基于格子Boltzmann方法和大涡模拟的颈动脉分叉狭窄流动并行计算

颈动脉斑块的形成与复杂的血流动力学因素密切相关,血液流动状况的精确模拟对颈动脉斑块的临床诊断具有重要意义。为了精确模拟脉动流场,在格子Boltzmann方法（LBM）的基础上,添加大涡模拟（LES）模型,建立了LBM-LES颈动脉模拟算法。利用医学图像重构软件,建立颈动脉狭窄真实几何模型,对颈动脉狭窄脉动流动进行了数值模拟,通过计算血液流动速度、壁面剪切应力（WSS）等,得出了有意义的流动结果,验证了LBM-LES对颈动脉狭窄后段血液流动研究的有效性。基于OpenMP编程环境,在高性能集群机全互联胖节点上进行了千万量级网格的并行计算,结果表明LBM-LES颈动脉模拟算法具有较好的并行性能。     

A parallel lattice Boltzmann method for large eddy simulation on multiple GPUs

To improve the simulation efficiency of turbulent fluid flows at high Reynolds numbers with large eddy dynamics, a CUDA-based simulation solution of lattice Boltzmann method for large eddy simulation (LES) using multiple graphics processing units (GPUs) is proposed. Our solution adopts the “collision after propagation” lattice evolution way and puts the misaligned propagation phase at global memory read process. The latest GPU platform allows a single CPU thread to control up to four GPUs that run in parallel. In order to make use of multiple GPUs, the whole working set is evenly partitioned into sub-domains. We implement Smagorinsky model and Vreman model respectively to verify our multi-GPU solution. These two LES models have different relaxation time calculation behavior and lead to different CUDA implementation characteristics. The implementation based on Smagorinsky model achieves 190 times speedup over the sequential implementation on CPU, while the implementation based on Vreman model archives more than 90 times speedup. The experimental results show that the parallel performance of our multi-GPU solution scales very well on multiple GPUs. Therefore large-scale (up to 10,240 $\times $ 10,240 lattices) LES–LBM simulation becomes possible at a low cost, even using double-precision floating point calculation.     

Lattice Boltzmann simulation of the dispersion of aggregated particles under shear flows

The lattice Boltzmann method (LBM) for multicomponent immiscible fluids is applied to simulations of the deformation and breakup of a particle-cluster aggregate in shear flows. In the simulations, the solid particle is modeled by a droplet with strong interfacial tension and large viscosity. The van der Waals attraction force is taken into account for the interaction between the particles. The ratio of the hydrodynamic drag force to cohesive force, I, is introduced, and the effect of I on the aggregate deformation and breakup in shear flows is investigated. It is found that the aggregate is easier to deform and to be dispersed when I is over 100.     

Lattice Boltzmann simulations of impedance tube flows

An original time-domain surface acoustic impedance condition for Lattice Boltzmann methods has been developed. The basis for this method is the extension proposed by Delattre et al. [Delattre G, Manoha E, Redonnet S, Sagaut P. Time-domain simulation of sound absorption on curved wall. 13th AIAA/CEAS Aeroacoustics conference, Rome, Italy, AIAA-2007-3493; 2007] of the z-transform approach suggested by Özyörük et al. [Özyörük Y, Long LN, Jones M. Time-domain numerical simulation of a flow impedance tube. J Comput Phys 1998;146:29-57]. Using this boundary condition that links the normal velocity and the pressure, the basic idea consists in calculating the Lattice Boltzmann populations at a boundary node thanks to the gradients of the fluid velocity. This paper describes the proposed LBM boundary conditions and its assessment on the NASA Langley flow-impedance tube with a constant depth ceramic tubular liner. We performed both single and broadband-frequency simulations, without mean flow and with sheared mean flows. Excellent agreement is shown with both experimental data and other simulation results at various frequencies up to a Mach number equal to 0.5.     

