Towards a Pragmatic Method for Prediction of Whipping: Wedge Impact Simulations using OpenFOAM Neil R. Southall 1 , Yongwon Lee 1 , Michael C. Johnson 1 , Spyros E. Hirdaris 2 and Nigel J. White 1 1 Structural Analysis and Hydrodynamics, Global Technology Centre, Lloyd’s Register, UK 2 Technology Centre Korea, Korea, Lloyd’s Register Asia ABSTRACT This paper presents an approach for the prediction of impact loads using the open-source Computational Fluid Dynamics (CFD) code OpenFOAM. Computational results displaying the time history of impact pressures and forces for drop tests of two dimensional wedges for 20 degree and 30 degree dead rise angles and varying tilt angles are compared against experimental results from WILS JIP-III. An incompressible Volume of Fluid (VOF) computational scheme is used to capture the free surface effects. The key numerical idealisation factors of the method employed are identified and the steps taken to overcome those are described. Preliminary results using a compressible flow solver are also presented and initial investigations into the sensitivity of the results to mesh density and domain size are described. KEY WORDS: Slamming; Computational Fluid Dynamics (CFD), Volume of Fluid (VOF) method; OpenFOAM. INTRODUCTION In recent years, the increasing demand for higher container carrying capacity has resulted in the rapid growth of container ship sizes. It is believed that the combination of relatively high speed and high length to beam aspect ratios of these open deck ships may result in whipping and springing becoming significant factors for the prediction of wave induced dynamic loads (Hirdaris et al., 2014). Whipping, defined as the vibrational response of the hull girder due to wave impact loads, may augment the extreme wave bending moments and shear forces (Lee et al., 2012). Hence, understanding of impact fluid flow phenomena and the development of practical fluid structure interaction methods for the prediction of the incident wave induced slamming loads on symmetric and antisymmetric ship dynamic response is considered to be an important research area within the context of rationally based ship design. Hull slamming occurs when the forefoot of a ship rises above the water surface and then submerges again with high vertical velocity. Impulsive pressure loads act on the hull, introducing dynamic excitation to the structure due to the large force applied in the order of milliseconds. Technology availability, theoretical knowledge and uncertainty quantification associated with the derivation and validation of impact induced loads is still developing. Model experiments are still considered to be the most reliable method for the prediction of slamming loads. However, they may be expensive and time consuming. On the other hand, with the increasing capacity of computational power advanced numerical methods (e.g. boundary elements or CFD), could suggest an alternative, as well as offering the significant advantage of providing detailed insight into impact flow phenomena. The third iteration of the WILS (Wave Induced Loads on Ships) Joint Industry Project was instigated with the objective to provide reliable model test data on wave induced impact loads of container ships for validation of commercial numerical codes and Classification Rule- based local slamming loads (e.g. Lloyd's Register, 2013). The first phase of experiments consisted of drop tests of wedge-shaped forms designed to provide a matrix of impact pressures and forces at a range of incidence angles, both for sensor validation and to provide data for validation of numerical results. Based on 2D wedge experiments carried out under WILS JIP-III proceedings at the facilities of the Korean Maritime and Ocean Engineering Research Institute MOERI (2013) this paper presents comparisons against the measured impact loads using the open-source Computational Fluid Dynamics code OpenFOAM. The aim of the study is to develop a pragmatic fluid structure interaction approach for the prediction of impact pressures and forces. As part of a wider research programme to develop a coupled time-domain ship motions and slamming method where minimizing runtime is a primary requirement, an initial investigation was made to optimise the solution in terms of computational performance. In the results presented both mesh density and the bounds of the numerical model are pushed beyond what would normally be considered good practice. This helped to establish the influence and credibility of key numerical idealisation factors. LITERATURE REVIEW The classic papers in the subject area of impact include von Karman (1929), Wagner (1932) and Stavovy and Chuang (1970) and these remain in widespread use for cases where severe impact loads do not introduce large deformations that modify the fluid motion. More recently, Korobkin et al. (2011) showed the evolution of the wetted body area in time is an important characteristic of the impact, which may influence the magnitude of the loads. Panciroli et al. (2011) found that the impact load during water entry of deformable wedges shows marked oscillations due to the mutual interaction between the structural deformations and the fluid flow. 806 Proceedings of the Twenty-fourth (2014) International Ocean and Polar Engineering Conference Busan, Korea, June 15-20, 2014 Copyright © 2014 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1 880653 91-3 (Set); ISSN 1098-6189 (Set) www.isope.org