34 research outputs found

    A study of particles-flow interactions based on the numerical solution of the Boltzmann equation

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    A study of particles-flow interactions based on the numerical solution of the Boltzmann equation

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    Advances in Heat and Mass Transfer in Micro/Nano Systems

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    The miniaturization of components in mechanical and electronic equipment has been the driving force for the fast development of micro/nanosystems. Heat and mass transfer are crucial processes in such systems, and they have attracted great interest in recent years. Tremendous effort, in terms of theoretical analyses, experimental measurements, numerical simulation, and practical applications, has been devoted to improve our understanding of complex heat and mass transfer processes and behaviors in such micro/nanosystems. This Special Issue is dedicated to showcasing recent advances in heat and mass transfer in micro- and nanosystems, with particular focus on the development of new models and theories, the employment of new experimental techniques, the adoption of new computational methods, and the design of novel micro/nanodevices. Thirteen articles have been published after peer-review evaluations, and these articles cover a wide spectrum of active research in the frontiers of micro/nanosystems

    On the entropic property of the Ellipsoidal Statistical model with the Prandtl number below 2/3

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    Entropic property of the Ellipsoidal Statistical model with the Prandtl number Pr below 2/3 is discussed. Although 2/3 is the lower bound of Pr for the H theorem to hold unconditionally, it is shown that the theorem still holds even for Pr<2/3\mathrm{Pr}<2/3, provided that anisotropy of stress tensor satisfies a certain criterion. The practical tolerance of that criterion is assessed numerically by the strong normal shock wave and the Couette flow problems. A couple of moving plate tests are also conducted

    Investigation of Lattice Boltzmann Methods applied to multiphase flows

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    The purpose of this thesis is the study of Lattice Boltzmann Methods (LBM), applied to multiphase flows. First, general principles of statistical physics and of Lattice Boltzmann Methods are introduced, followed by a historical review about Lattice Gas Automata. A state of the art of the multiphase flow simulation methods is then proposed, with a particular emphasize on diffuse interface methods. In particular, the phase field methods are introduced, and different methods allowing to numerically simulate surface tension are also presented. A second review concerning multiphase flow simulation in a Lattice Boltzmann framework is presented. More precisely, general principals are presented, and the four major methods, Color Gradient, Pseudo-Potential, Free Energy and HCZ, are successively presented. Lattice Boltzmann Methods advanced notions are then introduced, in particular, a Taylor expansion based method that allows to determine Lattice Boltzmann schemes equivalent macroscopic equation is described. A Gradient Color method theoretical study is proposed. First, an original reformulation of the algorithm allowing an improvement in computational efficiency is proposed. The Taylor expansion method is then applied to Gradient Color Method in order to determine the high order error induced by the numerical scheme. This expression allows to demonstrate how the degree of isotropy is essential to the scheme numerical stability. In particular, a numerical operator allowing to introduce an equation of states that differs from the athermal perfect gas equation is proposed. This operator efficiency is illustrated by being applied to academical testcases. The Taylor expansion method is also applied in order to show how the Color Gradient Method allows to solve an Allen-Cahn phase field equation. This theoretical result is then validated numerically. Finally, an original improved version of the Gradient Color Method is proposed. In this method, the efficient formulation and the isotropic Equation of State operator is used, and an original temporal correction term is proposed. This correction term improves the scheme numerical stability and allows to expands the method application range to higher density ratios. Finally, this method is validated through academical testcases

    Heat Transfer

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    Over the past few decades there has been a prolific increase in research and development in area of heat transfer, heat exchangers and their associated technologies. This book is a collection of current research in the above mentioned areas and describes modelling, numerical methods, simulation and information technology with modern ideas and methods to analyse and enhance heat transfer for single and multiphase systems. The topics considered include various basic concepts of heat transfer, the fundamental modes of heat transfer (namely conduction, convection and radiation), thermophysical properties, computational methodologies, control, stabilization and optimization problems, condensation, boiling and freezing, with many real-world problems and important modern applications. The book is divided in four sections : "Inverse, Stabilization and Optimization Problems", "Numerical Methods and Calculations", "Heat Transfer in Mini/Micro Systems", "Energy Transfer and Solid Materials", and each section discusses various issues, methods and applications in accordance with the subjects. The combination of fundamental approach with many important practical applications of current interest will make this book of interest to researchers, scientists, engineers and graduate students in many disciplines, who make use of mathematical modelling, inverse problems, implementation of recently developed numerical methods in this multidisciplinary field as well as to experimental and theoretical researchers in the field of heat and mass transfer