A lattice Boltzmann approach for free-surface-flow simulations on non-uniform block-structured grids

In this paper, we present a hybrid volume-of-fluid-based algorithm for the simulation of free-surface-flow problems. For the solution of the flow field, the lattice Boltzmann method is used. The additional advection equation for the volume-of-fluid (VOF) fill level is discretized with a classical finite volume method. For the interface reconstruction, a piecewise linear interface reconstruction in 3D has been implemented. The free-surface-tracking algorithm is embedded into the 3D, non-uniform, lattice-Boltzmann-based solver VirtualFluids; Freudiger et al. (2009) [1], Freudiger (2009) [2]. The advection algorithm is verified and validated with well-known advection test cases. For the validation of the free-surface algorithm, we present simulations of a breaking-dam benchmark.     

Multi-thread implementations of the lattice Boltzmann method on non-uniform grids for CPUs and GPUs

Two multi-thread based parallel implementations of the lattice Boltzmann method for non-uniform grids on different hardware platforms are compared in this paper: a multi-core CPU implementation and an implementation on General Purpose Graphics Processing Units (GPGPU). Both codes employ second order accurate compact interpolation at the interfaces, coupling grids of different resolutions. Since the compact interpolation technique is both simple and accurate, it produces almost no computational overhead as compared to the lattice Boltzmann method for uniform grids in terms of node updates per second. To the best of our knowledge, the current paper presents the first study on multi-core parallelization of the lattice Boltzmann method with inhomogeneous grid spacing and nested time stepping for both CPUs and GPUs.     

Lattice Boltzmann simulation of cerebral artery hemodynamics

A hemodynamics analysis approach that combines the level-set method for medical imaging processing and the Lattice Boltzmann method for flow simulation with patient-specific cerebral vasculature geometry is presented. The flow solver is validated by simulating a bent duct flow and is then applied to investigate blood flow in actual cerebral artery models. It is demonstrated that this approach is effective in studying complex hemodynamic flows.     

An immersed boundary method on Cartesian adaptive grids for the simulation of compressible flows around arbitrary geometries

In this article, we present an immersed boundary method for the simulation of compressible flows of complex geometries encountered in aerodynamics. The immersed boundary methods allow the mesh not to conform to obstacles, whose influence is taken into account by modifying the governing equations locally (either by a source term within the equation or by imposing the flow variables or fluxes locally, similarly to a boundary condition). A main feature of the approach which we propose is that it relies on structured Cartesian grids in combination with a dedicated HPC Cartesian solver, taking advantage of their low memory and CPU time requirements but also the automation of the mesh generation and adaptation. Turbulent flow simulations are performed by solving the Reynolds-averaged Navier–Stokes equations or by a Large-Eddy simulation approach, in combination with a wall function at high Reynolds number, to mitigate the cell count resulting from the isotropic nature of Cartesian cells. The objective of this paper is to demonstrate that this automatic workflow is fast and robust and enables to get quantitative aerodynamics results on geometrically complex configurations. Results obtained are in good agreement with classical body-fitted approaches but with a significant reduction of the time of the whole process, that is a day for RANS simulations, including the mesh generation.

     

基于3D-LBM的多相流快速模拟

对于流体和其他物体的交互, 提出了一种基于Lattice Boltzmann的建模和绘制方法。针对固、液交互提出了外力叠加机制, 考虑了障碍物对流体的单向作用; 在液、液交互时考虑了两种液体的相互作用力。采用GPU硬件加速技术对LBM算法进行了加速, 并采用基于屏幕空间的绘制技术对流体表面进行了绘制。实现了两种不相溶液体交互, 以及液体与固体交互场景的模拟。     

Stable free surface flows with the lattice Boltzmann method on adaptively coarsened grids

In this paper we will present an algorithm to perform free surface flow simulations with the lattice Boltzmann method on adaptive grids. This reduces the required computational time by more than a factor of three for simulations with large volumes of fluid. To achieve this, the simulation of large fluid regions is performed with coarser grid resolutions. We have developed a set of rules to dynamically adapt the coarse regions to the movement of the free surface, while ensuring the consistency of all grids. Furthermore, the free surface treatment is combined with a Smagorinsky turbulence model and a technique for adaptive time steps to ensure stable simulations. The method is validated by comparing the position of the free surface with an uncoarsened simulation. It yields speedup factors of up to 3.85 for a simulation with a resolution of 480³ cells and three coarser grid levels, and thus enables efficient and stable simulations of free surface flows, e.g. for highly detailed physically based animations of fluids.     

