This paper investigates the flow dynamics around two circular cylinders in side-by-side arrangement with different spacing ratios under a subcritical Reynolds number condition. A 3-D numerical model was developed with LES technique. Based on the LES results, three wake regimes were identified, i.e., single bluff body regime (T/D=1.2), biased flow regime (T/D=1.5-2) and parallel vortex streets regime (T/D=2.5-4). In the single bluff body regime with T/D =1.2, the stable deflection of gap flow is also observed which indicate that there may exist a transition state from the single bluff body regime to the biased flow regime. In biased flow regime, the pairing and merging process of the outer vortices with the inner vortices are analyzed. The occurrence of the flip-flopping phenomenon is found to be related to the merging tendency between gap-side vortices in narrow wake region and free-flow-side vortices in wide wake region, and the relative phase of gap side vortices in transient state. In the parallel vortex streets regime, the phase relation of the vortex shedding process was analyzed. The time proportions of the in-phase mode and anti-phase mode are found to be varied with spacing ratio. As the spacing ratio increases, the wakes behind the cylinders lose their dependency on the anti-phase mode. The results of the present study were compared with the existing results at other Reynolds numbers. It is found that vortex shedding manner during the flip-over transitions is closely related to the spacing ratios, and is independent of the Reynolds number.

Floodplain vegetation is of great importance in velocity distribution and turbulent coherent structure within compound open channel flows. As the large eddy simulation (LES) technique can provide detailed instantaneous flow dynamics and coherent turbulent structure predictions, it is of great importance to perform LES simulations of compound open channel flows with floodplain vegetation. In the present study, a wall-modeled large eddy simulation (WMLES) method was employed to simulate the compound open channel flows with floodplain vegetation. The vegetation-induced resistance effect was modeled with the drag force method. The WMLES model, incorporating the drag force method, was verified against flume measurements and an analytical solution of vegetated open channel flows. Numerical simulations were conducted with a depth ratio of 0.5 and four different floodplain vegetation densities (f_rk = 0, 0.28 m⁻¹, 1.13 m⁻¹ and 2.26 m⁻¹). The main flow velocity, secondary flow, bed shear stress and vortex coherent structure, based on the Q criterion, were obtained and analyzed. Based on the numerical results, the influences of floodplain vegetation density on the flow field and turbulent structure of compound open channel flows were summarized and discussed. Compared to the case without floodplain vegetation, the streamwise velocity in the main channel increased by 10.8%, 19.9% and 24.4% with the f_rk = 0.28 m⁻¹, 1.13 m⁻¹ and 2.26 m⁻¹, respectively. The results also indicated that, when the floodplain vegetation density increased, the following occurred: the velocity increased in the main channel, while the velocity decreased in the floodplain; the transverse momentum exchange was enhanced; and the strip structures were more concentrated near the junction area of compound open channel flows.

To investigate the characteristics of the bi-stable flow at subcritical Reynolds numbers, large eddy simulation is adopted to simulate the crossflow around two tandem circular cylinders at Re = 3900. The reattachment/co-shedding bi-stability is observed in the simulations with spacing ratios (L/D, L is the center-to-center cylinder spacing and D is the diameter) of 4.5 and 4.7. Statistical analyses are performed on the hydrodynamic coefficients, time-averaged flow fields, three-dimensional characteristics, wake pattern, and vortex shedding frequencies at different spacing ratio and time period. In addition, a detailed analysis and explanation were conducted on the secondary vortices identified in the reattachment flow regime, revealing that the secondary vortices, generated from the instability of the shear layer, significantly influence the variation in vortex shedding frequency over time. The reduced-order variational mode decomposition method is employed to decompose the flow field during the flow regime transitions, unveiling their spatial and temporal features. It is revealed that the shear layer instability and the low-frequency modulation behavior are the predominant factors triggering the bi-stable phenomenon at subcritical Reynolds numbers. This study aims to uncover triggering mechanisms underlying the bi-stable phenomenon in the flow around two tandem cylinders and provides valuable insight for relevant engineering applications.

This paper investigates the flow dynamics around two tandem circular cylinders with different spacing ratios (L/D, L is the center-to-center spacing and D is the cylinder diameter) under a subcritical Reynolds number of 3900. Large eddy simulations are conducted using a well-validated three-dimensional (3D) numerical model Zeng et al., [Phys. Fluids 35, 035102 (2023)] at eleven spacing ratios (L/D = 1.2, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, and 7). As the spacing ratio (L/D) increases, the flow field around the cylinders sequentially exhibits the following wake regimes: the extended-body regime (L/D = 1.2), the reattachment regime (L/D = 1.5–4), the transitional regime (L/D = 4.5), and the co-shedding regime (L/D = 5–7). The characteristics of each regime were qualitatively analyzed using parameters such as the hydrodynamic coefficients, the circumferential pressure coefficient, and the vortex shedding frequency. For the reattachment regime, the shear layer separation points and the reattachment points on the downstream cylinder are examined. Based on the instantaneous vorticity field, the reattachment regime is further classified into the alternate impact mode and the alternate attachment mode. In the co-shedding regime, there is a phase lag between the vortex shedding of the two cylinders, and it is found that this phase lag increases linearly with the spacing ratio.

To investigate the characteristics of the wake-induced galloping (WIG) phenomenon at subcritical Reynolds numbers, large eddy simulation (LES) is adopted to simulate the cross-flow around one-fixed-one-free tandem cylinders at Re = 3900 and 15 000. The WIG phenomenon is numerically reproduced when the downstream cylinder (DC) has single- or two-degree-of-freedom (2DOF), and is verified against the existing experimental data. The research reveals that the wake regime is the 2T mode, consistent with the superupper branch of the 2DOF vortex-induced vibration of a single cylinder. The WIG mechanism is attributed to the coupling between the cylinder oscillation and the upstream-shed vortices. The DC's oscillation offers space for the growth of the opposite-side vortices, enabling them to fully develop before reaching the DC. Subsequently, the interaction between the vortex structure and the side of the DC leads to surface pressure variations, propelling the cylinder to move in the opposite direction and establishing a stable galloping frequency and amplitude. Analysis also indicates that the galloping response is governed by the upstream cylinder's vortex shedding frequency and the wake regime, which is closely associated with the Reynolds number and inflow conditions.