Investigation of wake dynamics near the cylinder with the variation of length-to-diameter ratio

Abstract

The study investigates the impact of drag forces on underwater pipeline structures at junctions where diameter changes occur, utilizing three-dimensional numerical simulations to analyze flow patterns around dual-step cylinders at a Reynolds number of 5000. The numerical model is validated against benchmark studies for a plain circular cylinder at heating condition, demonstrating strong agreement. Subsequently, cylinder temperatures were varied to assess their influence on drag force. The study further explores the effect of varying the aspect ratio L/D from 1 to 3 on drag reduction. Analysis of the flow structures is conducted, focusing on vortex patterns, velocity fields, and force coefficients surrounding the cylinders. Additionally, streamwise and crosswise velocities are analyzed at different sectional planes of the step cylinder. Results indicate that the drag force diminishes with increasing aspect ratio, correlating with formation length (Lf). At lower aspect ratios, step-induced vortices migrate toward the wall, while at higher aspect ratios, they drift midway. This research provides insights for designing stepped diameter cylinder systems in the subcritical regime, contributing to optimized underwater pipeline structures.