Three-dimensional wake transition and hydrodynamics of a circular cylinder near a plane wall

The three-dimensional wake transition and hydrodynamic forces of a circular cylinder placed near a plane wall are investigated by using direct numerical simulation at a fixed Reynolds number of 300. The simulations span a wide range of gap ratios (G/D = 0.2-∞) with a constant boundary layer thickness (δ/D = 1.6), aiming to quantitatively elucidate the mechanisms linking wall proximity, wake development, and force variations. Using spanwise Fast Foourier Transform and autocorrelation on spatiotemporal lift fluctuations and streamwise vorticity, the wake transition is found to be strongly gap-dependent: two-dimensional flow dominates at small G/D (0.2-0.4), while increasing G/D induces a progression from finer streamwise vortices to a mixture of large- and small-scale vortices. Near-wall interactions accelerate the transition to three-dimensionality compared to the isolated case. Four distinct vortex shedding regimes are identified—suppressed shedding, single vortex street, weak double vortex shedding, and quasi-isolated wake—each governed by the interplay between cylinder shear layers and the boundary layer. Hydrodynamic force analysis reveals that drag and lift fluctuations, as well as the Strouhal number, increase with G/D, stabilizing as wall effects diminish. The evolution of streamwise velocity fluctuations near the wall further captures the strengthening of local instabilities and their role in shaping wake structures and force responses. These findings provide new quantitative insights into near-wall bluff body flows and serve as a foundation for flow control and design optimization strategies.