无网格法中边界畸变的控制与计算效率的提高

分析比较了常用的2种无网格法的形函数，即采用光滑粒子流体动力学（SPH）法与移动最小二乘（MLS）法构造的形函数，指出SPH形函数在特定情况下易在边界处产生畸变的原因，并提出了在边界外围设置虚节点以改善边界畸变的方法.分别通过配点法和无网格Galerkin（EFG）法计算了一维和二维算例，讨论了不同的边界条件处理方式对计算精度的影响，结果表明Lagrange乘子法处理边界条件的精度比点插值法高.在EFG法的一维悬臂梁算例分析中，讨论了节点支撑域半径和高斯积分阶次对计算量和计算精度的影响.分析表明，当使用单点高斯积分时，节点支撑域的变化易导致计算结果不稳定，提高高斯积分阶次能够降低计算结果对节点支撑域大小变化的敏感性并提高计算精度，但同时增加了计算量.     

Hypervelocity impacts on thin brittle targets: Experimental data and SPH simulations

The meteoroids and debris environment play an important role in the reduction of spacecraft lifetime. Ejecta or secondary debris, are produced when a debris or a meteoroid impact a spacecraft surface. Brittle materials are particularly sensitive to HVI in term of damages and amount of ejected matter: the ejected fragments total mass is in the order of 100 times bigger than the impacting mass. The French atomic energy commission (CEA) faces the same problem in the Laser MégaJoule project. The lasers optics will be bombarded by hypervelocity debris and shrapnel resulting from target disassembly. Two millimeter thick fused silica disposable debris shields (DDS) located in front of the main debris shields might be used to reduce very small shrapnel cratering on the main debris shields. The aim of this paper is to study the damaging and ejection processes that occur during HVI on thin brittle targets. A two-stage light-gas gun has been used to impact 2 mm DDS with 500 μm steel projectiles. Experimental characterization of ejected matter has also been performed: lightweight paperboards coated with adhesive have been used to collect ejected fragments including spalls. Numerical simulation using the smooth particle hydrodynamics (SPH) method of LS-DYNA and the Johnson Holmquist material model were performed. The results of these calculations are compared to experimental data which include the damage features in the targets (spalled zones and perforation hole) and the ejection clouds. Satisfying agreement between numerical and experimental simulations was obtained for damage characteristics and ejection phenomena.     

基于SPH法的正弦形消力池底板水跃现象数值模拟

利用光滑质点水动力学法(SPH方法)对正弦形消力池底板上的水跃现象进行数值建模,共模拟2种波形5种工况。将SPH方法的模拟值与已有文献的试验值作对比,验证数值模型及数值方法研究此类问题的可行性和适用性,并分析水面线、流速分布、跃长、共轭水深、消能率等水跃特性的变化规律。结果表明:SPH方法模拟结果与试验结果吻合度较高;水跃段流速分布不均匀、自由表面波动较大,且流层间存在相对运动从而形成旋滚,同等条件下随着弗劳德数的递增,掺气量和自由表面破碎现象越来越剧烈,旋滚的影响范围逐渐变大;正弦形底板消能率较光滑底板提高10%左右且所有工况消能率均在47%以上。     

基于周期性校正神经网络的血流血管壁耦合

针对传统的血流血管壁耦合难以兼顾计算效率和视觉真实感的问题,提出了一种基于周期性校正神经网络（Periodic-corrected Network,PcNet）的血流血管壁耦合数据驱动仿真方法。设计基于平滑粒子流体动力学（SPH）的血流粒子状态特征向量,对邻域血流粒子和血管壁代理粒子的混合贡献进行建模。提出一种半监督的神经网络--改进的周期性校正神经网络,预测每个粒子在下一帧的加速度。实验结果表明该仿真方法实现了快速、稳定、逼真的血流血管壁耦合。     

High-speed water impacts of flat plates in different ditching configuration through a Riemann--ALE SPH model

