Numerical simulation and theoretical analysis of vortex dipole impinging on airfoil

Strong vortex gusts are a primary cause of flight-vehicle control destabilisation. To investigate the underlying mechanism, direct numerical simulation and impulse theory are employed to systematically analyze the unsteady flow characteristics and aerodynamic performance of a vortex dipole impinging on a NACA0012 airfoil at zero angle of attack. The study finds that the centerline height of the vortex dipole is the core parameter determining the airfoil force and moment characteristics: the drag reaches its maximum when it is level with the leading edge; the lift, moment, and thrust peak when it is slightly above the leading edge; as the height increases further, the vortex-body interaction decays, and the aerodynamic disturbances approach zero. The physical mechanisms differ significantly across heights. In level case, the vortex dipole splits, and the induced secondary vortices combine with it to form a new dipole. This new structure interacts with the airfoil again through self-induced motion, generating oscillatory drag. In slightly high case, the negative-signed primary vortex interacts directly with the airfoil, causing its deformation, weakening, and trajectory deviation. This process leads the thrust to change from a peak to a trough, while the lift and moment undergo an asymmetric evolution from small negative values to large positive values. In high-spacing case, the motion path of dipole is almost unaffected by the presence of the airfoil. Analysis based on impulse theory further reveals the intrinsic relationship between aerodynamic forces and vortex motion: during collision, the change in the primary vortex impulse dominates the force generation; in non-collision cases, the primary vortex impulse remains nearly constant, and the rate of change of impulse from the induced secondary vortices determines the airfoil forces. Quasi-3D simulations with spanwise periodic boundary conditions reveal that an over-limit spanwise computational domain triggers strong 3D flow effects, causing remarkable attenuation of peak aerodynamic force and moment coefficients.