Interference effects on wakes of a cluster of pentad square cylinders in a crossflow: A lattice Boltzmann study

Abstract

This study examines the variations in wake structures and fluid forces around a cluster of five square cylinders. The cylinders in the cluster are arranged such that a larger cylinder (the main cylinder) is surrounded by four similar but small-sized cylinders, which are very common in marine risers and axial pipe shrouds. The analysis uses an in-house developed code based on the lattice Boltzmann method. The gap spacing (g*) between the main and the surrounding cylinders varies from 0 to 10 while maintaining a fixed Reynolds number (Re) of 200. The accuracy and consistency of the code are first tested through simulations that examine the flow behind a single cylinder, revealing strong concurrence with the existing data. In the next step, the flow characteristics of the pentad arrangement are analyzed. Five diverse flow modes, including the extended single bluff body flow, stable symmetric flow, chaotic irregular shedding, antiphase symmetric flow, and modulated periodic flow, are observed depending on the gap distances. It is found that the small cylinders in the downstream position from the primary cylinder experience a negative mean drag at narrow gaps. In contrast, those at the upstream position are subjected to a positive drag force regardless of the gap spacing. The main cylinder has a higher drag coefficient compared to other cylinders for all gap spacing cases. The shedding frequencies appear similar for upstream and downstream smaller-size cylinders at higher spacing values, while the shedding frequencies for the main cylinder significantly deviate from the surrounding small cylinders.