The piston mode fluid resonance in the narrow gap between a moored floating body and a bottom-mounted vertical wall is numerically investigated based on a two-dimensional potential flow model and viscous numerical simulations. This study focuses on understanding the effect of mooring stiffness on the coupling dynamics of the gap resonance and the sway or heave motion of the floating body in regular waves. Numerical studies show that the resonant wave amplitude in the gap is reduced by the sway and heave motions. The reduction is highly dependent on the mooring stiffness. Two resonant frequencies are confirmed, and both increase with the mooring stiffness. Different modes of motions are identified in terms of the phase difference between the oscillatory motions of the gap flow and the floating body. Higher harmonic components of responses are found for the specific mooring stiffness. The performance of potential flow models in predicting resonant responses is revisited based on the understanding that the overall damping effect consists of two parts: (1) radiation damping and (2) viscous dissipation. It is confirmed that a potential model is also able to produce reasonable predictions as radiation damping plays a dominant role, for example, at the second resonant frequencies of coupling the gap resonance with the sway motion. Otherwise, as viscous dissipation dominates radiation damping, noticeable over-predictions by a potential model occur as recognized before, for example, the present results at the second peak response of gap resonance with the heave motion. The relative viscous dissipation is quantified with the reflection coefficient of viscous numerical results, while the radiation damping is quantified based on a specially designed radiation potential model with inputs of viscous numerical solutions.

The cost of wave energy converters (WECs) can be reduced significantly by integrating WECs into other marine facilities, especially in sea areas with a mild wave climate. To reduce the cost and increase the efficiency, a hybrid WEC system, comprising a linear array (medium farm) of oscillating buoy-type WECs attached to the weather side of a fixed-type floating pontoon as the base structure is proposed. The performance of the WEC array is investigated numerically using a boundary element method (BEM) based on the linear potential flow theory. The linear power take-off (PTO) damping model is used to calculate the output power of the WEC array. The performance of the WEC array and each individual WEC device is balanced by using the mean interaction factor and the individual interaction factor. To quantify the effect of the pontoon, the hydrodynamic results of the WEC arrays with and without a pontoon are compared with each other. Detailed investigations on the influence of the structural and PTO parameters are performed in a wide wave frequency range. Results show that the energy conversion efficiency of a WEC array with a properly designed pontoon is much higher than that without a pontoon. This integration scheme can achieve the efficiency improvement and construction-cost reduction of the wave energy converters.

Modal decomposition techniques, flow field, and spectral analysis are employed to investigate the wake dynamics and destabilization mechanisms of a four-bladed marine propeller with or without a nozzle. Numerical simulations are conducted using the delayed detached eddy simulation model for the wake and the arbitrary mesh interface method for the blade rotation. The presence of the nozzle significantly reduces the wake's streamwise velocity, delays the wake destabilization, increases the wake length, and changes the morphologies of wake vortices. In particular, the hub vortex in the ducted propeller wake is broken down into chaotic turbulence by the perturbation of the backflow. Two modal decomposition methods, namely, proper orthogonal decomposition and dynamic mode decomposition, are used to decompose the vorticity magnitude in the rotor wake field. From modal analysis, the spatial scale of flow phenomena decreases with the increase in modal frequency. Underlying destabilization mechanisms in the wake correspond to some characteristic frequencies. The interaction of each sheet vortex with the previously shed tip (leakage) vortices occurs at blade passing frequency (BPF). The pairing of adjacent tip (leakage) vortices occurs at half-BPF. The long-wave instability of the hub vortex and the wake meandering are stochastic processes, each of which occurs at a frequency lower or equal to shaft frequency. These four destabilization mechanisms can approximately reconstruct the large-scale flow phenomena in the wake. Moreover, each sheet vortex's alternating connection and disconnection with the previously shed tip (leakage) vortices cause the short-wave instability of the tip (leakage) vortices and generate the secondary vortices. The radial expansion motion of large-scale helical vortices in the outer slipstream dominates the wake meandering phenomenon.

Two-degree-of-freedom Vortex-Induced Vibration (VIV) of two rigidly coupled circular cylinders of different diameters at a low Reynolds number of 250 is investigated numerically. While the diameter ratio and the mass ratio are kept constant, the study is focused on the effect of the position angle of the small cylinder on the lock-in regime of the VIV. Simulations are carried out for position angles α of the small cylinder ranging from 0° to 180° with an interval of 22.5° and the reduced velocities ranging from 1 to 15 with an increment of 1. In order to find the effect of the gap between the two cylinders on the vibration, two gap-to-diameter ratios (0 and 0.2) are considered. It is found that compared with a single cylinder case, the lock-in regime of the reduced velocity is widened significantly when the position angle of the small cylinder is α = 0°, 22.5°, 90°, or 112.5°. Pulsed beating phenomenon characterized by regular vibration with occasional high-amplitude disturbances at regular or irregular intervals is observed at G = 0 and α = 90°. At α = 135°, more than one lock-in regimes are observed in the computed range of reduced velocity for both gaps (G = 0 and 0.2). Setting a small gap (gap-to-diameter ratio of 0.2) between the two cylinders mitigates the vibration by narrowing the lock-in regime and reducing the vibration amplitude.