Numerical simulations of steady and oscillatory flows around multiple cylinders

This study numerically examines the flow features around multiple-circular cylinders and hydrodynamic forces on the cylinders. The research is motivated by both practical engineering applications and understanding of fundamental flow characteristics. The thesis structure and major findings from this study are summarized below.

Steady uniform flow around two identical circular cylinders of various arrangements at a subcritical Reynolds number (Re) is investigated in Chapter 2. The pitch distance between the cylinders (P) and the cylinders’ alignment to the cross flow are found to be highly influential on the pressure distributions on the cylinders, the vortex shedding frequency (St), and the enstrophy of the system. The change in pressure distribution leads to a variety of characteristics of the forces on both cylinders, including negative drag force, attractive and repulsive lift forces, and the suppression or amplification of these forces compared to the those on an otherwise isolated cylinder. Two distinct values of St are identified in the staggered arrangement based on force analysis. Flow regimes around the two-cylinder system with the change of P and alignment angle are identified and classified. It is found that three-dimensionality of the flow in the gap region and in the shared wake is considerably weakened by cylinder interference in two of the flow regimes.

Chapter 3 is highlighted by a total of seven flow regimes induced by steady flow around a four-cylinder array in a square arrangement, which are identified and mapped on the plane of Reynolds number and pitch distance. Each of the seven flow regimes has distinguished flow features, resulting from the interactions among the shear layers, vortices shed around cylinders and Kármán vortex streets behind the them. Some of the flow features around the four cylinders, such as the single bluff-body vortex shedding, binary vortex shedding, in-phase vortex shedding, biased vortex shedding, and vortex co-shedding & anti-phase synchronization, share similarities with flow features exhibited by flow around two side-by-side cylinders and flow around two tandem cylinders. Some of the flow features, such as triple vortex streets and the in-phase escape of vortices at the onset of vortex shedding from upstream cylinders found in the in-phase vortex shedding regime, are unique to the four cylinder system and have not been reported previously.

Three-dimensional (3-D) simulations are carried out in Chapter 4 to study the vortex shedding flow in the wake of four circular cylinders in a square configuration at one chosen pitch distance where four distinct flow regimes are identified. Physical mechanisms responsible for different flow regimes are proposed and discussed in details. Significant changes in the force coefficients, wake formation length and phase angle of the lift coefficients on the downstream cylinders are observed when the flow transits from one regime to another.

Sinusoidally oscillatory flow around four circular cylinders in the in-line square arrangement is modeled in Chapter 5 at oscillations with relatively low frequency and low amplitude. The primary aim is to investigate the influence of cylinder proximity on flow regimes. A captivating set of flow patterns is observed and identified, including six types of reflection symmetry to the axis of oscillation, two types of spatio-temporal symmetry and a series of symmetric breaking flow patterns. In general, at small gap distances, the four cylinders behave as a single porous body and therefore, the flow fields resemble to those around a single cylinder; while with the increase of gap distance, the individual behavior of each cylinder in the array starts to dominate the flow patterns and thus, the flow field shows a variety of symmetry states as a result of vortex interactions from each cylinder, but also is very prone to asymmetry. The force distributions present the similar features to those observed in flow fields.