Power flow boundary element analysis for multi-domain problems in vibrational built-up structures

A power flow boundary element methodology for the treatment of multi-domain problems is developed to predict the vibrational responses of coupled structures in medium-to-high frequency ranges. In the proposed methodology, a matrix equation is formulated by utilizing the power flow coupling relationships at the interface of the domains. This analysis method is successfully applied to the treatment of simply supported coupled beams and coupled plates. The vibration analysis of these structures is then considered for all wave components. The vibrational energy density and intensity of two coupled structures obtained through numerical simulations are compared with those of the original power flow analysis, and these results show good agreement.     

A conservative numerical method for the Cahn–Hilliard equation in complex domains

We propose an efficient finite difference scheme for solving the Cahn–Hilliard equation with a variable mobility in complex domains. Our method employs a type of unconditionally gradient stable splitting discretization. We also extend the scheme to compute the Cahn–Hilliard equation in arbitrarily shaped domains. We prove the mass conservation property of the proposed discrete scheme for complex domains. The resulting discretized equations are solved using a multigrid method. Numerical simulations are presented to demonstrate that the proposed scheme can deal with complex geometries robustly. Furthermore, the multigrid efficiency is retained even if the embedded domain is present.     

Structure of a tethered polymer under flow using molecular dynamics and hybrid molecular-continuum simulations

We analyse the structure of a single polymer tethered to a solid surface undergoing a Couette flow. We study the problem using molecular dynamics (MD) and hybrid MD-continuum simulations, wherein the polymer and the surrounding solvent are treated via standard MD, and the solvent flow farther away from the polymer is solved by continuum fluid dynamics (CFD). The polymer represents a freely jointed chain (FJC) and is modelled by Lennard-Jones (LJ) beads interacting through the FENE potential. The solvent (modelled as a LJ fluid) and a weakly attractive wall are treated at the molecular level. At large shear rates the polymer becomes more elongated than predicted by existing theoretical scaling laws. Also, along the normal-to-wall direction the structure observed for the FJC is, surprisingly, very similar to that predicted for a semiflexible chain. Comparison with previous Brownian dynamics simulations (which exclude both solvent and wall potential) indicates that these effects are due to the polymer–solvent and polymer–wall interactions. The hybrid simulations are in perfect agreement with the MD simulations, showing no trace of finite size effects. Importantly, the extra cost required to couple the MD and CFD domains is negligible.     

基于面元法的水平轴风力机数值模拟

使用基于速度面元法的势流数值模拟方法,以NREL PhaseⅥ为例进行了叶片气动载荷和风轮近尾流场的数值模拟。将势流数值模拟、叶素动量理论和计算流体力学CFD方法的计算结果与实验数据进行了对比分析。结果表明使用速度面元法计算风轮绕流场具有较高的计算精度和求解效率,为大规模风力机群的流场计算和出力预报提供支撑。     

Numerical Simulation of Dendritic Solidification with Convection: Two-Dimensional Geometry

A front tracking method is presented for simulations of dendritic growth of pure substances in the presence of flow. The liquid–solid interface is explicitly tracked and the latent heat released during solidification is calculated using the normal temperature gradient near the interface. A projection method is used to solve the Navier–Stokes equations. The no-slip condition on the interface is enforced by setting the velocities in the solid phase to zero. The method is validated through a comparison with an exact solution for a Stefan problem, a grid refinement test, and a comparison with a solution obtained by a boundary integral method. Three sets of two-dimensional simulations are presented: a comparison with the simulations of Beckermann et al. (J. Comput. Phys.154, 468, 1999); a study of the effect of different flow velocities; and a study of the effect of the Prandtl number on the growth of a group of dendrites growing together. The simulations show that on the upstream side the dendrite tip velocity is increased due to the increase in the temperature gradient and the formation of side branches is promoted. The flow has the opposite effect on the downstream side. The results are in good qualitative agreement with published experimental results, even though only the two-dimensional aspects are examined here.     

Transition to chaos in a confined two-dimensional fluid flow

For a two-dimensional fluid in a square domain with no-slip walls, new direct numerical simulations reveal that the transition from steady to chaotic flow occurs through a sequence of various periodic and quasiperiodic flows, similar to the well-known Ruelle-Takens-Newhouse scenario. For all solutions beyond the ground state, the phenomenology is dominated by a domain-filling circulation cell, whereas the associated symmetry is reduced from the full symmetry group of the square to rotational symmetry over an angle pi. The results complement both laboratory experiments in containers with rigid walls and numerical simulations on double-periodic domains.     

