Abbas Khayyer

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.

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.

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.

Space potential particles to enhance the stability of projection-based particle methods

Particle methods have been seldom verified by a Karman vortex simulation, which is commonly performed as a typical benchmark in computational fluid dynamics. This is mainly due to a difficulty in suppression of occurrence of unphysical voids manifested usually in a strong vortex on account of definition of free surface by the Lagrangian tracking framework with inconsistency in volume conservation. This paper presents a simple and effective scheme as a free-surface boundary condition of projection-based particle methods, namely the MPS (moving particle semi-implicit) and Incompressible SPH (ISPH) methods to handle the free surface with consistency in volume conservation. The new scheme is introduced into the Poisson pressure equation (PPE) with consideration of a potential in void space as space potential particle (SPP), to reproduce physical motions of particles around free surface through a particle–void interaction. The enhancing effect of the newly proposed SPP scheme is shown by simulating a few numerical tests, including a whirling water flow, a two-phase surfacing flow, and a set of Karman vortex simulations.

On the state-of-the-art of particle methods for coastal and ocean engineering

ABSTRACT The article aims at providing an up-to-date review on several latest advancements related to particle methods with applications in coastal and ocean engineering. The latest advancements corresponding to accuracy, stability, conservation properties, multiphase multi-physics multi-scale simulations, fluid-structure interactions, exclusive coastal/ocean engineering applications and computational efficiency are reviewed. The future perspectives for further enhancement of applicability and reliability of particle methods for coastal/ocean engineering applications are also highlighted.

Corrected First-Order Derivative ISPH in Water Wave Simulations

The smoothed particle hydrodynamics (SPH) method is a meshless numerical modeling technique. It has been applied in many different research fields in coastal engineering. Due to the drawback of its kernel approximation, however, the accuracy of SPH simulation results still needs to be improved in the prediction of violent wave impact. This paper compares several different forms of correction on the first-order derivative of ISPH formulation aiming to find one optimum kernel approximation. Based on four benchmark case analysis, we explored different kernel corrections and compared their accuracies. Furthermore, we applied them to one solitary wave and two dam-break flows with violent wave impact. The recommended method has been found to achieve much more promising results as compared with experimental data and other numerical approaches.

Numerical Investigation of the Morphological Dynamics of a Step-and-Pool Riverbed Using DEM-MPS

AbstractIn mountain streams, riverbeds with a series of coarse gravel steps are common. The sequence flow pattern with a chute and pool is usually formed on series of steps, where a transition regi...

An MPS-based particle method for simulation of multiphase flows characterized by high density ratios by incorporation of space potential particle concept

Abstract Simulation of multiphase flows characterized by high density ratios is targeted by using a refined particle method. The proposed method is founded on Moving Particle Semi-implicit (MPS) method which is a projection-based particle method. The proposed method comprises of a decoupled two-step computational algorithm through application of a consistent scheme for coupling the light/heavy phases by incorporating the concept of Space Potential Particles (SPP). The proposed coupling scheme guarantees the continuity of pressure and space (volume conservation) at the phase interface without any need for commonly applied density smoothing/averaging schemes or application of numerical stabilizing terms. Verification of the proposed multiphase particle method is conducted through the simulations of four benchmark tests and the method is shown to possess acceptable accuracy and convergence properties.