The flow through tandem square cylinders was investigated at a Reynolds number of 100 for oscillation amplitudes A = 0.1D to 0.7D and gaps L = 2.0D, 5.0D, and 6.0D, where D is the width of the cylinders. A moving reference frame method combined with the spectral/hp element method was employed to simulate the two-dimensional flow in the lock-in regime. Fluid forces, vorticity fields, power spectrum density, and pressure distribution were first investigated. Since surface pressure is directly connected with fluid forces, pressure and velocity field were synchronously analyzed by employing optimal dynamic mode decomposition. An underlying link between fluid forces and coherence modes was then uncovered. The results reveal that the move-induced forces and flow structures strongly depend on gaps and amplitudes in the lock-in regime. With respect to the dynamic mode decomposition analysis, odd-order modes contribute to lift forces, while even-order modes result in drag forces. The flow structures are dominated by at most three modes; as the amplitude increases, the high-order mode energy increases, coinciding with corresponding power spectrum density results of forces. Typical 2S, 2P, and C(2S) wakes were observed for various gaps and two representative amplitudes (A/D = 0 and 0.7), and their dominant modes show distinctive differences that lead to different local pressure shapes on the cylinders. It is the combined effects of local mode shape and global mode energy that account for the change in fluid forces for various gaps and two oscillating amplitudes.

This study investigates the flutter response of a rectangular cylinder model with an aspect ratio of 5 at the Reynolds number Re = 100 via direct numerical simulation. The effects of two key parameters, i.e., the moment of inertia and reduced flow velocity, on the aerodynamic performance and dynamic responses of the cylinder in the state of torsional flutter are investigated. To reveal the flutter mechanism, the high-order dynamic mode decomposition (HODMD) analysis is conducted to decompose the flow field. The results show that both an increase in the moment of inertia and a higher reduced flow velocity lead to a larger torsional amplitude and a corresponding decrease in torque. At the same time, the primary frequency decreases and the size of the shedding vortex gradually enlarges. The vortices shed from the leading edge and the trailing edge of the model form a 2P wake pattern. The leading-edge vortex is significantly larger than the trailing-edge vortex in terms of strength and size. The leading edge plays a dominant role and only contributes to the odd-order HODMD modes while the even-order modes are deemed inconsequential. As the moment of inertia increases, the total energy of the higher-order modes increases, which has the same results as the power spectral density of torque, reflecting increased nonlinearity and complexity of the system. Similarly, increasing the reduced flow velocity at the same moment of inertia has similar excitation effects.

This paper focuses on a typical bluff body, a rectangular cylinder with an aspect ratio of 5, and quantitatively analyzes the fluid force coefficients and Strouhal number (St) under different angles of attack (α) and Reynolds numbers (Re) through numerical simulations. The higher-order dynamic mode decomposition method is used for coherence mode analysis, and Floquet instability analysis is used to identify the 3D transitional angle of attack (α3D) at different Re as well as the Floquet modes at various α. The results show that increasing the angle of attack significantly raises the mean drag coefficient (C¯D) and fluctuating lift coefficient (C′L), while the St number gradually decreases. This leads to the emergence of vortices and complex flow structures of different scales and frequencies at the rear edge of the rectangular cylinder and its lower cavity region, accompanied by increased downstream turbulence intensity. Furthermore, the 3D transitional angle of attack (α3D) decreases with increasing Re. Two types of Floquet modes are identified within the range of Re and α, namely, Mode A and Quasi-Periodic Mode QP. This study provides important insights and theoretical support for a deeper understanding of flow evolution problems.

Flow-induced vibration (FIV) of twin square cylinders in a tandem arrangement was numerically investigated at Reynolds numbers 200 and gap L/D = 2.0, 4.0, and 6.0 (D is the side length of the cylinders). Fluid-structure-coupled Koopman mode analysis method was developed to synchronously identify the coherence flow and structural modes. Then, the energy transfer between cylinders and Koopman modes was analyzed to uncover the underlying mechanism of FIV. The results showed that at L/D = 2.0 and 4.0, only soft lock-in vortex-induced vibration (VIV) was observed. The oscillating amplitude for L/D = 4.0 was much higher than that of L/D = 2.0, due to the interference effects induced by fully developed gap vortices. As L/D = 6.0, VIV and galloping coexisted. For the coherence mode, the primary flow mode induced by the vortex shedding dominated the flow field at L/D = 2.0 and 4.0. The direct mode energy dominated the energy transfer process. The upstream cylinder (UC) contributed to the negative work done and thus tended to stabilize the vibration; in contrast, the downstream cylinder (DC) exhibited the opposite behavior. In the galloping branch at L/D = 6.0, both the flow field and structural response contained three main modes: one vortex-shedding-induced mode and two vibration-induced modes. For the direct mode energy, owing to the interference effects, DC contributed to more positive work done than UC by the vibration-induced modes. The vortex-induced mode was governed by DC and afforded negative work done. Moreover, all the coupled mode energy was almost equal to zero.

Under strong winds, the effect of sudden windbreak transition (WT) on high-speed trains is severe, leading to a deterioration of train aerodynamics and sudden yawing motion of the car body. To address these problems, based on a high-speed train and the specific geometric conditions derived from Xinjiang railway, first, the impact of a WT on the train and reasons for sudden changes in aerodynamic forces were determined by flow structural analysis. Furthermore, based on a multibody system dynamic model, the dynamic responses to WT were analysed. The results show that the impacts of WT were the strongest on the head car. WT had a strong effect on the train due to the unreasonable structural shape and the insufficient height of the windbreak in the transition region. This led to a strong push effect on the train; subsequently, the train’s dynamic characteristics deteriorated.

A rectangular cylinder, with an aspect ratio of 5, is a widely used bluff body in engineering practice. It undergoes intricate dynamical behavior in response to minute alterations in the flow angle of attack (α). These modifications invariably precipitate the failure of wake control for classical flow control methods with various α values. In this study, global linear instability, adjoint method, and sensitivity analysis are employed to identify the optimal position for flow control. It is found that the sensitive region gradually transitions from the leeward side to the downwind side of the model as α and Reynolds number (Re) increase. So, we set up airflow orifices for flow control in both positions. Jet flow control on the leeward side effectively inhibits vortex shedding (α ≤ 2°). High-order dynamic mode decomposition is employed to reveal the inherent mechanism of control. Suction control on the downside effectively mitigates the shear layer separation phenomenon induced by the altered spatial structure associated with higher α. A novel zero-net-mass-flux wake control, bionics-based breathe-valve control (BVC), is proposed to optimize the control effect. BVC is applicable for various α and Re, with optimal effectiveness achievable through jet velocity adjustments. The prediction-control approach in this investigation provides a targeted method to mitigate flow-induced vibration.