Numerical investigations on water entry and/or exit problems using a multi-resolution Delta-plus-SPH model with TIC

Abstract

As a classical Fluid-Structure Interaction (FSI) problem, water entry and/or exit of marine structures involve large deformations of free-surface and splashing, which have long been a great challenge for numerical modeling. Smoothed Particle Hydrodynamics (SPH) is a meshless Lagrangian particle method that has natural advantages in dealing with the non-linear variation of free-surface and moving interfaces. However, the characteristics of weakly-compressible SPH itself may lead to deficiencies such as pressure noise, tensile instability, heterogeneous particle distributions and computational inefficiency. Therefore, in this work, an acoustic damper term is formulated in the momentum equation to eliminate the amount of acoustic pressure waves induced by liquid impacts. Meanwhile, the Adaptive Particle Refinement (APR) is applied to obtain sufficiently fine particle resolution and reduce the computational cost. For the tensile instability issues induced by negative pressures, especially at the water exit stage, the Tensile Instability Control (TIC) is adopted into the SPH simulations to suppress its adverse impact. Furthermore, the Particle Shifting Technique (PST) is beneficial to the two schemes above, and it is also employed in this study. That is to say, PST can effectively solve the problem of particle disorder, which increases the accuracy and robustness of APR and reduces the momentum conservation error caused by TIC. Finally, we propose an improved Free-Surface Detection Method (FSDM) to allow the flow separation from the structure surface at the water entry stage that is impacted by PST. The fairly good agreements between the numerical results and the experimental data indicate the present SPH model can be treated as a reliable numerical tool for accurately solving such problems of structures penetrating fluid interfaces.