599 research outputs found
Lattice Boltzmann method for computational aeroacoustics on non-uniform meshes: a direct grid coupling approach
The present study proposes a highly accurate lattice Boltzmann direct
coupling cell-vertex algorithm, well suited for industrial purposes, making it
highly valuable for aeroacoustic applications. It is indeed known that the
convection of vortical structures across a grid refinement interface, where
cell size is abruptly doubled, is likely to generate spurious noise that may
corrupt the solution over the whole computational domain. This issue becomes
critical in the case of aeroacoustic simulations, where accurate pressure
estimations are of paramount importance. Consequently, any interfering noise
that may pollute the acoustic predictions must be reduced.
The proposed grid refinement algorithm differs from conventionally used ones,
in which an overlapping mesh layer is considered. Instead, it provides a direct
connection allowing a tighter link between fine and coarse grids, especially
with the use of a coherent equilibrium function shared by both grids. Moreover,
the direct coupling makes the algorithm more local and prevents the duplication
of points, which might be detrimental for massive parallelization. This work
follows our first study (Astoul~\textit{et al. 2020}) on the deleterious effect
of non-hydrodynamic modes crossing mesh transitions, which can be addressed
using an appropriate collision model. The Hybrid Recursive Regularized model is
then used for this study. The grid coupling algorithm is assessed and compared
to a widely-used cell-vertex algorithm on an acoustic pulse test case, a
convected vortex and a turbulent circular cylinder wake flow at high Reynolds
number.Comment: also submitted to Journal of Computational Physic
Large Eddy Simulation of Turbulent Flows Using the Lattice Boltzmann Method
Turbulent flow is a complex fluid phenomenon because of its disordered and chaotic flow patterns. Analysis of such flows presents practical significance and is widely performed using either experiments or simulations. The numerical simulation, or computational fluid dynamics (CFD) is one powerful technique; traditionally, it is based on the Navier-Stokes equations. A novel numerical approach called the lattice Boltzmann method (LBM) has developed quickly over the past decades, and this method is based on an entirely different mechanism. The current thesis seeks to present an investigation of turbulent flows that was performed using the LBM
Development of Immersed Boundary-Phase Field-Lattice Boltzmann Method for Solid Multiphase Flow Interactions
Ph.DDOCTOR OF PHILOSOPH
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Modeling Cardiovascular Hemodynamics Using the Lattice Boltzmann Method on Massively Parallel Supercomputers
Accurate and reliable modeling of cardiovascular hemodynamics has the potential to improve understanding of the localization and progression of heart diseases, which are currently the most common cause of death in Western countries. However, building a detailed, realistic model of human blood flow is a formidable mathematical and computational challenge. The simulation must combine the motion of the fluid, the intricate geometry of the blood vessels, continual changes in flow and pressure driven by the heartbeat, and the behavior of suspended bodies such as red blood cells. Such simulations can provide insight into factors like endothelial shear stress that act as triggers for the complex biomechanical events that can lead to atherosclerotic pathologies. Currently, it is not possible to measure endothelial shear stress in vivo, making these simulations a crucial component to understanding and potentially predicting the progression of cardiovascular disease. In this thesis, an approach for efficiently modeling the fluid movement coupled to the cell dynamics in real-patient geometries while accounting for the additional force from the expansion and contraction of the heart will be presented and examined. First, a novel method to couple a mesoscopic lattice Boltzmann fluid model to the microscopic molecular dynamics model of cell movement is elucidated. A treatment of red blood cells as extended structures, a method to handle highly irregular geometries through topology driven graph partitioning, and an efficient molecular dynamics load balancing scheme are introduced. These result in a large-scale simulation of the cardiovascular system, with a realistic description of the complex human arterial geometry, from centimeters down to the spatial resolution of red-blood cells. The computational methods developed to enable scaling of the application to 294,912 processors are discussed, thus empowering the simulation of a full heartbeat. Second, further extensions to enable the modeling of fluids in vessels with smaller diameters and a method for introducing the deformational forces exerted on the arterial flows from the movement of the heart by borrowing concepts from cosmodynamics are presented. These additional forces have a great impact on the endothelial shear stress. Third, the fluid model is extended to not only recover Navier-Stokes hydrodynamics, but also a wider range of Knudsen numbers, which is especially important in micro- and nano-scale flows. The tradeoffs of many optimizations methods such as the use of deep halo level ghost cells that, alongside hybrid programming models, reduce the impact of such higher-order models and enable efficient modeling of extreme regimes of computational fluid dynamics are discussed. Fourth, the extension of these models to other research questions like clogging in microfluidic devices and determining the severity of co-arctation of the aorta is presented. Through this work, a validation of these methods by taking real patient data and the measured pressure value before the narrowing of the aorta and predicting the pressure drop across the co-arctation is shown. Comparison with the measured pressure drop in vivo highlights the accuracy and potential impact of such patient specific simulations. Finally, a method to enable the simulation of longer trajectories in time by discretizing both spatially and temporally is presented. In this method, a serial coarse iterator is used to initialize data at discrete time steps for a fine model that runs in parallel. This coarse solver is based on a larger time step and typically a coarser discretization in space. Iterative refinement enables the compute-intensive fine iterator to be modeled with temporal parallelization. The algorithm consists of a series of prediction-corrector iterations completing when the results have converged within a certain tolerance. Combined, these developments allow large fluid models to be simulated for longer time durations than previously possible.Engineering and Applied Science
Modeling Electrokinetic Flows with the Discrete Ion Stochastic Continuum Overdamped Solvent Algorithm
In this article we develop an algorithm for the efficient simulation of
electrolytes in the presence of physical boundaries. In previous work the
Discrete Ion Stochastic Continuum Overdamped Solvent (DISCOS) algorithm was
derived for triply periodic domains, and was validated through ion-ion pair
correlation functions and Debye-H{\"u}ckel-Onsager theory for conductivity,
including the Wien effect for strong electric fields. In extending this
approach to include an accurate treatment of physical boundaries we must
address several important issues. First, the modifications to the spreading and
interpolation operators necessary to incorporate interactions of the ions with
the boundary are described. Next we discuss the modifications to the
electrostatic solver to handle the influence of charges near either a fixed
potential or dielectric boundary. An additional short-ranged potential is also
introduced to represent interaction of the ions with a solid wall. Finally, the
dry diffusion term is modified to account for the reduced mobility of ions near
a boundary, which introduces an additional stochastic drift correction. Several
validation tests are presented confirming the correct equilibrium distribution
of ions in a channel. Additionally, the methodology is demonstrated using
electro-osmosis and induced charge electro-osmosis, with comparison made to
theory and other numerical methods. Notably, the DISCOS approach achieves
greater accuracy than a continuum electrostatic simulation method. We also
examine the effect of under-resolving hydrodynamic effects using a `dry
diffusion' approach, and find that considerable computational speedup can be
achieved with a negligible impact on accuracy.Comment: 27 pages, 15 figure
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