Abstract:
Computational fluid dynamics (CFD) has become a powerful tool for solving complex
fluid flow problems in the last few decades. The relation between the flow regime
transformation and its corresponding hydrodynamics forces on the staggered cylinders
has not been studied at low Reynolds number (Re), both experimentally and
numerically. In order to do so, numerical investigations are conducted to study the
unsteady flow past square cylinders arranged in staggered configurations. Six distinct
staggered geometries are constructed for which simulations are carried out by using two
dimensional single relaxation time lattice Boltzmann method (SRT-LBM). The
combined effect of Re and separation ratio (g*) on the flow physics and hydrodynamics
forces is systematically studied. For this study, physical parameters of practical
importance are chosen in the range of 20 ≤ Re ≤ 160 and 0 ≤ g* ≤ 7. The SRT-LBM
was validated by comparing the unsteady flow around an isolated cylinder and two
cylinders with available experimental and numerical data. Results are presented in the
form of instantaneous vorticity contour visualization, time-histories of drag and lift
forces, power spectra of lift signals and hydrodynamic forces. For all chosen geometries
critical spacing ratio and Reynolds number are identified. It is observed that with
increase in spacing ratios between the cylinders the critical Reynolds number for the
onset of vortex shedding phenomena also increases. Different flow regimes are
observed for different staggered geometries. Chaotic flow is one of the common flow
regime. The transformation of the flow occurs due to the interaction between primary
and secondary frequencies; the latter frequency strongly depends on the spacing ratio
between cylinders. The effect of jet flows on the wake structure mechanism (shed
vortices) behind the cylinders is studied in detail. The results show that both Re and g*
have a significant effect on the hydrodynamic forces. The flow regime maps for varying
Re and g* are proposed for all considered problems. All these results, most of which
have been obtained for the very first time, are of fundamental significance in engineering applications.