Lattice Boltzmann method simulation of electroosmotic stirring in a microscale cavity

The suitable surface modification of microfluidic channels can enable a neutral electrolyte solution to develop an electric double layer (EDL). The ions contained within the EDL can be moved by applying an external electric field, inducing electroosmotic flows (EOFs) that results in associated stirring. This provides a solution for the rapid mixing required for many microfluidic applications. We have investigated EOFs generated by applying a steady electric field across a square cavity that has homogenous electric potentials along its walls. The flowfield is simulated using the lattice Boltzmann method. The extent of mixing is characterized for different electrode configurations and electric field strengths. We find that rapid mixing can be achieved by using this simple configuration which increases with increasing electric field strength. The mixing time for water-soluble organic molecules can be decreased by four orders of magnitude by suitable choice of wall zeta potential and electric field. We dedicate this paper to the memory of our colleagues Professors Kevin Granata and Liviu Librescu who fell tragically on April 16, 2007 while answering their call to serve higher education. They continue to inspire us. AM gratefully acknowledges support from Jadavpur University under the World Bank funded Technical Education Quality Improvement Programme of the Government of India and the hospitality of the Virginia Tech ESM Department where he conducted a portion of this work.     

Lattice Boltzmann modeling of microchannel flows in the transition flow regime

Owing to its kinetic nature and distinctive computational features, the lattice Boltzmann method for simulating rarefied gas flows has attracted significant research interest in recent years. In this article, a lattice Boltzmann (LB) model is presented to study microchannel flows in the transition flow regime, which have gained much attention because of fundamental scientific issues and technological applications in various micro-electro-mechanical system (MEMS) devices. In the model, a Bosanquet-type effective viscosity is used to account for the rarefaction effect on gas viscosity. To match the introduced effective viscosity and to gain an accurate simulation, a modified second-order slip boundary condition with a new set of slip coefficients is proposed. Numerical investigations demonstrate that the results, including the velocity profile, the non-linear pressure distribution along the channel, and the mass flow rate, are in good agreement with the solution of the linearized Boltzmann equation, the direct simulation Monte Carlo (DSMC) results, and the experimental results over a broad range of Knudsen numbers. It is shown that taking the rarefaction effect on gas viscosity into consideration and employing an appropriate slip boundary condition can lead to a significant improvement in the modeling of rarefied gas flows with moderate Knudsen numbers in the transition flow regime.     

Large eddy simulation of pulsating flow over a circular cylinder at subcritical Reynolds number

The pulsating cross-flow over a single circular cylinder at the subcritical Reynolds number Re_D = 2580 is studied with the large eddy simulation (LES) technique using the standard Smagorinsky model as well as a dynamic model in which the test filtered quantities are evaluated through a truncated Taylor series expansion. The filtered equations are discretised using the finite volume method in an unstructured, collocated grid arrangement with a second-order accurate method, in both space and time. The predictions are compared against very detailed experiments for mean velocities and Reynolds stresses that were performed in a duct of cross-section 72 mm × 72 mm using the PIV technique. The effects of mesh refinement close to the cylinder as well as of subgrid scale model are also examined. The numerical predictions are in very good agreement with the measurements in terms of mean as well as turbulence quantities. The instantaneous flow patterns of the flow field are examined and the effect of the external flow pulsation on the wake characteristics such as vortex formation length, vortex strength, Strouhal number as well as the lift and drag coefficients is quantified. The vortex formation length is decreased while the mean drag, as well as the rms values of the drag and lift coefficients increase significantly under pulsating flow conditions. The performance of the LES technique is analysed in the light of the wake characteristics.     

