50 / 2019-08-28 14:13:39

Vortex Dynamics in the Wake of Two Dual-Step Cylinders in Side-by-Side Arrangement

vortex structures, dual-step cylinder, side-by-side

In recent years, complex cylindrical structures evolve with nonuniform geometries in real-life engineering practice such as marine risers with a series of intermediate buoyancy modules. Due to a discontinuity in the diameters, the flow field and wake vortex characteristics of such a stepped cylinder are significantly different from those of a uniform cylinder. Based on the different shedding frequencies appearing behind the stepped cylinder, the vortices shed from the smaller cylinder, the larger cylinder, and the steps are the S-cell, L-cell, and N-cell, respectively. The S-cell has the highest shedding frequency, and the frequencies of the L- and N-cells are sequentially decreased. Owing to the difference in the shedding frequency of vortex cells, a variety of vortex interactions occur within vortex cells and at the cell boundaries. For example, the half-loop connections exist between the two vortices of the opposite sign within one cell while the direct cross-boundary connections occur between the two different vortex cells. At the same time, two streamwise vortex structures, the junction vortex and the edge vortex rotating in the opposite directions at the root of the smaller cylinder, were observed.
In this study, flows past two dual-step cylinders in the side-by-side arrangement is numerically investigated by using the Nektar++. Vortex structures in the wake of the dual-step cylinders are visualized by using the isosurfaces of λ_2 – the second largest eigenvalue of the symmetric tensor S2 + Ω2, where S and Ω are, respectively, the symmetric and asymmetric parts of the velocity gradient tensor ∇u. Vortex connections are studies on the purpose of shedding new light on the complicated vortex dynamics.