation during the airfoil oscillation, observed at an intermediate mean angle of attack of 4 deg, generates an unusual shock wave movement, resulting in a unique lift loop for the variation in the angle of attack. The study also demonstrated a good prediction accuracy of the URANS with the SA model for the unsteady aerodynamics around the pitching airfoil with the shock and shock-induced boundary-layer separation.