    A multi-compartmental mathematical model of the postprandial human stomach : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Anatomy and Physiology at Massey University, Palmerston North, New Zealand

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    Computational fluid dynamics of the human stomach helps to understand the gastric processes such as trituration, mixing, and transit of digesta. Their outcomes give greater insight into the design of food and orally dosed drug delivery system. Current models of gastric contractile activity are primarily limited to the gastric antrum and assume global values for the various physiological characteristics. This thesis developed a unified compartmental gastric model with correctly informed anatomical and physiological data. The gastric geometry incorporated the actions of multiple compartments, such as the gastric fundus, body, antrum, pyloric canal, proximal duodenal cap, and the small intestinal brake. Lattice-Boltzmann Method (LBM) is used to simulate the fluid dynamics within the stomach. This thesis quantified the effects of transgastric pressure gradient (TGPG) between the fundus and the duodenum, the effect of antral propagating contraction (APC) amplitude, and the viscosity of the gastric contents on gastric flow, mixing, and gastric emptying. The results of this work suggest that TGPG influences gastric emptying where as APCs do not play major role in gastric emptying. Flow rate without TGPG obtained in this work agrees with previous work (Pal et al., 2004); however, it is higher in the presence of a TGPG. Results show that APCs promote recirculation, and the amplitude of APC is vital in this regard. The 'pendulating' flow of gastric content observed in this work is reported previously in duplex sonography experiments (Hausken et al., 1992). This work quantified the gastric shear rates (0.6 - 2.0 /s). This work also suggests that the viscosity of the content influences gastric fluid dynamics. This work is a simplified first step towards a 3D gastric model. Hence, these simulation studies were performed under two simplifications: dimensionality and rheology, i.e., we have assumed a Newtonian fluid flow in 2D gastric geometry. A 3D gastric model with more rheologically realistic fluid to explore the pseudoplastic fluid dynamics within the stomach in the future is recommended

    Computational modelling of cellular blood flow in complex vascular networks.

