Moving-particle Semi-implicit Method Researh And Application Based On Large Eddy Simulation

Modeling free surface flows is of great significance in the field of ship and ocean engineering. However, these flows are difficult to simulate because the surface boundary conditions are specified on arbitarity moving surface. Some methods in Eulerian grid are flexible and robust to trace free surface in simulation, but with those method, Navier-Stokes equations are solved on a fixed grid, which brings the problem of numerical diffusion because the existence of the advection term in Navier-Stokes equations, especially when the deformation of free surface is very large and complicated or the free surface splashes. Moving-Particle Semi-Implicit Method (MPS) is a new meshless numerical method which bases on Lagrange particle, due to the advantage in free surface simulation, the MPS method plays more and more important role in numerical research. Studies show that the MPS method has broad prospects in the area of Naval Architecture and Ocean Engineering. But there are some deficiencies in its theory to restrict the development of MPS method. This paper studies deficiencies of MPS theory, and proposes some technologies for these deficiencies. With these technologies, the MPS method is coupled with a sub-particle scale (SPS) turbulence model to simulate sloshing problems with large deforming free surface. This paper has the following content andinnovations:1. Additional correction term for particles collision or pass through phenomena. Because the Lagrange behavior of particles, there are lots of particles collision phenomena in MPS simulation, some of these collision phenomena will pass through the boundary and lead to a serious consequence, but with the additional correction term and appropriate distance for correction, there are only few particles collision phenomena in simulations, and the probability of pass through phenomena is few.2. Behavior of kernel function in MPS method. Kernel function is a very important part in MPS theory, and it is one of the only a few adjustable parameters in MPS method. In early studies, better results are got by adjust kernel functions, but there has no systemic study on kerner functions'behavior. So the study on behavior of kernel function can help to select the kernel function for simulations.3. Pressure fluctuation phenomena, an area-time average method for pressure fluctuation phenomena, and the effect of the area-time average method. The unacceptable pressure fluctuation is an obvious defect in MPS method, and it has a negative influence to the development of MPS method. In most studies by MPS, more attention are putted on the free surface, pressure results are neglected because the fluctuation. An area-time average method is proposed in this paper, different area average type and different time average period are discussed. With different area-time average methods, the fluctuation of pressure is solved, the result is quit good and acceptable, and that makes MPS method more suitable for ocean engineering problem.4. A new mixed free surface traced method for MPS method, which based on neighbors search. MPS method uses the numerical density to trace the free surface, and it is efficiency. But due to the algorithm of MPS method, there are lots of non-free surface particles are traced as free surface particles. That will cause mistakes in simulation and make the pressure fluctuation more serious. The new mixed free surface traced method bases on neighbors search, so it solves the mistreatment problem in theory. Because its efficiency to use numerical density for free surface, the neighbors search method is mixed used with numerical density method, the neighbors search method is used when the mistreatment problem always happen, so the mixed free surface traced method solved the mistreatment problem with efficiency.5. Modified MPS-LES method for sloshing problem with violent surface. Most ocean engineering problems are problems with turbulence flow, and most of the early studies which bases on MPS method are laminar flow, so it is meanful to simulate the practical cases with turbulent flow. In this paper, the MPS method is extended to the large eddy simulation (LES) by coupled with a sub-particle-scale (SPS) turbulence model, the effect of the SPS turbulence is found as the Reynolds stress terms in the filtered momentum equation, and the Smagorinsky model is introduced to describe the Reynolds stress terms. 3 different sloshing problems are simulated by MPS-LES method successfully, these cases are a resonance sloshing rectangular tank, a modeled resonance rolling LNG tank longitudinal section and a full-scale resonance mix movement LNG tank transverse section.Results show, these new technologies or methods adapted to the MPS method and improved the original one. With these technologies, the modified MPS-LES method is good at the simulation of sloshing with large deforming free surface. The additional correction term make the modified MPS-LES method avoided nearly the entire pass through phenomena; behavior of kernel function is helpful to select a kernel function before simulation; the new mixed free surface traced method work well and no more mistreatment; different area-time average methods solved pressure fluctuation well, the pressure is good and very close to pressure of experiments or pressure by other methods.