Catamaran wetdeck slamming : a numerical and experimental investigation

High-speed catamarans have, over the past two decades, extended their service areas from protected waters to the open ocean where impacts with waves can result in structural damage. The work detailed in this thesis investigates the hydrodynamic loads experienced by wave-piercer catamarans during water impacts using a combination of experimental and numerical techniques. This work is aimed at addressing the lack of high-quality three-dimensional (3D) experimental data suitable for benchmarking catamaran vessels impacting with water in a 3D regime, as well as establishing an understanding of the key elements influencing the severity of wetdeck slamming loads. It also aims to evaluate the accuracy of numerical techniques by utilising Computational Fluid Dynamics (CFD) simulations to predict the magnitude of wetdeck slamming forces and pressure distributions, thus allowing ship designers to improve catamaran hull designs. A quasi two-dimensional (quasi-2D) simulation of a wedge shaped hullform impacting with water was validated against existing free-fall experimental data and compared to previously published numerical simulations using Smoothed Particle Hydrodynamics (SPH), with the CFD results showing better agreement with the experimental data than the SPH predictions. CFD simulations were then used to investigate the behaviour of a quasi-2D catamaran hull section with a centrebow during water-entry. The computed 2D vertical acceleration and slamming pressures are comparable to previously published drop test experimental data. With the lack of existing 3D water-impact experimental validation data for wave-piercer catamaran hullforms, two series of water-impact experiments were performed to investigate the hydrodynamic loads experienced by a generic wave-piercer catamaran hullform with two interchangeable centrebow sections during water impacts. The experiments, which focused on the characterisation of the unsteady slam loads on an arched wetdeck, were conducted using a Servo-hydraulic Slam Testing System (SSTS) allowing the model to penetrate a body of water at a range of constant speeds and two trim angles. The systematic and random uncertainties associated with the controlled speed test results are quantified in detail. These experiments therefore provide a new dataset for the slam pressure distributions and forces on the arched wetdeck structure of catamaran vessels. Strong relationships between slam force peaks and impact velocity are observed as a function of relative impact angle and centrebow geometry, with a possible reduction for a newly-developed centrebow. The three dimensionality of the water flow in these slam test events is characterised. It was also found that the limited pressure measurements along the archway were not representative of wetdeck slamming loads. High localised pressure is affected by jet formation or localised flow effects. Total slamming load is governed by the relative impact velocity and the rate of change of added mass and is not necessarily strongly related to localised pressure distributions. The 3D CFD simulations provide information on the different techniques and settings required to accurately model such unsteady events. The CFD simulations were able to accurately characterise 3D wetdeck slamming loads of catamaran vessels and quantify the splitting force (i.e. the component of slamming force that mainly acts on demihulls and centrebow in the transverse direction) that occurs concurrently with the wetdeck slamming event.