基于SPH方法的湿润性固壁边界条件模拟研究

固体壁面由于表面特殊结构和材料属性,时常表现出对交界面上水体的吸附作用,而这一特征对微小水体作用尤为明显。本文提出了一种湿润性固壁边界条件的计算方法,即假设壁面粒子的亲水性以及毛细吸附作用统一表现为对支持域内流体粒子的吸附力。基于光滑粒子流体动力学（SPH）方法,模拟了静态液滴在不同湿润性壁面上的变形至稳定过程。模拟了液滴撞击疏水壁面的过程,将液滴的运动过程分为碰撞、铺展、回缩和回弹四个阶段,分析各阶段壁面受力分布情况。研究表明：根据模拟液滴静态接触角的变化特点,本文湿润性固壁边界条件可以较好的反映出壁面湿润性;液滴撞击输水表面的模拟数据与试验结果趋势上吻合良好;壁面压力波伴随着液滴的铺展和回缩传播并衰减;只有在回弹后期液滴即将脱离壁面时壁面拉力起主导作用,其余各时刻壁面均以压力为主。     

基于SPH方法的表面张力修正算法及其应用

针对传统SPEI方法中基于CSF模型的表面张力算法,在计算边界、尖角等粒子缺失部位的曲率时存在偏差较大,且粒子秩序较差,对大变形问题表面张力计算精度较低的问题,在Morris提出的表面张力SPH方法基础上,通过引入CSPM方法对边界法向的计算和曲率的计算进行修正,得到了表面张力修正方程组.应用本文方法模拟了水溶液中初始...     

Oldroyd-B黏弹性液滴弹跳行为的改进SPH模拟

基于光滑粒子流体动力学SPH(Smoothed Particle Hydrodynamics)方法对Oldroyd-B黏弹性液滴撞击固壁面产生的弹跳行为进行了模拟与分析。首先,为了解决SPH模拟黏弹性自由表面流出现的张力不稳定性问题,联合粒子迁移技术提出了一种改进SPH方法。然后,对Oldroyd-B黏弹性液滴撞击固壁面产生的铺展行为进行了改进SPH模拟,与文献结果的比较验证了方法的有效性。最后,通过降低Reynolds数捕捉到了液滴的弹跳行为;并在此基础上,分析了液滴黏度比、Weissenberg数和Reynolds数对液滴弹跳行为的影响。结果表明,改进SPH方法可有效地模拟黏弹性自由表面流问题;液滴黏度比、Weissenberg数和Reynolds数对液滴最大回弹高度均有显著的影响。     

基于光滑粒子流体动力学的滑油泵数值仿真

基于光滑粒子流体动力学方法,构建齿轮泵壳体及内部流体的粒子模型,对内啮合齿轮泵在不同工况下的流量特性进行数值模拟。首先设置了均匀分布的油泵出入口压强,计算得到的流量结果与试验结果吻合,且在中低转速下流量与转速呈线性关系;针对高转速下油泵流量降低的问题,通过适当减小周期模型内部的流体粒子数反映流量降低,获得了与试验值相吻合的连续转速流量的模拟结果;针对影响齿轮泵性能的间隙和空化现象,结合模型特点给出了相应的近似处理方法。通过以上研究,将SPH方法成功地应用于滑油泵问题的分析计算。     

45钢柱壳爆炸膨胀断裂的SPH模拟分析

金属柱壳爆炸膨胀断裂存在拉伸、剪切及拉剪混合等多种断裂模式,目前其物理机制及影响因素还不清晰。本文中采用光滑粒子流体动力学方法（smoothed particle hydrodynamics, SPH）对45钢柱壳在JOB-9003及RHT-901不同装药条件下的外爆实验进行了数值模拟,探讨柱壳在不同装药条件下发生的剪切断裂、拉剪混合断裂模式及其演化过程,模拟结果与实验结果一致。SPH数值模拟结果表明:在爆炸加载阶段,随着冲击波在柱壳内、外壁间来回反射形成二次塑性区,沿柱壳壁厚等效塑性应变演化呈凸形分布,壁厚中部区域等效塑性应变较内、外壁大;在较高爆炸压力（JOB-9003）作用下,柱壳断裂发生在爆轰波加载阶段,损伤裂纹从塑性应变积累较大的壁厚中部开始沿剪切方向向内、外壁扩展,形成剪切型断裂模式;而在RHT-901空心炸药加载下,虽然裂纹仍从壁厚中部开始沿剪切方向扩展,但随后柱壳进入自由膨胀阶段,未断区域处于拉伸应力状态,柱壳局部发生结构失稳,形成类似“颈缩”现象,裂纹从剪切方向转向沿颈缩区向外扩展,呈现拉剪混合断裂模式。拉伸裂纹占截面的比例与柱壳结构失稳时刻相关。可见,柱壳断裂演化是一个爆炸冲击波与柱壳结构相互作用的过程,不能简单将其作为一系列膨胀拉伸环处理。     

