Kinematic regimes and wake topologies of a two-degree-of-freedom airfoil with distinct heave and pitch stiffnesses in flow-induced vibrations

Numerical simulations are performed to investigate the flow-induced vibration of a two-degree-of-freedom airfoil with coupled heave and pitch motions and distinct structural stiffnesses. The simulations are conducted for a controlled two-dimensional low Reynolds number baseline at Re=ρU∞c/μ=400, Ma=0.1, Xe=0.35c, and zero structural damping (ch=cθ=0). The structural parameter space is organized by the reduced velocities Uh*=U∞/(fhc) and Up*=U∞/(fθc) over 2.5≤Uh*≤10 and 3≤Up*≤10.5. The global response map is divided into four kinematic regimes, denoted by Hd, HPm, Pd, and Nv, together with four periodic and two intermittent temporal sub-regimes. The resulting map is not a simple stiffness map: large responses occur only where the fluid forcing coordinates the heave and pitch branches into a coherent oscillatory state. The synchronized periodic responses collapse onto a common dominant frequency and are characterized by closed heave–pitch trajectories and a nearly constant phase difference close to π/2, whereas the intermittent responses show phase drift, broadband residual content, and incomplete trajectory recovery. Representative cases further show that the kinematic organization is accompanied by distinct wake families ranging from attached flow and 2S like states to 2P + S like, 2P + C like, and intermittently modulated wakes. By establishing a phenomenological link between the global kinematic response, phase coordination, and representative wake topology, this regime map provides a compact baseline for interpreting synchronization and wake transitions in the present passive airfoil system.