Lattice Boltzmann simulation of gas flow over micro-scale airfoils

In order to understand aerodynamic issues related to design and performance of micro air aircraft, in this paper isothermal low Reynolds number flows over micro-scale airfoils are simulated using the kinetic lattice Boltzmann method [Niu et al., Phys. Rev. E 76 (2007) 036711 ]. The sample of the micro-scale airfoil investigated is a flat plate with a 5% thickness ratio. Investigation shows that low Reynolds number flows over the micro-scale airfoils are viscous and compressible, and that rarefied effects in these kinds of flows are dominant. It is also found that the lift coefficients of the micro-scale plate airfoils are always smaller than the drag coefficients of them at Reynolds numbers less than 100, and this observation is consistent with the previous studies.     

Lattice Boltzmann simulation of solution chemistry for crevice corrosion

We apply the two-dimensional lattice Boltzmann method (2D LBM) to the simulation of solution chemistry for crevice corrosion. The 2D distributions of pH and electrical potential are obtained by the numerical simulation. The critical value of pH which brings about the rapid crevice corrosion and the incubation period until the solution reaches this pH are estimated from the simulation results. It is found that the estimated pH and incubation period are in nearly agreement with experiment.     

Lattice Boltzmann simulation of lid-driven flow in trapezoidal cavities

In this paper, an incompressible lattice Bhatnagar–Gross–Krook (LBGK) model proposed by Guo et al. is used to simulate lid-driven flow in a two-dimensional isosceles trapezoidal cavity. Due to the complex boundary of the trapezoidal cavity, here the extrapolation scheme proposed by Guo et al. is used to treat curved boundary. In our numerical simulations, the effects of the Reynolds number (Re) and the top angle θ on the strength, center position and number of vortices in the isosceles trapezoidal cavities are studied. Re is varied from 100 to 15,000, and the top angle θ ranges from 50 to 90. Numerical results show that, as Re increases, the phenomena in the cavity become more and more complex, and the number of the vortexes increases. We also found that the vortex near the bottom wall breaks up into two smaller vortices as θ increases up to a critical value. Furthermore, as Re is increased, the flow in the cavity undergoes a complex transition (from steady to the periodic flow, and finally to the chaotic flow). At last, the scope of critical Re for flow transition from steady to periodic state, and from periodic to chaotic state is presented for different top angles θ.     

Lattice Boltzmann simulation of lid-driven flow in deep cavities

In this paper, the lattice Boltzmann equation (LBE) method is applied for simulation of lid-driven flow in a two-dimensional, rectangular, deep cavity. First, the code is validated for the standard square cavity, and then the results of a deep cavity are presented. Steady results are presented for deep cavities with aspect ratios of 1.5–4, and Reynolds numbers of 50–3200. Several features of the flow, such as the location and strength of the primary vortex, and the corner-eddy dynamics are investigated and compared with previous findings from experiments and theory. Steady results for deep cavities show the existence of corner eddies at the bottom, which coalesce to form a second primary-eddy as the cavity aspect-ratio is increased above a critical value. However, at relatively high Reynolds numbers, the second primary-eddy is formed via a rapid transition of an unsteady wall-eddy. The predicted results from LBE simulations are shown to be consistent with experiments and theory.     

Large eddy simulation of turbulent open duct flow using a lattice Boltzmann approach

Large eddy simulations of turbulent open duct flow are performed using the lattice Boltzmann method (LBM) in conjunction with the Smagorinsky sub-grid scale (SGS) model. A smaller value of the Smagorinsky constant than the usually used one in plain channel flow simulations is used. Results for the mean flow and turbulent fluctuations are compared to experimental data obtained in an open duct of similar dimensions. It is found that the LBM simulation results are in good qualitative agreement with the experiments.