超高速碰撞产生等离子体的毁伤特性研究

以热力学为基础,结合化学反应速率方程,推导得出描述超高速碰撞产生的等离子体电子密度与系统内能关系的物理方程组. 应用自行编写的二维光滑粒子流体动力学(smoothed particle hydrodynamics, SPH) 程序求解了此方程组,在模拟超高速碰撞过程中计算产生的等离子体,实现对超高速碰撞产生等离子体的数值模拟.进行铝球超高速碰撞双层铝板的数值模拟研究,给出与实验对比的结果. 统计前后两次碰撞产生等离子体的电量,发现碰撞较薄的前板产生了较少的电荷而一次碎片云对较厚后板的碰撞产生了大量的电荷. 交换前后两板的位置. 进行相同速度碰撞的模拟,分析结果发现一次碎片云碰撞后板产生的等离子体电量远小于第一次的模拟结果,而前板碰撞产生的等离子体电量要高于第一次模拟结果,由此可见,超高速碰撞产生等离子体的总电量不仅与弹丸的质量和碰撞速度有关,与薄板的厚度也有很大的关系,通过一次碎片云与第二层板碰撞可以产生远高于弹丸碰撞单层板产生的电量,可以提高等离子体产生效率,增强对航天器的电磁毁伤.      

SPH方法研究空气在平板入水中的影响

飞机水上迫降以及船体在高速行驶时,均会发生结构物砰击水面的现象,当结构物底面较为平坦,砰击水面时会捕获空气,在板底形成空气垫。研究空气垫的形成及影响,有助于对结构物进行更准确的载荷分析。该问题的研究涉及到气固液三相的相互作用,在数值模拟中仍是一个挑战,本文使用光滑粒子流体动力学(SPH)方法对该问题进行模拟。首先,与实验结果进行对比;其次,系统地研究了平板形成空气垫以及砰击水面的过程,考查了空气、水和平板的动能变化。对比了在SPH模拟中考虑空气与不考虑空气得到的水压力场和平板所受压力,说明了空气对结果的影响;最后,研究了平板宽度对空气垫形成的影响,从而进一步影响加速度。     

基于修正光滑粒子流体动力学算法的低能量耗散数值波浪水池开发

目前, 无网格光滑粒子流体动力学SPH粒子法在波浪与结构物相互作用研究方面得到广泛应用, 但该方法模拟波浪远距离传播时, 常常面临严重的能量耗散问题, 导致波高非物理性降低, 给大范围海域、长时间作用下的波-物耦合作用研究带来一定困难. 对此, 本文采用一种核函数修正算法, 在确保粒子间相互作用对称性的同时, 改进压力梯度离散项的计算精度, 设法解决SPH方法中能量非物理性耗散的难题. 相较于前人减缓能量非物理性衰减的方法, 本文的修正SPH算法避免了自由液面搜索等复杂处理过程, 并能保证动量守恒特性. 数值结果中, 采用振荡液滴、规则波、不规则波等算例, 验证本修正SPH算法的准确性和有效性. 结果表明, 该修正SPH算法能准确模拟振荡液滴形态变化, 且动能保持较好守恒性. 通过数值水池与物理水池两者规则波与不规则波结果的对比分析表明, 基于本文修正SPH算法建立的数值波浪水池具有较好的抗能量衰减效果, 能实现长时间、远距离波浪传播的准确模拟. 此外, 本算法能在低光滑长度系数条件下, 实现精确模拟, 将极大缩减三维SPH模拟的时间, 从而节约计算成本.      

