Simulation on mass transfer at immiscible liquid interface entrained by single bubble using particle method

Numerical simulation of the SURC-2 and SURC-4 MCCI experiments by MPS method

Core melting and molten migration behavior are hot and difficult issues in the field of nuclear reactor severe accident research. The Moving Particle Semi-implicit (MPS) meshless method has potential to simulate free-surface and multiphase flows. In this study, the MPS method was utilized to simulate the melting process of UO2-Zr rod-type fuel elements. The models of heat conduction with phase change, simplified UO2-Zr eutectic reaction, viscous flow and surface tension were implemented with the framework of standard MPS method. Then, the improved MPS code was used to simulate and analyze the process of high-temperature melting and characteristics of molten migration and solidification in the coolant channel, aiming at revealing the severe accidents for light water reactors (LWR), particularly the early core damage. The results showed that compared with the case of higher initial temperature, when the initial temperature of molten UO2 is lower, more molten UO2 will solidify on the surface of rod cluster, and the blockage of upper flow channel caused by molten UO2 is more serious. In addition, this study also demonstrated the potential of the MPS method for the study of complicated severe accident phenomena in not only traditional LWR but also advanced nuclear reactors in the future.

This paper proposes a control method for swarm robots to avoid collisions via fluid dynamics. MPS (Moving Particle Semi-implicit) method, which is a type of particle method, is used to reflect fluid dynamics to swarm robots. To adapt the MPS method to swarm robots, we consider distributed calculation of the MPS method. Specifically, the simultaneous equations for pressure calculation in the MPS method are solved by DCG (Distributed Conjugate Gradient) method. Since the DCG method is a distributed iterative solution method, the trade-off between computational complexity and communication traffi is important. We cluster swarm robots to reduce the total communication traffic, taking into account the relationship between distributed calculation and communication traffic. Numerical experiments are conducted to demonstrate the effectiveness of fluid collision avoidance against various shaped obstacles.

Direct numerical simulation of incompressible multiphase flow with vaporization using moving particle semi-implicit method

In the postulated severe accident of a light water reactor, the molten core material (corium) may be released to the reactor containment floor and the Molten Core - Concrete Interaction (MCCI) may take place. This complex phenomena may involve dehydration, thermal decomposition, and ablation of concrete, redox reactions of oxidic and metallic phases of corium, solidification and remelting of corium, mixing of molten concrete with corium and so on. Evaluation of the resulting concrete floor ablation is important as it may threaten the containment boundary. The Moving Particle Semi-implicit (MPS) method discretizes fluids and solids with calculation points (particles) in a Lagrangian manner and has been extensively developed for analyses of various MCCI tests with consideration of crust formation at the melt-concrete interface. Due to its Lagrangian nature, there is always some randomness to each particle position and Particle Shifting (PS) is one of the numerical techniques, which corrects such randomness by slightly shifting the particle position from a particle dense region to a coarse region to sustain numerical accuracy of the MPS method. Such slight correction does not usually have any physical impacts on the calculation unless the particle velocity becomes extremely small and duration of the phenomena becomes extremely long. In this study, a new crust model is being developed to analyze MCCI, which involves continuous concrete ablation with presence of the crust layer between the corium and the concrete walls, which may gradually move with the slow concrete wall ablation process over long time. The new crust model must enable accumulation of physical displacement of the crust particle over long time (i.e., enable physical creeping) while preventing accumulation of numerical displacement of the crust particles over long time (i.e., preventing numerical creeping), Hence, in the new crust model, the PS has been effectively disabled for the crust particles. Qualitative validity of such numerical modeling was confirmed through some trial analyses of VULCANO-VBS test using a set of tentative calculation conditions and parameters, which should be carefully revised for future quantitative discussions including validation of the analysis results with experimental results.

Moving Particle Semi-implicit (MPS) method is a particle method which has been developed to analyze incompressible flows with free surfaces. Fluids are represented by particles which move in fully Lagrangian description. Since grids are not necessary, complex motion of free surfaces can be calculated without grid tangling or numerical diffusion. In the MPS method, a differential equation system is transformed to particle dynamics using the models representing differential operators. Incompressibility is solved by the Poisson equation of pressure to keep the particle number density constant. Solid mechanics is also modeled by particles and fluid-structure interaction can be solved. Calculation examples are provided in this paper: collapse of a water column, melt droplet fragmentation, sloshing with tank deformation, nucleate boiling and droplet breakup. These examples show that the MPS method is a powerful tool to investigate complex behaviors of free surfaces and multiphase flows.

Lagrangian multiphase modeling of sand discharge into still water

Coupling of rigid body dynamics and moving particle semi-implicit method for simulating isothermal multi-phase fluid interactions

Comparison of MPS and SPH methods for solving forced motion ship flooding problems

Comparison investigations of numerical simulations of incompressible viscous flows by the SPH (smoothed particle hydrodynamics) and the MPS (moving particle semi-implicit) are presented. A dam-break problem is chosen as the test case. In the calculation with the SPH method, weakly compressible model is used, i.e. WCSPH, which describes water as a nearly incompressible fluid, while in MPS method, the pressure Poisson equation is introduced to keep the density of fluid to be constant. The numerical results show that the two particle methods are robust and flexible, numerical results qualitatively agree with the experimental data. It can be seen that both the SPH method and the MPS method can be easily applied to the complex free surface flow problems.

Moving Particle Semi-implicit (MPS) method uses particles and their interactions to simulate incompressible flow and it is a promising meshless method for multiphase flow simulation. In order to simulate the micro-bubble generation in micro-channel, surface tension model in MPS is improved by introducing fourth order central difference scheme for the calculation of unit normal vector. Numerical results for the oscillation of macro and micro droplets are in good agreement with theoretical prediction, which confirmed the validation of our model. By introducing the improved surface tension model into MPS method, the micro-bubble generation in T-shaped micro-channel was simulated successfully. The reasonable agreement between numerical simulations with visualization experiment confirmed the capacity of MPS with the improved surface tension model for the microgravity or micro-scale two-phase flow, which is dominated by surface tension effect. Finally, micro-bubble generations in different micro-channels are simulated. It is found that bubble size will decrease with increasing liquid flow rate and increase with increasing gas flow rate. Compared with 45° bifurcation micro-channel, T-shaped micro-channel can generate bubble smaller and faster.

When a liquid droplet impacts on a solid surface, it recoils to the center of that surface after reaching its maximum spreading diameter. The mechanism of droplet recoiling is not fully understood. To simulate this recoiling of a droplet, a particle method is a good choice because it does not require grids for simulating fluid motions, and can easily handle a large deformation of fluid. In this study, the coupled method of rigid body dynamics and the moving particle semi-implicit (MPS) method (Park and Jeun, 2011) was used to calculate three-dimensional droplet impingement. Also, the previous surface tension model for MPS (Nomura et al., 2001) was revised to get a more realistic surface tension force. A two-step calculation was performed. In the first step, a MPS calculation was performed with particles that were considered to have no mass or volume. In the second step, rigid body dynamics came into the calculation and considered the diameters of particles being slightly lesser than the initial distance between particles. In this study, the calculated results were compared with the measured data (Kim and Chun, 2000) and the recoiling lengths of droplets for the various initial impingement speeds were estimated.

A 6M digital twin for modeling and simulation in subsurface reservoirs

Investigation on influence of crust formation on VULCANO VE-U7 corium spreading with MPS method

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