16th International Conference on Composite Structures ICCS16 A. J. M. Ferreira (Editor) c  FEUP, Porto, 2011 VISCOELASTICITY EFFECT IN WATER-ENTRY PROBLEMS: COMPARISON BETWEEN EXPERIMENTAL AND SPH RESULTS R. Panciroli ∗ , S. Abrate † , G. Minak ∗ and A. Zucchelli ∗ ∗ DIEM, Department of Mechanical Engineering, Alma Mater Studiorum - Università di Bologna, Italy e-mail: riccardo.panciroli@unibo.it, giangiacomo.minak@unibo.it, a.zucchelli@unibo.it web page: http://www.ing.unibo.it † Department of Mechanical Engineering & Energy Processes Southern Illinois University, Carbondale, IL 62901, USA e-mail: abrate@engr.siu.edu Key words: Viscoelasticity, SPH, Experimental mechanics, Fluid/Structure interaction. Summary. Since Wagner developed the first analytical solution to water-entry problems [1], much effort has been spent on this topic. Literature is plenty of original works, and many of those are very effective when dealing with the water entry of simple-shaped structures impact- ing the surface with pure vertical velocity. Some models are even capable to account for the so called hydroelasticity caused by the deflection of the impacting object that may lead to changes in the fluid response and consequently in the overall impact dynamics [2, 3]. However these analytical models are limited to the analysis of simple-shaped object impacting vertically onto a free and initially calm surface. To be able to study more realistic situations, many numerical methods capable to couple the fluid dynamics with the structural response have been used [4, 5, 6]. Others tried to develop fully-coupled numerical solutions, which carries the advantage to solve fluid and solid domain at a time. Smoothed Particle Hydrodynamics (SHP) is one of these techniques; one of its major advantages is the ease of treating fluids presenting a free surface, together with the possibility to interact with FEM models, while its major limitation is the very expensive computational time, which limits its application to small bidimensional models. This study investigates hydrody- namic impacts by means of experiments and numerical SPH simulations, mainly focusing on the real applicability of the SPH method to study hydroelastic phenomena. While fluid/structure interaction problems have been widely studied in literature by means of SPH models [7, 8, 9], many article show inaccurate results and lack of details, up to the point that this technique can be considered not fully suitable for this purpose. In this research we initially validate the most used models to implement the water properties (e.g. Polynomial and Gruneisen), analysing their capability to reproduce the real water behaviour. We later investi- gate the water-entry problems of rigid bodies, whose results are compared with Von Karman and Wagner’s analytical solutions, showing that SPH is actually capable to correctly simulate the impact dynamics. However, the accuracy of the solution in terms of pressure at the fluid- solid interface was found to be highly influenced by various parameters, like element size and