Hydrodynamic Study of Flow Past Cylinders with Different Diameters at High Reynolds Number

Abstract

Deepwater floating systems such as risers, jacket legs, etc., are coupled dynamical systems that undergoes severe hydrodynamic loading when subjected to harsh ocean environment conditions. The flow profiles of these floating structures exhibit complex flow patterns due to its interaction with ocean currents. In the present study, we present the numerical investigations of flow dynamics between the cylinders of different diameters in tandem, side-by-side and staggered arrangements for gap ratios varying from 0.75D to 4D at Re ranging from 3 × 105 to 1.2 × 106. Present study is focused on understanding the flow dynamics, which comprises of flow interferences, shedding of vortices and interaction of shear layers for the cylinders in different locations. Two-dimensional simulations are performed by adopting Reynolds Averaged Naviers–Stokes (RANS) approach and employing k-ω SST turbulence model. Hybrid mesh elements are adopted to capture the complex flow profiles. The elements closer to the cylinder wall are treated with boundary layer condition based on the y+ study performed. For various configurations parameters such as flow fields, time-averaged drag force and pressure coefficients are delineated in the study, for two cylinders with different diameters. To further, understand the effect of spacing ratio and the influence of large diameter cylinder over the smaller diameter cylinder a parametric study has been conducted. A comparative analysis is performed for Strouhal number and drag coefficient at various spacing ratio between the two cylinders. Earlier investigators observe a strong dependency of flow field over spacing ratio and position of the cylinders. The present study exhibits the variation of hydrodynamic forces for the large and small diameter cylinders which also delineates the significant changes in the flow profile. Numerical investigations are carried out by using RANS approach and adopting k-ω SST turbulence model. The adopted numerical approach is validated with the available measurements. Followed by a detailed analysis of lift, drag, and pressure forces are delineated for both small and large diameter cylinders. The hydrodynamic forces alter significantly for different cylinder positions. At side-by-side arrangement, the Cd for large and small diameter cylinder is higher for lower values of gap ratios and decreases as the gap ratio increases. Moreover, significant pressure fluctuations are observed for the cylinder at the wake side of the windward side cylinder, due to the wake interference.