激光散斑图样的光流场分析研究

提出了一种基于动态图像处理的新的电子散斑计量方法－光流场分析法，从新的角度出发讨论了运用图像序列分析的概念秋解决散斑图样测量的可行性和简便性。并且解决了当散斑图像不满足进行光流场分析的约束条件时的测量问题，即通过多通道的图像处理方法来改变图像连续特性，从而扩大了测量范围。     

A level-set continuum method for two-phase flows with insoluble surfactant

A level-set continuum surface force method is presented to compute two-phase flows with insoluble surfactant. Our method recasts the Navier–Stokes equations for a two-phase flow with insoluble surfactant as “one-fluid” formulation. Interfacial transport and interfacial jump conditions are treated using the level-set method and the discrete Dirac function. Based on the density-weighted projection method, a stable semi-implicit scheme is used to decouple the velocity components in solving the regularized Navier–Stokes equations. It allows numerical simulations for a wide range of viscosity ratios and density ratios.Numerical simulations on single drop deformation in a 2D shear flow are presented. Simulations on two drop interaction shows that surfactants can play a critical role in preventing drop coalescence. A fully 3D simulation demonstrating the physical interactions of multiple surfactant-laden drops is presented.     

Adapting the spectral vanishing viscosity method for large-eddy simulations in cylindrical configurations

During the last decade the spectral vanishing viscosity (SVV) method has been adopted successfully for large-eddy-type simulations (LES) with high-order discretizations in both Cartesian and cylindrical coordinate systems. For the latter case, however, previous studies were confined to annular domains. In the present work, we examine the applicability of SVV in cylindrical coordinates to flows in which the axis region is included, within the setting of an exponentially convergent spectral element–Fourier discretization. In addition to the ‘standard’ SVV viscosity kernel, two modified kernels with enhanced stabilization in the axis region are considered. Three fluid flow examples are considered, including turbulent pipe flow. The results, on the one hand, show a surprisingly small influence of the SVV kernel, while on the other, they reveal the importance of spatial resolution in the axis region.     

KIVA程序网格生成算法的改进及雷诺应力模型计算

针对KIVA程序中网格生成不能适用于复杂区域的缺点,对其提出了一种改进算法,该算法采用Laplace椭圆方程,生成计算区域的网格,具有收敛快、适用于复杂区域的特点。该方法只要给定边界点,就可以自动生成内部网格,无需差值形成内部网格。针对几种部同的内燃机燃烧室采用雷诺应力(DSM)模型,计算了压缩、膨胀、进气和排气过程的内流场,计算结果正确给出了缸内的湍流各向异性的特点。     

Numerical investigation of the tangential stress effects on a fluid flow structure in a partially open cavity

Mathematical and numerical modeling of fluid flows in the domains with free boundaries under co-current gas flow is widely investigated nowadays. A stationary problem of fluid motion in a rectangular cavity with a non-deformed free boundary is studied in a two-dimensional statement. The tangential stresses created on the free boundary by an adjoint gas flow are considered to be a driving force for a fluid motion. The influence of the cavity geometry (cavity aspect ratio) and of the free boundary (length of the open part of the boundary) on the velocity field is investigated numerically. The simulations are carried out for different values of the gas Reynolds numbers. The characteristic values for the flow parameters as well as geometrical characteristics described in this paper are motivated by the main features of the CIMEX-1 experiments prepared for the International Space Station. The paper presents examples of the fluid flow structure in the open cavities and conclusions.     

An examination of the flow induced by the motion of many buoyant bubbles

The results of direct numerical simulations of the motion of many three-dimensional buoyant bubbles in periodic domains are examined. The bubble motion is computed by solving the full Navier-Stokes equations by a parallelized finite difference/front tracking method that allows a fully deformable interface between the bubbles and the ambient fluid and the inclusion of surface tension. The governing parameters are selected such that the average rise Reynolds number is about 25. Two cases are examined. In one, the bubbles are nearly spherical; in the other, the bubbles rise with an ellipsoidal shape. The ellipsoidal bubbles show a much larger fluctuation velocity and by visualizing the flow field it is possible to show that the difference is due to larger vorticity generation and stronger interactions of the deformable bubbles. The focus here is on the early stage of the flow, when both the spherical and the deformable bubbles are nearly uniformly distributed.     

