Hitoshi Gotoh

Enhancement of stability and accuracy of the moving particle semi-implicit method

As a Lagrangian gridless particle method, the MPS (Moving Particle Semi-implicit) method has been proven useful in a wide range of engineering applications. Up to now, most of MPS applications have been limited to problems with compressive stress states. This paper investigates the performance and stability of MPS method in simulation of more general hydrodynamic problems, including those characterized by tensile stresses or by changes in the stress states. It is shown that MPS-based simulations are prone to become destabilized in presence of attractive interparticle forces. This instability appears to be similar to the so-called tensile instability in SPH method. Two new modifications, namely, a modified Poisson Pressure Equation and a corrective matrix for pressure gradient model, are proposed to resolve this shortcoming. These two new modifications together with two previously proposed improvements are shown to stabilize and enhance the performance of MPS method.

SPH-LES MODEL FOR NUMERICAL INVESTIGATION OF WAVE INTERACTION WITH PARTIALLY IMMERSED BREAKWATER

The reflection and transmission characteristics of regular waves by a partially immersed curtain-type breakwater have been studied by the experiment and numerical model in the paper. Non-overtopping and overtopping of the breakwater by the incident wave were considered to compare different wave dissipation efficiencies. An incompressible Smoothed Particle Hydrodynamics (SPH) theory combined with a Large Eddy Simulation (LES) model was employed as the numerical tool. The SPH method is robust for tracking free surfaces without numerical diffusion and the LES model is capable of analyzing turbulence and eddy vortices during wave-breakwater interactions. A good agreement between computational and experimental wave profiles verifies the accuracy of the SPH-LES model. The computations also disclose that the wave energy dissipation is mainly attributed to the turbulence production and vortex shedding during the wave transmission and reflection processes.

Enhancement of performance and stability of MPS mesh-free particle method for multiphase flows characterized by high density ratios

The paper presents an enhanced stabilized MPS (Moving Particle Semi-implicit) method for simulation of multiphase flows characterized by high density ratios. The developed method benefits from four previously developed schemes [1] as well as a novel one proposed for accurate, consistent modeling of density at the phase interface. The new scheme can be considered as an extended version of a commonly applied density smoothening scheme and is shown to keep the sharpness of spatial density variations while enhancing the stability and performance of simulations. Further, the paper highlights the importance of applying a Taylor series consistent scheme for calculation of pressure gradient in multiphase MPS-based simulations. By presenting a simple perturbation analysis, it is shown that some commonly applied MPS-based pressure gradient models are prone to increase the level of unphysical perturbations at the phase interface leading to numerical instabilities. The original MPS gradient model with a Gradient Correction [1] is shown to provide stable and accurate results even in case of violent multiphase flows characterized by high density ratios.

DEVELOPMENT OF CMPS METHOD FOR ACCURATE WATER-SURFACE TRACKING IN BREAKING WAVES

A Corrected Moving Particle Semi-implicit (CMPS) method is proposed for the accurate tracking of water surface in breaking waves. The original formulations of standard MPS method are revisited from the view point of momentum conservation. Modifications and corrections are made to ensure the momentum conservation in a particle-based calculation of viscous incompressible free-surface flows. A simple numerical test demonstrates the excellent performance of the CMPS method in exact conservation of linear momentum and significantly enhanced preservation of angular momentum. The CMPS method is applied to the simulation of plunging breaking and post-breaking of solitary waves. Qualitative and quantitative comparisons with the experimental data confirm the high capability and precision of the CMPS method. A tensor-type strain-based viscosity is also proposed to further enhanced CMPS reproduction of a splash-up.

Turbulence particle models for tracking free surfaces

Two numerical particle models, the Smoothed Particle Hydrodynamics (SPH) and Moving Particle Semi-implicit (MPS) methods, coupled with a sub-particle scale (SPS) turbulence model, are presented to simulate free surface flows. Both SPH and MPS methods have the advantages in that the governing Navier-Stokes equations are solved by Lagrangian approach and no grid is needed in the computation. Thus the free surface can be easily and accurately tracked by particles without numerical diffusion. In this paper different particle interaction models for SPH and MPS methods are summarized and compared. The robustness of two models is validated through experimental data of a dam-break flow. In addition, a series of numerical runs are carried out to investigate the order of convergence of the models with regard to the time step and particle spacing. Finally the efficiency of the incorporated SPS model is further demonstrated by the computed turbulence patterns from a breaking wave. It is shown that both SPH and MPS models provide a useful tool for simulating free surface flows.

