A two-dimensional numerical and experimental study of piston and sloshing resonance in moonpools with recess

Abstract

The piston and first sloshing modes of two-dimensional moonpools with recess are investigated. Dedicated forced heave experiments are carried out. Different recess lengths are tested from $1/4$ to $1/2$ of the length of the moonpool at the mean waterline. A theoretical model to calculate the natural frequencies is developed based on linearized potential flow theory and eigenfunction expansion. Two numerical methods are implemented: a boundary element method (BEM) and a Navier–Stokes solver (CFD). Both the BEM and CFD have linearized free-surface and body-boundary conditions. As expected, the BEM over-predicts the moonpool response significantly, in particular at the first sloshing mode. The CFD is in general able to predict the maximum moonpool response adequately, both at the piston and first sloshing modes. Both numerical methods fail to predict the Duffing-type behaviour at the first sloshing mode, due to the linearized free-surface conditions. The Duffing behaviour is more pronounced for the largest recess. The main source of damping in the proximity of the first sloshing mode is discussed.

An experimental and numerical study of the resonant flow between a hull and a wall

“…As such, two-dimensional Navier-Stokes computations are more practical to study the problem and determine the flow field over the entire wave tank as demonstrated by Moradi, Zhou & Cheng (2015) and Lu et al (2020), for instance. This also avoids possible issues related to the coupling of viscous solvers to capture flow separation with inviscid solutions as pursued by Kristiansen & Faltinsen (2012) and Ravinthrakumar et al (2019). Notably, Lu et al (2020) considered a barge heaving and swaying next to a vertical wall and presented vorticity contours to support the conclusion that vortex shedding around the square bilge dominated the damping of the gap response.…”

“…The problem has often been simplified by generating idealised two-dimensional flow conditions in a wave tank. Kristiansen & Faltinsen (2008) and Ravinthrakumar et al (2019), for instance, used the forced heave response of two rigidly connected hull sections separated by a small gap to study the moonpool problem. The terminal and side-by-side problems have been investigated using fixed hulls positioned either close to a wall (Kristiansen & Faltinsen 2009) or separated by a small gap (Tan et al 2019), subjected to surface waves.…”

Time domain modelling of a Helmholtz resonator analogue for water waves

“…We can notice that compressed air breakwaters related to OWC have been also envisioned (Linton & Evans 1990). More recently, studies have been conducted on vertical openings through ship hulls, called moonpools (Molin 2001;Ravinthrakumar et al 2019), and gaps in between two ships side by side in offloading operations (Molin et al 2018;Zhao et al 2020). In these configurations, the lowest resonance corresponds to a piston mode associated with a velocity potential varying linearly along the vertical direction in the device, be it the gap or the moonpool.…”

Mooring Analysis of a Floating OWC Wave Energy Converter

“…Resonance can occur in various modes, including piston and sloshing types, both of which have been seen to have some influence on the heave and pitch motions of structures with moonpools [18]. A comparison of numerical and experimental results indicates that potential flow solvers over-estimate the magnitude of the internal free-surface oscillations, as the damping is controlled by flow separation at the sharp corners of the moonpool inlet [19,20]. Most commercial potential flow codes allow for the inclusion of a thin-surface object within the moonpool, known as a lid, with a controllable damping coefficient to mitigate the oscillations of the free surface within the moonpool.…”