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    Microcirculatory disorders are associated with some of the most prevailing medical conditions in modern society, e.g., cancer and cardiovascular disease (CVD). Early detection and effective treatment of these diseases require an in-depth knowledge of the changes in the haemodynamic environment preceding fatal deteriorating conditions. However, such a knowledge is difficult to obtain merely relying on experiments, on account of the overwhelming complexity of blood flow at the microscale that is sometimes beyond the capability of contemporary experimental techniques. Alternatively, computational modelling provides a potent tool to uncover the missing details of haemodynamics at the microcirculation level. Thanks to the advent of information era which fosters growingly powerful computing facilities and architectures, the progress that has been made on blood flow modelling over recent years is unprecedented. Notwithstanding, exhaustive modelling of blood flow at the microcirculation level incorporating all blood constituents remains daunting. Existing studies employing a range of models are only possible via invoking simplifications justified under different assumptions. However, one important assumption for many modelling studies, namely that blood in the microcirculation can be approximated by a homogeneous non-Newtonian fluid, has been increasingly challenged. The reason is that the microscopic behaviour of red blood cells (RBCs) as the primary blood constituent is found to non-trivially modify key rheological properties of blood flow at the microscale, such as its effective viscosity, cell-free layer (CFL) and wall shear stress (WSS). To ultimately facilitate the translation of scientific investigations to real medical applications, the cellular character of microcirculatory blood flow has to be properly considered by computational models. Bearing the above challenges in mind, the present PhD embarks on a venture to research the complex behaviour of cellular blood flow under microcirculatory conditions, capitalising on a recently developed computational tool equipped with high-level parallelisation. This computational thesis sets out to answer several important questions, ranging from the rich dynamics of individual RBCs to the collective phenomena of RBC suspensions in either microvascular networks or microfluidic mimicries. The current three-dimensional (3D) computational model is based on the lattice Boltzmann method (LBM) coupled with the immersed boundary method (IBM) for high-level resolution of discrete RBCs, which are modelled as Lagrangian membranes using the finite-element method (FEM). In the thesis, an concise introduction of the computational model is given in Chapter 4. Before applied to research projects, the model has been systematically validated against existing numerical or experimental observations. Three benchmark tests of close relevance to the scope of microscale blood flow are selected for demonstration and discussion in Chapter 5. The main body of this thesis (Chapters 6–8) reports several novel aspects of blood flow at the microscale including, but not limited to, the non-inertial focusing of RBCs under low Reynolds number as revealed in Chapter 6, the excessive haemodilution induced by CFL asymmetry as revealed in Chapter 7, and the strong association between RBC perfusion and vascular patterning as revealed in Chapter 8. Some confusion about or misinterpretation of well-known effects in the community has also been clarified, such as the spatial scaling of hydrodynamic lift in non-circular channels, the development length of CFL in typical microfluidic flows, and the existence of high- /low-flow attraction near bifurcating geometries. Quantitative or qualitative agreement has been achieved through elaborated comparison with supplementary experiments from my collaborators or with established empirical models in the literature. Starting from blood flow in a single microchannel, Chapter 6 highlights an exceedingly large CFL development length even under low inertia, which is greater than 28 times channel hydraulic diameter (Dh) in simulation and 46Dh in experiment (experimental data from my collaborator in Glasgow, UK). This finding suggests that microfluidic designs need to be longer if their purpose is to investigate localised microscopic behaviour of a dilute suspension without interference from entrance effects or upstream disturbances. On a network level where the RBCs flow through bifurcating microchannels arranged biomimetically following Murray’s law, Chapter 7 identifies ideal partitioning of RBCs at symmetric bifurcations (agreeing with predictions of a classic empirical model derived from in vivo data), but biased partitioning when significant CFL asymmetry arises in inter-bifurcation branches. Furthermore, the breakdown of CFL symmetry leads to severe haemo-dilution/concentration in the bifurcating network. In Chapter 8, the computational framework is applied to model blood flow in realistic microvasculatures of developmental mouse retina and demonstrates an unreported highly heterogeneous distribution of RBCs in the post-sprouting vascular network. Remarkably, a strong association between vessel regression and RBC depletion is uncovered, driven by the effect of plasma skimming. The association is further confirmed by in vivo observation of simultaneous vascular remodelling alongside blood perfusion using a developmental zebrafish model (experimental data from my collaborator in Berlin, Germany). In summary, this thesis provides insights for the design of improved microfluidic devices and the conception of haemodynamic mechanisms governing the onset and progression of microcirculatory disorders. Additionally, the computational model successfully applied to various biological or biomimetic scenarios in this thesis justifies itself as a feasible and reliable tool for practical simulation of microcirculatory blood flows and may seek wider applications of its own accord

    Segregation in High Concentration Flows

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    Sand injectites are observed in a wide range of locations and settings, both modern and ancient but little is known about the processes controlling their formation. The scale of these injections range from mm to km in size and represent the forceful injection of fluidised sand into host strata. Due to the difficulty of observing in-situ events and relative paucity of outcrop data interpretations, understanding of the flow processes during fluidisation pipe formation is lacking. Existing fluidisation models provide mechanisms for fluidisation but remain simplistic and do not capture the full dynamics nor the range of characteristics which are observed to vary both spatially and temporally across the system during the formation of sand injectites. Fluidisation theory relies on an understanding of both the velocity characteristics and the concentration characteristics of a fluidisation event but comprehensive evidence of these quantities has not previously been available. The novel application of experimental techniques in both two dimensions and three dimensions in this thesis provides both high resolution velocity data for the formation and quasi-steady state of fluidisation pipes along with high resolution concentration data for the first time. Complementing this, the novel application of numerical modelling provides insight into the early stages of void formation and demonstrates a new methodology for investigating flow processes during fluidisation. The products of the fluidisation events modelled are presented providing a direct link between fluidisation processes and products for reference in interpreting outcrop data. Residual morphologies are evidenced resulting in explanations of the poor detection rate of sand injections. New models of fluidisation and void formation are presented based on the extensive characterisation of a fluidisation event achieved across multiple methodologies
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