滴状模式下液桥形成及断裂的电流体动力学特性研究

对电场作用下微通道荷电液滴脱落过程中液桥形成及断裂的显微演变特征进行了可视化实验研究.借助时空分辨率较高的高速摄像技术精确捕捉了电场作用下液桥形成及断裂的界面演化过程,研究了液桥的界面结构变化及其断裂的动力学显微演变行为,获得了时间特征数、电邦德数及半月面形成角对液桥长度及断裂顺序的作用规律.实验结果显示,液桥断裂长度取决于黏度与表面张力之比,而受荷电弛豫时间的影响甚微,低电压工况下各实验介质液桥相对长度的变化并不明显,而在较高电压工况下相对液桥长度的增长速度加快.随着电邦德数的不断增加,液桥长度的变化在较高邦德数下更为明显且存在突变区,此时伴随着雾化模式的转变,表明液桥的突变恰恰是雾化模式过渡的信号.不同物性介质的射流过渡行为由于液桥上下游形成角的变化而存在较大差异.对于无水乙醇介质,电邦德数的增加使滴状模式首先过渡到纺锤模式,而对于生物柴油,滴状模式后会首先出现脉动模式而非纺锤模式.      

多相流动的光滑粒子流体动力学方法研究综述

光滑粒子流体动力学(smoothed particle hydrodynamics, SPH)具有粒子方法的无网格和全拉格朗日特征, 适用于具有界面大变形、不连续性和多物理场的多相流的高精度模拟. SPH方法模拟多相流已有大量报道, 具体的实现方式也大不相同. 本文首先阐述了采用SPH方法模拟流体的基本控制方程, 以及求解过程中需要考虑的流体压力求解、表面张力、固体边界等问题. 整理和总结了基于SPH方法进行多相流模拟的主要实现方式: (1)双流体模型的拉格朗日求解器: 两相离散为两组独立SPH粒子, 并用显式相间作用耦合两相; (2)多相SPH方法: SPH方法对多相流模拟的自然延伸, 相间作用由SPH参数隐式描述; (3) SPH与其他离散方法的耦合: 差异较大的两相各自采用不同离散方法, 发挥不同拉格朗日方法的优点; (4) SPH和基于网格方法的耦合: 网格方法处理简单的单相流动主体, 获得精度和效率间的平衡. 另外, 还在模拟参数物理化等方面论述了与SPH方法模拟多相流相关的一些改进和修正方法, 并在最后讨论和建议了提高多相流SPH模拟效率和精度的措施.      

Approximate Wall Boundary Conditions in the Large-Eddy Simulation of High Reynolds Number Flow

The near-wall regions of high Reynolds numbers turbulent flows must be modelled to treat many practical engineering and aeronautical applications. In this review we examine results from simulations of both attached and separated flows on coarse grids in which the near-wall regions are not resolved and are instead represented by approximate wall boundary conditions. The simulations use the dynamic Smagorinsky subgrid-scale model and a second-order finite-difference method. Typical results are found to be mixed, with acceptable results found in many cases in the core of the flow far from the walls, provided there is adequate numerical resolution, but with poorer results generally found near the wall. Deficiencies in this approach are caused in part by both inaccuracies in subgrid-scale modelling and numerical errors in the low-order finite-difference method on coarse near-wall grids, which should be taken into account when constructing models and performing large-eddy simulation on coarse grids. A promising new method for developing wall models from optimal control theory is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bingham流体与固壁、液面斜撞击的数值模拟

在交错网格中用MAC方法求解二维不可压N S方程 ,对圆截面液柱与固壁、液面斜撞击问题进行了数值模拟 ,得到了自由面随时间演化的图像。主要考察了 :(1)固壁和液面 ;(2 )牛顿、非牛顿流体 ;(3)碰撞入射角 ;(4 )Bingham流体近似本构式中参数q0 、K对计算结果的影响。结果表明 :液柱与液面碰撞形成的自由面更复杂。碰撞初期 ,Bingham流体和水的自由面相似 ;但碰撞后期 ,Bingham流体的自由面相对简单。对液柱与液面碰撞自由面的影响较大。本文条件下 ,当K 2 0时 ,K对自由面的影响不大 ;当q0 增大时 ,自由面变得相对简单。     

Simulation of Transverse Combustion Instability in a Multi-Injector Combustor using the Time-Domain Impedance Boundary Conditions

The time-domain impedance boundary condition (TDIBC) is used as a reduced-order model (ROM) in large-eddy simulation (LES) to study self-sustained transverse oscillations in an experimentally studied high-pressure, shear coaxial multi-injector combustor. This work is an extension of the recent study using ROM-LES to simulate a single-element combustor that exhibited longitudinal instability. Here, we focus on transverse instability in a seven-injector combustor. The fuel and oxidizer inlets are truncated and the conventional inflow boundary conditions at the original inlet are replaced by an impedance describing function (IDF) in the form of a reflection coefficient that couples with LES through characteristic based boundary conditions at the truncated inlet. The impedance model is also generalized to include the effects of entropy fluctuations at the inflow. The hybrid ROM-LES simulations are compared with LES simulations with the full combustor geometry. Results show very good agreement and confirm that the use of TDIBC within LES is a viable tool to account for complex acoustic/boundary interaction in a physical way without explicitly solving the full geometry at LES level. Some simplifications and approximations have to be invoked and these constraints are also discussed.     