LAGRANGIAN PARTICLE METHOD FOR SIMULATION OF WAVE OVERTOPPING ON A VERTICAL SEAWALL

A particle method, or a gridless Lagrangian method, shows the high performance in describing the complicated behavior of water surface with the fragmentation and coalescence of water. In this paper, a wave overtopping process on a vertical seawall is numerically simulated on the basis of the Navier-Stokes equation with surface-tension term, which is discretized by the MPS (moving particle semi-implicit) method belonging to the category of the particle method. An improvement of the listing process of neighboring particle is introduced to reduce the computational load. Wave overtopping process in the experiments are well reproduced by the MPS method. The predictions of the MPS method of the overtopping volume agree well with the experimental results.

LAGRANGIAN SIMULATION OF BREAKING WAVES USING PARTICLE METHOD

A Lagrangian numerical simulation of breaking waves is performed by the moving particle semi-implicit (MPS) method, in which the Navier-Stokes equation is discritized based on the interaction of pa...

SIMULATING COUPLED MOTION OF PROGRESSIVE WAVE AND FLOATING CURTAIN WALL BY SPH-LES MODEL

The coupled motion between progressive wave and floating curtain-wall type breakwater is simulated by an incompressible Smoothed Particle Hydrodynamics (SPH) method combined with a Large Eddy Simulation (LES) model. The Naviers-Stokes equations in Lagrangian form are solved using a two-step split method. The method first integrates the velocity field in time without enforcing incompressibility. Then the resulting deviation of particle density is projected into a divergence-free space to satisfy incompressibility. The SPH method is convenient for describing free surfaces and moving boundaries of the floating body. The LES model is capable of dealing with turbulence during the wave-breakwater interactions. The translation and rotation of the floating curtain wall is calculated through an additional rigid-body-tracking subroutine. The computed wave profiles and hydrodynamic forces by the current model are in good agreement with those reported in the literature. Finally, the wave transmission and reflection characteristics are analyzed and compared with numerical results in which the breakwater is fixed.

On particle-based simulation of a dam break over a wet bed

This research presents particle-based simulations of a dam break on a wet bed by standard and improved versions of three particle methods, namely moving particle semi-implicit (MPS), incompressible smoothed particle hydrodynamics (ISPH) and weakly compressible smoothed particle hydrodynamics (WCSPH). The improved versions of these three methods are corrected MPS/ISPH (CMPS/CISPH) with a higher-order source term and WCSPH with a moving least square (MLS) density re-initialization. Direct comparisons of test photos and their corresponding simulation snapshots are made in terms of the reproduced free surface profile and simulated mixing processes. The work highlights potential capabilities of particle methods in reproducing detailed features of a dam break on a wet bed. A new viscosity reduction function is proposed for an efficient reproduction of backward breaking by WCSPH with an artificial viscosity term. The effect of bed friction is taken into account by applying a frictional drag force term usually applied in dissipative particle dynamics.

Comparative study on accuracy and conservation properties of two particle regularization schemes and proposal of an optimized particle shifting scheme in ISPH context

Abstract The paper provides a comparative investigation on accuracy and conservation properties of two particle regularization schemes, namely, the Dynamic Stabilization (DS) [1] and generalized Particle Shifting (PS) [2] schemes in simulations of both internal and free-surface flows in ISPH (Incompressible SPH) context. The paper also presents an Optimized PS (OPS) scheme for accurate and consistent implementation of particle shifting for free-surface flows. In contrast to PS, the OPS does not contain any tuning parameters for free-surface, consistently resulting in perfect elimination of shifting normal to an interface and resolves the unphysical discontinuity beneath the interface, seen in PS results.