Modified relaxation time Monte Carlo method for continuum-transition gas flows

Gas flows in the continuum-transition regime often occur in micro-electro-mechanical systems. The relaxation time Monte Carlo (RTMC) method was modified by using an ellipsoid statistical model and a multiple translational temperature model in the BGK model equation to simulate continuum-transition gas flows. The modified RTMC method uses a simplified form of the generalized relaxation time, which is related to the macro velocity and the local Knudsen number. The results for Couette flow and Poiseuille flow in microchannels predicted using the modified RTMC and the DSMC are in good agreement with the modified RTMC being much faster than the DSMC for continuum-transition gas flow simulations.     

Strain softening in stretched DNA

The microscopic mechanics of DNA stretching was characterized using extensive molecular dynamics simulations. By employing an anisotropic pressure-control method, realistic force-extension dependences of effectively infinite DNA molecules were obtained. A coexistence of B and S DNA domains was observed during the overstretching transition. The simulations revealed that strain softening may occur in the process of stretching torsionally constrained DNA. The latter observation was qualitatively reconciled with available experimental data using a random-field Ising model.     

Computational bluff body aerodynamic noise prediction using a statistical approach

A new method for calculating the aerodynamic noise generated by bluff bodies is presented in this paper. The methodology uses two-dimensional, unsteady Reynolds averaged Navier Stokes turbulent flow simulations to calculate the acoustic source terms. To account for turbulent flow effects that are not resolved by the flow simulation, a statistical approach has been developed and applied to introduce narrow band random noise. Spanwise de-correlation of flow information is accounted for using a correction method based on a de-correlation length scale. Curle’s compact acoustic analogy is used to calculate the far-field noise. To illustrate the effectiveness of the method, the turbulent flow and noise about two test cases are calculated and compared with experimental results from the literature.     

Lattice-Boltzmann accuracy in pore-scale flow simulation

We investigate the possibility of using nominally second-order-accurate techniques for resolving flow about solid boundaries as a means of improving accuracy and reducing grid resolution requirements in pore-scale simulations. An LBGK method is used to calculate flow in several geometries of increasing complexity, using a first-order accurate and two nominally second-order-accurate methods for no-slip boundaries. The geometries include uniform flow past an isolated sphere, quadratic flow past a sphere near a wall, flow through a BCC array of spheres, and through a randomly packed bed of spheres. The packed bed flows are also used to compare hydrodynamic dispersion results. The results confirm second-order-accurate behavior where Navier–Stokes flows are clearly developed. However 3D pore-scale simulations involve a trade-off between resolution of the flow and the number of pore spaces, and there is a resolution threshold, below which certain flow features, such as recirculation, are not resolved. We conjecture that most simulations will tend to operate near this threshold because of the competing demands for resolution and statistical accuracy. We consider local flow features and the velocity distribution, in addition to hydraulic permeability and drag, to provide a fuller understanding of accuracy near this threshold.     

A front-tracking/ghost-fluid method for fluid interfaces in compressible flows

A front-tracking/ghost-fluid method is introduced for simulations of fluid interfaces in compressible flows. The new method captures fluid interfaces using explicit front-tracking and defines interface conditions with the ghost-fluid method. Several examples of multiphase flow simulations, including a shock–bubble interaction, the Richtmyer–Meshkov instability, the Rayleigh–Taylor instability, the collapse of an air bubble in water and the breakup of a water drop in air, using the Euler or the Navier–Stokes equations, are performed in order to demonstrate the accuracy and capability of the new method. The computational results are compared with experiments and earlier computational studies. The results show that the new method can simulate interface dynamics accurately, including the effect of surface tension. Results for compressible gas–water systems show that the new method can be used for simulations of fluid interface with large density differences.     

A parallel Vlasov solver based on local cubic spline interpolation on patches

A method for computing the numerical solution of Vlasov type equations on massively parallel computers is presented. In contrast with Particle In Cell methods which are known to be noisy, the method is based on a semi-Lagrangian algorithm that approaches the Vlasov equation on a grid of phase space. As this kind of method requires a huge computational effort, the simulations are carried out on parallel machines. To that purpose, we present a local cubic splines interpolation method based on a domain decomposition, e.g. devoted to a processor. Hermite boundary conditions between the domains, using ad hoc reconstruction of the derivatives, provide a good approximation of the global solution. The method is applied on various physical configurations which show the ability of the numerical scheme.