Xinhai Xu

Non-equilibrium steady states of entangled polymer mixtures under shear flow

By solving the full equations of an extended two-fluid model in two dimensions, we give the first numerical study revealing non-equilibrium steady states in sheared entangled polymer mixtures. This research provides answers for some fundamental questions in sheared binary mixtures of entangled polymers. Our results reveal that non-equilibrium steady states with finite domain size do exist, and apparent scaling exponents L ∥ ~ γ · − 1 . 05 and L ⊥ ~ γ · − 1 are found over six decades of shear rate. Since the wall effects get involved in our simulations, the dependence of average domain size on system size cannot be strictly eliminated. In addition, as an obvious influence of viscoelasticity, the polymer viscosity η p appears to induce linear translation of the fitted lines. Through two-dimensional numerical simulations, we show the detailed dynamic evolution of microstructure in binary polymer mixtures with asymmetric composition under shear flow. It is found that the phase patterns are significantly different from symmetric fluids studied previously. Finally, we also identify the importance of wall effects and confirm the irreplaceable role of inertia for a non-equilibrium steady state.

Multi-scale simulation of non-equilibrium phase transitions under shear flow in dilute polymer solutions

The phase transition of complex fluids is intrinsically a multi-scale problem. In this paper we propose a multi-scale two-fluid model that couples a coarse-grained microscopic method to the two-fluid framework for studying multi-phase fluids under shear flow. In this model the macroscopic viscoelastic stress is calculated by tracking the massive microscopic Brownian configuration fields in the simulation box. Both the macroscopic and microscopic equations are solved using a modified PISO iterative algorithm based on a finite volume discretization scheme. Our 2D numerical results reproduce numerous dynamic phenomena reported in literature and show that the theoretical model presented here could be a promising multi-scale approach to numerically study multi-phase viscoelastic fluids under flow.

Coupling Strategies Investigation of Hybrid Atomistic-Continuum Method Based on State Variable Coupling

Different configurations of coupling strategies influence greatly the accuracy and convergence of the simulation results in the hybrid atomistic-continuum method. This study aims to quantitatively investigate this effect and offer the guidance on how to choose the proper configuration of coupling strategies in the hybrid atomistic-continuum method. We first propose a hybrid molecular dynamics- (MD-) continuum solver in LAMMPS and OpenFOAM that exchanges state variables between the atomistic region and the continuum region and evaluate different configurations of coupling strategies using the sudden start Couette flow, aiming to find the preferable configuration that delivers better accuracy and efficiency. The major findings are as follows: the region plays the most important role in the overlap region and the “4-layer-1” combination achieves the best precision with a fixed width of the overlap region; the data exchanging operation only needs a few sampling points closer to the occasions of interactions and decreasing the coupling exchange operations can reduce the computational load with acceptable errors; the nonperiodic boundary force model with a smoothing parameter of 0.1 and a finer parameter of 20 can not only achieve the minimum disturbance near the MD-continuum interface but also keep the simulation precision.

A New Method to Simulate Free Surface Flows for Viscoelastic Fluid

Free surface flows arise in a variety of engineering applications. To predict the dynamic characteristics of such problems, specific numerical methods are required to accurately capture the shape of free surface. This paper proposed a new method which combined the Arbitrary Lagrangian-Eulerian (ALE) technique with the Finite Volume Method (FVM) to simulate the time-dependent viscoelastic free surface flows. Based on an open source CFD toolbox called OpenFOAM, we designed an ALE-FVM free surface simulation platform. In the meantime, the die-swell flow had been investigated with our proposed platform to make a further analysis of free surface phenomenon. The results validated the correctness and effectiveness of the proposed method for free surface simulation in both Newtonian fluid and viscoelastic fluid.

WI-USHER: A grid-based parallel algorithm for particle insertion in hybrid atomistic-continuum method

The hybrid atomistic-continuum coupling method based on domain decomposition serves as an important tool for the micro-fluid simulation. There exists a certain degree of parallelism load imbalance when directly using the USHER algorithm in the domain decomposition–based hybrid atomistic-continuum coupling method. In this article, we propose a grid-based parallel algorithm for particle insertion, named WI-USHER, to improve the efficiency of the particle insertion operation when restricting the size of the region to be inserted or with higher number density. The WI-USHER algorithm slices the region to be inserted into finer grids with proper spacing scale, marks parts of finer grids in black according to three exclusive rules, that is, Single Particle Occupation (SPO), Single Particle Coverage (SPC), and Multi-Particles Coverage (MPC), and finds the target insertion point in the remained white grids. We use two test cases to show the superiority of our WI-USHER algorithm over the USHER algorithm. The WI-USHER algorithm performs lower averaged force evaluation times, which decreases from O ( 10 4 ) to O ( 10 3 ) compared to the USHER algorithm when the number density of slightly high to high value. The percentage of the total parallel simulation time processed by the particle insertion operation decreases from 23.5% to 3% compared to the USHER algorithm.