The violent water entry of flat plates is investigated using a Riemann-arbitrary Eulerian-Lagrangian(ALE) smoothed particle hydrodynamics(SPH) model. The test conditions are of interest for problems related to aircraft and helicopter emergency landing in water. Three main parameters are considered: the horizontal velocity, the approach angle(i.e., vertical to horizontal velocity ratio) and the pitch angle, a. Regarding the latter, small angles are considered in this study. As described in the theoretical work by Zhao and Faltinsen(1993), for small a a very thin, high-speed jet of water is formed, and the time-spatial gradients of the pressure field are extremely high. These test conditions are very challenging for numerical solvers. In the present study an enhanced SPH model is firstly tested on a purely vertical impact with deadrise angle α=4°. An in-depth validation against analytical solutions and experimental results is carried out, highlighting the several critical aspects of the numerical modelling of this kind of flow, especially when pressure peaks are to be captured. A discussion on the main difficulties when comparing to model scale experiments is also provided. Then, the more realistic case of a plate with both horizontal and vertical velocity components is discussed and compared to ditching experiments recently carried out at CNR-INSEAN. In the latter case both 2-D and 3-D simulations are considered and the importance of 3-D effects on the pressure peak is discussed for α=4° and α=10°.     

SPH modeling of fluid-structure interaction

This work concerns numerical modeling of fluid-structure interaction(FSI) problems in a uniform smoothed particle hydrodynamics(SPH) framework. It combines a transport-velocity SPH scheme, advancing fluid motions, with a total Lagrangian SPH formulation dealing with the structure deformations. Since both fluid and solid governing equations are solved in SPH framework, while coupling becomes straightforward, the momentum conservation of the FSI system is satisfied strictly. A well-known FSI benchmark test case has been performed to validate the modeling and to demonstrate its potential.     

SPH numerical investigation of the characteristics of an oscillating hydraulic jump at an abrupt drop

This paper shows the results of the smooth particle hydrodynamics(SPH) modelling of the hydraulic jump at an abrupt drop,where the transition from supercritical to subcritical flow is characterised by several flow patterns depending upon the inflow and tailwater conditions. SPH simulations are obtained by a pseudo-compressible XSPH scheme with pressure smoothing; turbulent stresses are represented either by an algebraic mixing-length model, or by a two-equation k-ε model. The numerical model is applied to analyse the occurrence of oscillatory flow conditions between two different jump types characterised by quasi-periodic oscillation,and the results are compared with experiments performed at the hydraulics laboratory of Bari Technical University. The purpose of this paper is to obtain a deeper understanding of the physical features of a flow which is in general difficult to be reproduced numerically,owing to its unstable character: in particular, vorticity and turbulent kinetic energy fields, velocity, water depth and pressure spectra downstream of the jump, and velocity and pressure cross-correlations can be computed and analysed.     

基于CUDA的大规模流体实时模拟

流体模拟是计算机图形学中一个重要课题。使用基于粒子的光滑流体动力学SPH(smoothed particle hydrodynamics)方法模拟大规模流体的运动需要大量的粒子模拟流体,计算量巨大,传统的方法很难达到实时性要求。为了解决该问题,使用NVIDIA的并行计算架构CUDA(Compute Unified Device Architecture)将SPH方法的全部处理过程在GPU上实现,充分利用了GPU并行计算的性能优势。使用Z-order排列改进已有的并行邻域搜索算法,并通过优化数据结构及存储器分配,有效缓解了SPH方法在GPU架构上的性能瓶颈。实验结果表明,该方法能实时逼真地模拟大规模流体,与已有的GPU方法相比处理速度有显著的提升。     

一种基于SPH方法的人员疏散混合模型及模拟

为减少室内火灾环境下人员伤亡,研究了人员疏散计算机模拟问题.将人员运动视为一种流体运动,利用流体力学原理,构建一种新的人员疏散混合模型,并在其中引入了一种从众模式.同时,采用光滑粒子流体动力学方法对模型离散化,从而减少了计算量.不同工况下进行的疏散模拟实验表明,所提出的模型及算法能够较准确地模拟出实际疏散的现象.     

孤立波浅化过程的SPH数值模拟

本文应用光滑粒子流体动力学(SPH)方法建立了二维数值波浪水槽,研究了孤立波沿斜坡的传播、变形和破碎过程。通过数值计算分析了波浪沿斜坡传播过程中自由表面的变化特征,给出了斜坡上不同位置处波面变化的历时曲线,并应用文献记录的试验结果对数模结果进行了验证。得到了波高的沿程变化趋势、破波点的位置和破碎波高以及不同破碎形态下波浪自由表面的变化特征,显示了SPH方法在处理较复杂自由表面流动问题方面的优越性。