Influence of boundary layer thickness and gap ratios on three-dimensional flow characteristics around a circular cylinder in proximity to a bottom plane

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Three-dimensional flow characteristics around a horizontal circular cylinder near a plane boundary are investigated using a Direct Numerical Simulation (DNS) at the Reynolds number of 350. The cylinder has a gap-to-diameter ratio G/D = 0.2, 0.6 and 1.0, with a boundary layer thickness-to-diameter ratio δ/D = 0, 0.7 and 1.6. The hydrodynamic forces, pressure distributions, vortex shedding patterns, wake transitions and flow three-dimensionality features are presented and discussed. The hydrodynamic forces and pressure distributions are strongly dependent on G / D and δ / D . The mean drag coefficient decreases with increasing δ / D, whereas the mean lift coefficient increases with decreasing G / D. The pressure coefficient distribution around the cylinder changes from being asymmetric to symmetric as G / D is increased from 0.2 to 1.0, resulting in a development from the suppressed state to the fully developed state of the vortex shedding. As for the vortex dynamics, the strength and size of primary vortices decrease with increasing δ / D . The crossing trajectories exhibiting an inversion on the position of clockwise and counterclockwise vortices can only be observed in the wake when G / D is greater than a critical value below which the vortex shedding is completely suppressed, and when the cylinder is partially immersed into the boundary layer ( G / D < δ / D < G / D + 1). An intrinsic switching between the single- and dual-frequency modes of the drag coefficient is found to be related to a complex interaction between the boundary layer and the near-wake vortex shedding. For a specific G / D , the flow three-dimensionality represented by the spanwise kinetic energy is found to decrease with increasing δ / D . For a specific δ / D , the spanwise kinetic energy shows a non-monotonic trend with the variation of G / D . • 3-D flow characteristics around a circular cylinder near a bottom plane are investigated. • New switching between single- and dual-frequency modes of drag forces is found. • Influence of boundary layer thickness and gap ratios on wake transitions is examined. • Evolutions of vortex strength/interactions and flow three-dimensionality are highlighted. • Interesting V-type oblique vortex shedding pattern is observed through the wake transition.