The flow patterns around and the forces acting on an oscillating circular cylinder submerged in a still fluid at rest have been studied by both experiments and numerical simulations. Experiments has been carried out on fluiddynamic forces on and visualized flow patterns around a stationary circular cylinder which was set in a planar oscillatory flow generated by a U-tube water tank with the Keulegan Carpenter (KC) numbers up to 34 and the beta value of 95. An ALE (Arbitrary Lagrangian-eulerian) finite difference method has bene employed to simulate the flow around an oscillating circular cylinder submerged in a still water. The computations have been carried out under the assumption of 2- and 3-dimensional, unsteady, incompressible and viscous flow. The results predicted by the numericla simulations are compared with the experimental ones. Although flow-patterns around a circular cylinder are complicatedly produced by both the vrtex-shedding from a circular cylinder and the planar structures of "longitudinal vortices", "asymmetric region," transverse strees" and "double pair", all can be successfully reproduced and identified in the present computations, and be confirmed to agree well with the visualized fow patterns. It is also certified to be good agreements between the predicted and measured in-line forces, i.e., the values of drag and inertia coefficients, CD, CM, represented by the well-known Morison's equation and the fluctuating transverse force ()lift force C_Lrms across a range of KC values. Furthermore, it is noteworthy that the values of transverse force depending on KC numbers correspond well to the process of flow pattern variation.
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