This study conducts experimental investigations into wake-induced vibration (WIV) of a circular cylinder placed downstream of an oscillating cylinder. Surprisingly, it is observed that the previously identified WIV phenomenon, characterized by a sustained increase in amplitude at higher reduced velocities, does not occur when the upstream cylinder oscillates at large amplitudes. Instead, a different phenomenon, which we refer to as the ‘wake-captured vibration’, becomes dominant. The experiments reveal a negative correlation between the vortex-induced vibration amplitude response of the upstream cylinder and the WIV amplitude response of the downstream cylinder. Through a quasi-steady and linear instability analysis, the study demonstrates that the previously proposed wake-displacement mechanism may not be applicable for predicting the cylinder WIV response in the wake of an oscillating body. This is because the lift force gradients across the wake, measured through stationary cylinder experiments, decrease significantly when the upstream cylinder vibrates at higher amplitudes. Consequently, actively controlled vibration experiments are conducted to systematically map the hydrodynamic properties of the downstream cylinder vibrating in the wake of an oscillating cylinder. The findings align with observations from free-vibration experiments, and help to explain the amplitude and frequency response of WIV. Additionally, wake visualization through particle image velocimetry is conducted to provide further insights into the complex wake and vortex–body interactions.

The insight to the hydrodynamics of the flapping hydrofoils can be utilized to comprehend the fluid-mechanical mechanisms for the variation of the hydrofoil's propulsion performance, and optimize the design of the bio-inspired underwater robots. This paper numerically investigates the hydrodynamic performance of a two-dimensional NACA 0012 (defined by National Advisory Committee for Aeronautics) hydrofoil undergoing combined pitching and heaving motions, based on the discrete vortex method. A novel boundary method which couples the Joukowsky transformation and the circle theorem is used to deal with the vortex elements inside the hydrofoil surface. The numerical method is validated by the experiments from the available relative cases by comparing the hydrodynamic forces and the wake patterns with the pure pitching and pure heaving hydrofoil. Via the vortex dynamics, the discussion is carried out to explain the thrust deterioration within the large Strouhal number and large pitching amplitude in the combined motion. Further, a parameter optimization study is conducted under different heaving amplitudes and pivot locations. When the heaving amplitude is 1.25c and the pivot location is at 0.45c, the propulsion efficiency of the flapping hydrofoil is found maximized. Finally, by combining the time history of hydrodynamic forces and the formation and release of the leading-edge vortex and trailing-edge vortex in the vorticity field, this paper explains the reason why the optimal efficiency is obtained.

Some very useful methods for suppressing the flow-induced vibration (FIV) of a single cylinder are known to potentially have a limited efficiency for tandem-arrangement cylinders. In this paper, three splitter plates uniformly attached around a cylinder with an angle of 120° are proposed to suppress the FIVs of both a single cylinder and two tandem-arrangement cylinders in a wind tunnel at Re = 4000–45,200. The splitter plates’ length to diameter ratios, L/Ds (where L is the length of the splitter plate and D is the cylinder diameter), are set from 0.1 to 0.8. The results show that the proposed method not only effectively suppresses the vortex-induced vibration (VIV) for a single cylinder, but also successfully mitigates the wake-induced galloping (WIG) for two tandem-arrangement cylinders. The vibrations of the single cylinders are effectively suppressed, consistently achieving suppression efficiencies over 95% for L/Ds = 0.2–0.8, with a notable peak efficiency of 98.4% at L/D = 0.2. For the two tandem-arrangement cylinders at S/D = 4.0 (where S is the center-to-center spacing between the two cylinders), the suppression efficiencies of the upstream cylinder exceed 96% for L/D = 0.2–0.8, with an optimal efficiency of 97.4% at L/D = 0.6. The downstream cylinder exhibits vibration only at L/Ds = 0.1, 0.2, and 0.4, resulting in suppression efficiencies of 80.3%, 67.1%, and 91.0%. The vibrations remain fully suppressed throughout the entire reduced velocity range for L/Ds = 0.6–0.8, reaching an optimal efficiency of 98.7% at L/D = 0.6. Three regimes of f_s/f_n characteristics can be classified for the single cylinder, and the wake structures show that shear layers develop along the front plate before attaching on the cylinder and are then offset to either side of the cylinder by the two rear splitter plates, contributing to the absence of periodic vortex shedding.