Statistical Structure at the Wall of the High Reynolds Number Turbulent Boundary Layer

The one and two-point statistical structure of very high Reynolds number turbulence in the surface layer near a rigid `wall' is analysed. The essential mechanisms for turbulent eddies impinging on the wall are studied using linearised rapid distortion theory, which show how the mean shear and blocking actions of the surface act first independently and then, over the life time of the eddy, interactively. Previous analytical results are reinterpreted and some new results are derived to show how the integral length scales, cross correlations and spectra of the different components of the turbulence are distorted depending on the form of the spectra of eddies above the surface layer and how they are related to motions of characteristic eddy structures near the surface. These results are applied to derive some quantitative and qualitative predictions in the surface layers (SL), where the eddies are affected by local shear dynamics, and in the `eddy surface layer' (ESL) where quasi independents loping elongated eddies interact directly with the wall, and where there is a large range of wave number within which the spectra of the horizontal velocity components are proportional to k ⁻¹. The longest eddies in the boundary layer occur near the wall. Field experiments agree with the theoretical model predictions for the quite different forms for the spectra, cospectra and cross correlations for the vertical and horizontal components of the velocity field. By showing that in SL the energy exchange between the large and small scale eddies is local(`staircase') energy cascade, whereas in ESL there is a direct nonlocal (`elevator-like')energy transfer to the small scales, it is shown why the thickness of the ESL increases over rougher surfaces and as the Reynolds number decreases. This revised version was published online in August 2006 with corrections to the Cover Date.     

轴流压气机端壁附面层的理论预测

从Ｎ－Ｓ方程出发，推导了轴流压气机端壁边界层动量积分方程，并提出了一种适用范围广泛的叶片力亏损模型，结合边界层的卷吸方程和速度分布表达式，可以方便的预测压气机环壁边界层的发展，与传统的叶片力亏损模型相比，该方法的计算准确性和适用范围均得到提高。     

Numerical Simulation of the Buoyancy-Driven Bouncing of a 2-D Bubble at a Horizontal Wall

The rise of a buoyant bubble and its interaction with a target horizontal wall is simulated with a 2-D numerical code based on the Boundary Element Method (BEM). Developed from a viscous potential flow approximation, the BEM takes into account only the part of the energy dissipation related to the normal viscous stresses. Hence, a simple analytical model based on lubrication approximation is coupled to the BEM in order to compute the drainage of the interstitial liquid film filling the gap between the bubble and the near wall. In this way the bubble–wall interaction is fully computed: the approach stage, the bubble deformation stage and, depending on the values of the Reynolds number and the Weber number, the rebound and the bubble oscillations. From computation of both the bubble interface motion and the liquid velocity field, a physical analysis in terms of energy budget is proposed. Though, in the present study, the bubble under consideration is basically supposed to be a 2-D gaseous cylinder, a comparison between our numerical results and the experiments of Tsao and Koch (1997) enlightens interestingly the physics of bouncing.     

Necessary Conditions at the Boundary for Minimizers in Incompressible Finite Elasticity

New necessary conditions for energy-minimizing states of an incompressible, elastic body are found. These must hold at boundary points at which dead-load traction data is prescribed.     

Fluent环境中近壁面微空泡溃灭的仿真计算

基于FLUENT软件环境,采用VOF模型和非稳态方法求解Navier-Stokes方程,模拟了近壁面的空泡溃灭过程,同时计算了空泡溃灭处与壁面的距离对射流强度的影响.结果表明：在近壁面,空泡将形成非对称溃灭,因水锤作用,引发高速水射流在壁面产生高压而形成空蚀破坏;基于FLUENT环境的计算结果与已有的实验和计算结果相符,为研究空泡溃灭和空蚀机制提供了类比的数值计算方法.