The boundary conditions represented by polygons in moving particle semi-implicit (MPS) method (Koshizuka and Oka, Nuclear Science and Engineering, 1996) have been widely used in the industry simulations since it can simply simulate complex geometry with high efficiency. However, the inaccurate particle number density near non-planar wall boundaries dramatically affects the accuracy of simulations. In this paper, we propose an initial boundary particle arrangement technique coupled with the wall weight function method (Zhang et al. Transaction of JSCES, 2015) to improve the particle number density near slopes and curved surfaces with boundary conditions represented by polygons in three dimensions. Two uniform grids are utilized in the proposed technique. The grid points in the first uniform grid are used to construct boundary particles, and the second uniform grid stores the same information as in the work by Zhang et al. The wall weight functions of the grid points in the second uniform grid are calculated by newly constructed boundary particles. The wall weight functions of the fluid particles are interpolated from the values stored on the grid points in the second uniform grid. Because boundary particles are located on the polygons, complex geometries can be accurately represented. The proposed method can dramatically improve the particle number density and maintain the high efficiency. The performance of the previously proposed wall weight function (Zhang et al.) with the boundary particle arrangement technique is verified in comparison with the wall weight function without boundary particle arrangement by investigating two example geometries. The simulations of a water tank with a wedge and a complex geometry show the general applicability of the boundary particle arrangement technique to complex geometries and demonstrate its improvement of the wall weight function near the slopes and curved surfaces.

Surface tension is a dominant force in gas-liquid flows within micro/nano spaces, such as gas-liquid flows of micro-electro-mechanical systems, nano-in-print processes, and resin coating processes. We developed a surface tension model for a particle method to simulate gas-liquid flows within micro/nano spaces; the inter-particle force between particles was defined to simulate interactions at gas-liquid and liquid-solid interfaces. The inter-particle force was given by the derivation of potential energy and was related to the surface tension by Bakker's equation. The surface tension was modeled as volume force in the Navier-Stokes equation, and the volume force was changed in proportion to the curvatures of interfaces. The surface tension model was verified for pressure in a drop, oscillation of a square drop, and wall adhesion. The predicted pressure in a drop quantitatively agreed well with those given by the Young-Laplace equation. The oscillation of a square drop was qualitatively simulated. In the wall-adhesion verification, effects of different contact angles on drop deformations were quantitatively simulated. We clarified that our new surface tension model was useful for simulating gas-liquid interfaces.

The fuel spray of an injector for automobile engines contains multiscale free surfaces: liquid films formed at the fuel-injector outlet, ligaments generated by liquid-film breakup, and droplets generated from the ligaments within the secondary-drop-breakup region. To simulate these multiscale free surfaces, we developed a method that combines two types of simulation. The liquid-film breakup near the injector outlet was simulated by using a particle method, and the secondary-drop breakup after the liquid-film breakup was simulated by using a discrete droplet model (DDM). The injection conditions of DDM were the distributions of droplet diameters and velocities calculated in the liquid-film-breakup simulation. We applied our method to simulate the spray from a collision-type fuel injector. The simulated liquid-film breakup near the injector outlet and behavior of the secondary-drop breakup qualitatively agreed with measurements. Furthermore, the errors of the mean droplet diameters between the simulations and the measurements were less than 12%. This shows that our method is effective for fuel spray simulation.

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