Role of Nonmonotonic Constitutive Curves in Extrusion Instabilities

Flow instabilities of non-Newtonian fluids severely hamper the quality of products during various chemical processes, such as fibre spinning, extrusion, and film blowing. The origin of extrusion instability has been studied over many decades. However, no consensus has been reached among the research community so far. In this paper, the possible cause of extrusion instabilities is explored using the finitely extensible nonlinear elastic conformation-dependent (FENE-CD) model with a nonmonotonic constitutive curve. Many well-documented experimental phenomena are reproduced in our simulations, and it could be concluded that the nonmonotonic constitutive curve plays an essential role in extrusion instabilities. In addition, the results imply that the die exit singularity may generate or magnify oscillations.

A Hybrid Decomposition Parallel Algorithm for Multi-scale Simulation of Viscoelastic Fluids

The method of Brownian configuration fields (BCF) is a promising multi-scale approach for the simulationof viscoelastic fluids, however, it is a computationally expensive method, which restricts its application in complex scenarios. Therefore, it is of great importance to optimize the parallel implementation in order to improve computational efficiency. In this paper we propose a hybrid decomposition parallel algorithm named: MCDPar, whichenables the simulation problem to bedecomposed simultaneously over mesh cells andthe Brownian configuration fields. Compution processes are split into multiple groups and Brownian configuration fields are equally associated with these groups. Meanwhile, within each group the processes are concurrently executed based on the traditional mesh decomposition approach. Finally we implemented the MCDPar algorithm in a micro-macro numerical solver based on OpenFOAM. Experimental results show that the micro-macro simulation time of viscoelastic fluids issignificantly reduced with improved scalability and parallel efficiency. In the test case with Nf = 2000 and Ncell = 262144, the speedup of the MCDParis up to 9.23x with a 7.5x increase in number of cores compared to the original parallel algorithm.

The way to develop software towards exascale computing

This paper preliminarily discusses how to develop software towards exascale computing. Two typical development models are studied, one is “all-round contract” which means all the functional modules are implemented in a single framework, and the other is “co-design” which using the existing packages to realize functional modules. As a widely used CFD software whose numerical solving module is implemented in a typical “all-round contract” manner, OpenFOAM is concerned in this work. After redesigning and reimplementing the solving module of OpenFOAM in a “co-design” way by inserting PETSc, the source lines of the code decrease dramatically, which improves the development efficiency. Tests of two CFD benchmark cases and a practical large-scale case on a 128-node cluster show that the newly implemented numerical module also has a higher solving efficiency, compared with the original numerical module in OpenFOAM. Therefore, it is recommended to develop software in a “co-design” manner towards exascale computing, and softwares already implemented in “all-round contract” way should also be reconsidered.

A multi-user performance analysis framework for CFD simulations

This paper proposes a multi-user performance analysis framework for computational fluid dynamics (CFD) simulations. Independent performance datasets are given according to different requirements of computer programmers, algorithm developers and application developers. The performance analysis framework is designed based on the general procedure of CFD processing: pre-processing, problem solving and post-processing. Optimisation of the data acquisition is carried out based on data dependence to realise lightweight. A profiler is implemented in OpenFOAM based on the framework, and experiments are conducted to verify the performance information extraction and analysis. The results on a subsystem of Tianhe-1A indicate that the multiuser performance tool can probe the performance information successfully during the parallel execution. And from the performance results, we found that the direction of mesh partition turns out to be a critical factor of simulation performance for CFD problems.

The Curve Boundary Design and Performance Analysis for DGM Based on OpenFOAM

OpenFOAM is a widely used numerical simulation software, and Discontinuous Galerkin method (DGM), a high-order numerical method, has been developed on OpenFOAM. In order to obtain meaningful numerical simulations, curve boundary is needed, but it has not been implemented on OpenFOAM. In this paper, based on codeStream function of original OpenFOAM, we design and implement curve boundary interface with reference to the interface of original OpenFOAM, so that users can use C++ code to describe curve boundary. Furthermore, in order to move the high-order points on the linear boundary to the curve boundary, we propose an algorithm to move each high-order point to a specific position on the curve, where the normal of this position passes through the origin point. Experimental results based on the flow around a cylinder show that curve boundary is needed by DGM numerical simulation, and DGM high-order simulation is much more efficient than DGM low-order. Typically, when the error of drag coefficient is about 0.03, the DGM high-order can save \(89.6\%\) time cost and \(83.0\%\) memory cost.