Numerical simulation of two large spheres moving in vertical turbulent pipe flow

Abstract

This study investigates the dynamics of two large spheres in a vertical turbulent pipe flow using Reynolds-Averaged Navier–Stokes (RANS) simulations. We focus on axial, radial, and tangential velocities and the effects of initial separation distances between the spheres. The simulation domain features a 13-meter-long pipe with a 200 mm diameter. The model's validation against experimental data shows an average systematic error of 6.6%. The results reveal that the initial distance between two spheres has a minimal impact on their axial velocities but significantly affects radial velocities, particularly for smaller separations. The spheres reach terminal velocities similar to a single sphere, although interactions between the two spheres lead to increased energy dissipation, resulting in lower velocities. The study also shows that in upward turbulent flow, the axial distance between spheres generally increases over time, with higher mean fluid velocities leading to faster separation due to more pronounced wake effects. The typical "drafting, kissing, and tumbling" (DKT) behaviour is not observed, as the bottom sphere can act as a "windbreak", reducing the direct influence of the wake from the upper one. Radial velocities exhibit significant initial fluctuations due to strong interactions, which dissipate quickly at higher mean velocities. Tangential movements appear to be minimal.