Unsteady wake dynamics and flow regimes of a freely rotating cylinder with a splitter plate

This study investigates the unsteady wake dynamics of a freely rotating cylinder with a centroid pin and a splitter plate of length ℓ/D∈[0.25, 1.0], across Reynolds numbers (25≤Re≤500). Using an immersed boundary method, the nonlinear interactions between cylinder vortex shedding and plate tip vortices are resolved. The cylinder splitter-plate system exhibits five distinct flow regimes. In regime 1 (Re<25), the system stabilizes on the centerline with symmetric vortices. Symmetry breaking at Re=25 (regime 2) results in a shift to a stable non-zero mean rotation angle (θM), enhanced by up to 35% for short plates (ℓ/D = 0.25). A Hopf bifurcation at Re≥50 (regime 3) initiates small-amplitude periodic oscillations from vortex shedding. In regime 4 (100≤Re≤300), (θM) becomes nearly Re-independent as inertial and wake-induced back-flow forces balance, although oscillation amplitudes vary nonlinearly due to strong wake–structure coupling. For Re>300 (regime 5), multiple shedding frequencies cause a rapid rise in rotation amplitude (θA) and intensified unsteadiness from secondary vortex interactions. The rotation angle is driven by (i) asymmetric vortex formation on the plate, (ii) turbulent wake fluctuations, and (iii) a feedback mechanism between the wake and the flow separation angle. The birth, growth, and decay of various vortical structures through lifecycle analysis explain non-uniform pressure and velocity distributions, leading to significant drag and lift variations. By linking vortex dynamics, fluid forces, and bifurcation behavior, this study explains how splitter-plate geometry modulates vortex shedding, offering new insights for improved flow-control strategies.