Experimental And Numerical Investigation Of Fluid Resonance In Narrow Gap Between Multiple Maritime Structures

Narrow gaps between marine structures can be found in such as the very large floating structures and the multi-floating structures. Under some certain circumstances, the incident waves with specific frequency may induce severe resonant oscillation of water column in the narrow gap, which is known as the gap resonance problem. Meanwhile, the floating structures may suffer significantly increasing wave forces, which is a serious threat to the safety of the structures. To study the characteristics and mechanism of fluid resonance in narrow gap between the marine structures, a series of laboratory tests and numerical analysis were carried out in this work.It was known that the conventional potential flow model significantly over-estimates the resonant wave amplitudes in the narrow gap due to the ignorance of mechanical energy dissipation. The viscous flow model is able to produce predictions as accurate as model tests in terms of both resonant frequency and resonant amplitude. However, the CFD simulations were also shown rather time-consuming. By introducing appropriate artificial damping effects, modifying the free surface boundary condition and adding artificial energy dissipation domain in fluid, the numerical predictions of resonant wave amplitude by potential flow model can be improved. The two-dimensional physical tests were conducted by considering both box-box and box-wall with narrow gap and subject to linear incident water waves. Experimental results show that when the incident wave frequency is near the resonant frequency, a certain dependency between the wave height in narrow gap and the incident wave height can be found. It was found that the variation of resonant wave height with incident wave height can be approximated by a power function, which implies that the change of incident wave height leads to the changes of mechanical energy dissipation. According to the results of model tests, the dependency between resonant wave height and incident wave height is under the influence of the geometric parameters of the models and the set-up.To study the physical energy dissipation from the vortex caused by structure angle in gap resonance, laboratory tests were conducted by considering twin round-corner boxes with a narrow gap. And the characteristics of fluid resonance in narrow gap at various radiuses of the corners were observed. The results illustrate that when the box corner changes from the rectangular shape to round, the resonant frequency almost remains but the resonant wave height increases significantly, which means the angles of the square box have some important influences on the physical dissipation. As the corner radius increases, the resonant frequency keeps increasing accordingly, while the resonant wave height decreases but still significantly higher than the case of rectangular corners. To some extent, it reveals the mechanism in how the pointed corners of the structures influence physical dissipation in gap resonance. The change of resonant frequency is associated with the variation of fluid mass in narrow gap. With the increase of the radius of the round corners, the resonant frequency rises since fluid mass in narrow gap decreases. In addition, the dependency between resonant wave height and incident wave height will change when the shape of box corners varies.In this work, laboratory tests were conducted by considering a simplified physical model of a fixed floating rectangular box in front of a vertical wall with narrow gap and subject to linear incident water waves. The influence of box draft, gap width and incident wave height and frequency on the hydrodynamics of fluid resonance in gap was investigated. The experimental results can be used for the validation of numerical models and the calibration of damping coefficients. Moreover, mechanical energy dissipation in the flow was studied through the analysis of variation of hydrodynamic coefficients with the incident wave frequency and height. The maximum resonant wave height was observed to reach even up to more than7times of the incident wave height. The reflection coefficient in the box-wall system shows complex and non-linear dependence on the body draft, gap width and incident wave height, which sheds insights on mechanical energy dissipation in the resonant response.Aiming for numerical solutions of the problem, the governing equation established within the framework of the potential flow theory was respectively solved by the method of matched eigenfunction expansions and boundary element method (BEM) combining with the boundary conditions. To begin with, according to the results of the physical model tests, the damping (resistance) coefficients of numerical models were calibrated in this paper. Then, the numerical models were employed to analysis the dependency between characteristics of fluid resonance and the geometric parameters of the models in detail. Also, the wave forces on the structures were studied. Numerical results suggest that under the fluid resonance in the narrow gap between twin round-corner boxes, the non-dimensional horizontal wave force is as high as2.5. In box-wall system subject to water waves, it shows the resonant frequency decreases with the increase in body draft, box breadth and gap width. Near the resonant frequency, the non-dimensional horizontal wave force on the structures is as high as4, and increases when the box draft deepens.