The low efficiency and complex aerodynamic characteristics of vertical-axis wind turbines (VAWTs) have limited their industrial development and application. An active variable pitch method can improve the aerodynamic performance of VAWTs, and different pitch control strategies yield varying improvements. In this study, a novel pitch control strategy for constant blade angle of attack (AoA) has been proposed to improve the aerodynamic performance of Darrieus H-type VAWTs. Numerical simulations have been performed to investigate the performance of this pitch control strategy by using the computational fluid dynamics (CFD) method based on an in-house Navier–Stokes (N–S) equations solver. The turbulence model is the k−ω shear stress transport (SST) model. The inviscid spatial discretization scheme is the fifth-order modified weighted essentially non-oscillatory (WENO-Z) scheme. The effect of pitch control factor κ and amplitude angle A of this novel pitch control strategy on the power coefficient Cp, instantaneous torque coefficient CT of single blade and flow fields has been investigated from a low tip speed ratio (TSR) of 1.44 to high TSR of 3.3. The extensive studies show that this novel pitch control strategy can effectively improve the aerodynamic performance of VAWTs, especially at relatively low TSRs, and the optimal values of κ and A are 3° and 14°, respectively. The maximum absolute increase in power coefficient Cp can reach 0.2444 at TSR of 1.68, and the average absolute increase for all TSRs is 0.1346.

Motivated by the extraordinary maneuvering capabilities by natural creatures, the unsteady fluid dynamics of the flapping air- and hydrofoils has been of keen sustained interest in recent decades, which are critical to the performance of the biomimetic thrusters. For the hydrofoil pitching with large amplitudes, in which situation the flow is massively separated from foil, the vortex evolutions and their implications on the hydrodynamic properties have not been described clearly. This paper numerically investigates the hydrodynamic properties of a large-amplitude 32° pitching NACA (National Advisory Committee for Aeronautics) 0012 hydrofoil, with Strouhal number 0.006–1.30. Focused on the vortex evolutions and their implications on the hydrodynamic responses, four wake types, i.e., leading-edge vortex (LEV)–trailing-edge vortex pair wake, von Kármán-like wake, inverted von Kármán wake, and deflected wake, are classified and discussed in detail, via vorticity patterns and the surface pressure distribution. The increasing Strouhal number delays the formation and the advection of the LEV and drastically alters the vortex dynamics. The asymmetric wake reinforces the thrust generation, which facilitates the transition from the drag to thrust. In addition, the effect of the pitching amplitude on the thrust generation and the wake types is also investigated, with pitching amplitude 15°–40°. With the pitching amplitude increases, the thrust and the efficiency monotonously reduce, and the thresholds for different wake types are also postponed.

Non-vertical flexible structures, such as inclined and curved cylinders, are encountered in deep-sea applications and exhibit more complex vortex-induced vibration (VIV) responses than vertical cylinders due to geometric non-uniformity and flow-curvature interaction. These complexities challenge both full-scale computational fluid dynamics (CFD) and reduced-order models, emphasizing the need for an accurate and efficient CFD-based approach. This study presents a meshless numerical methodology employing strip-theory-based discrete vortex method (SDVM) to simulate the VIV in non-vertical flexible cylinders. The full-scale flow is approximated by a sequence of flow strips, each resolved using SDVM without empirical hydrodynamic coefficients. Structural dynamics are modeled using a tensioned Bernoulli–Euler beam and solved in time domain. The methodology is validated against two benchmark experiments: an inclined flexible cylinder and a steel catenary riser (SCR) in uniform flow. Simulations verify the independence principle up to 60° inclination. For the SCR, a mixed standing-traveling wave pattern is observed, with downward energy transfer from the top. Additionally, a novel modal group switching behavior, characterized by abrupt shifts in the dominant frequency and amplitude, is also identified over a broad range of incoming flow velocities. The proposed methodology offers an efficient and accurate tool for simulating VIV in complex offshore structures, bridging the gap between simplified reduced-order models and full-scale CFD simulations.