MULTI-SCALE TWO-PHASE WAVE-STRUCTURE INTERACTION USING ADAPTIVE SPH COUPLED WITH QALE-FEM

近年来，波浪流体力学和波体相互作用的模拟取得了很大的进展。基于势流理论的波衍射分析现在已成为频域线性和二阶理论的标准。在完全非线性分析的时域中，最通用、最稳健、最有效的方法是位流的任意拉格朗日-欧拉有限元法（QALE-FEM）方法，它能够覆盖20x20波长的三维域，在8核处理器上过夜运行20个波周期。然而，该方法是单相的，具有无旋转、无粘性流体的物理限制。极端载荷和冲击或撞击通常涉及破碎波条件，其中多相（空气-水-固体）行为很重要。对于结构或物体内部具有复杂物理特性的剧烈流动，需要另一种方法。流体体积（VOF）方法的进展已应用于波浪与柱和海岸结构的相互作用。然而，最近使用光滑粒子流体力学（SPH）方法取得了对剧烈波浪结构的最重大进展。由于SPH在处理具有倾覆、飞溅和身体相互作用的自由表面流动方面的通用性，多年来一直是一个有前途的研究领域。EPSRC资助的一种不可压缩、无散度公式（ISPH）解决了单相流的数值收敛、稳定性和噪声压力等问题。用广义移位算法消除粒子分布中的不稳定性，证明了几种脉冲试验用例具有高压力精度的数值收敛性。显然，准确预测高压对流体-体相互作用至关重要。这已经扩展到两相流，其中包括可压缩空气相，在初步试验中取得了良好的结果。SPH的主要缺点是计算时间长，因为在三维中需要大量的粒子，0（1000万- 10亿），每个粒子的邻居相互作用数量多，所需的时间步长相对较小。变化的粒径和有效的邻域搜索不能使三维中许多波长的域维度在许多波周期内运行，而通常需要极端的波体相互作用。在前人工作的基础上，本项目将对波浪-结构相互作用的ISPH和QALE-FEM进行并行开发，然后进行耦合，以两种方式实现高效计算：动态自适应粒度，满足最小误差条件或解决一些物理特性，如密度或涡度。作者在二维上已经取得了令人满意的初步结果。利用QALE-FEM等有效求解方法耦合内部SPH域与外部非线性势流域。动态自适应粒度也应用于SPH领域。

Considerable advances have been made in the modelling of wave hydrodynamics and wave-body interaction in recent years. Wave diffraction analysis based on potential flow theory is now standard with linear and second-order theory in the frequency domain. In the time domain for fully nonlinear analysis arguably the most versatile, robust and efficient method is the arbitrary Lagrange-Euler finite-element method (QALE-FEM) approach for potential flow, capable of covering 3-D domains of say 20x20 wavelengths in plan with 20 wave periods on overnight runs on an 8-core processor. However, the method is single-phase with the physical limitation of an irrotational, inviscid fluid. Extreme loads and impacts or slam generally involve breaking wave conditions where multi-phase (air-water-solid) behaviour is significant. An alternative approach is required for violent flows with complex physics local to a structure or body. Progress with volume-of-fluid (VOF) methods has been applied to wave interactions with columns and coastal structures. However, the most significant advances for violent wave-structure have recently been made using the Smoothed Particle Hydrodynamics (SPH) method. SPH has been an area of promising research for some years due to its versatility in dealing with free-surface flows with overturning, splashing and body interaction. Recent problems with numerical convergence, stability and very noisy pressure have been resolved for single-phase flow through EPSRC funded work through an incompressible, divergence-free, formulation (ISPH). Numerical convergence with high pressure accuracy for several impulsive test cases has been demonstrated with generalised shifting algorithms for eliminating instabilities within the particle distributions. Clearly, predicting high pressure accurately is vital for fluid-body interaction. This has been extended to two-phase flow with the incorporation of a compressible air phase with good results in preliminary tests. The main disadvantage of SPH is the computational time due to the large number of particles required in 3-D, O(10 million - 1 billion), the large number of neighbour interactions per particle, and the relatively small time steps needed. Variable particle sizing and efficient neighbour searching do not enable domain dimensions of many wavelengths in 3-D to be run for many wave periods as generally required for extreme wave-body interaction. Building on previous work, in this project ISPH and QALE-FEM for wave-structure interaction will be developed in parallel and then coupled achieving efficient computation in two ways:1. Dynamic adaptive particle sizing, satisfying minimum error conditions or resolving some physical characteristic, e.g. density or vorticity. Promising preliminary results have been obtained in 2-D by the proposers.2. Coupling an inner SPH domain with an outer nonlinear potential flow domain using an efficient solution method such as QALE-FEM. Dynamic adaptive particle sizing should also be used in the SPH domain.