Showing papers on "Fluid dynamics published in 2013"

Fluid dynamics of bacterial turbulence.

Abstract: Self-sustained turbulent structures have been observed in a wide range of living fluids, yet no quantitative theory exists to explain their properties. We report experiments on active turbulence in highly concentrated 3D suspensions of Bacillus subtilis and compare them with a minimal fourth-order vector-field theory for incompressible bacterial dynamics. Velocimetry of bacteria and surrounding fluid, determined by imaging cells and tracking colloidal tracers, yields consistent results for velocity statistics and correlations over 2 orders of magnitude in kinetic energy, revealing a decrease of fluid memory with increasing swimming activity and linear scaling between kinetic energy and enstrophy. The best-fit model allows for quantitative agreement with experimental data.

Theory of Simple Liquids: with Applications to Soft Matter

Abstract: Comprehensive coverage of topics in the theory of classical liquids Widely regarded as the standard text in its field, Theory of Simple Liquids gives an advanced but self-contained account of liquid state theory within the unifying framework provided by classical statistical mechanics. The structure of this revised and updated Fourth Edition is similar to that of the previous one but there are significant shifts in emphasis and much new material has been added. Major changes and Key Features in content include: Expansion of existing sections on simulation methods, liquid-vapour coexistence, the hierarchical reference theory of criticality, and the dynamics of super-cooled liquids. New sections on binary fluid mixtures, surface tension, wetting, the asymptotic decay of pair correlations, fluids in porous media, the thermodynamics of glasses, and fluid flow at solid surfaces. An entirely new chapter on applications to 'soft matter' of a combination of liquid state theory and coarse graining strategies, with sections on polymer solutions and polymer melts, colloidal dispersions, colloid-polymer mixtures, lyotropic liquid crystals, colloidal dynamics, and on clustering and gelation. Expansion of existing sections on simulation methods, liquid-vapour coexistence, the hierarchian reference of criticality, and the dynamics of super-cooled liquids. New sections on binary fluid mixtures, surface tension, wetting, the asymptotic decay of pair correlations, fluids in porous media, the thermodynamics of glasses, and fluid flow at solid surfaces. An entirely new chapter on applications to 'soft matter' of a combination of liquid state theory and coarse graining strategies, with sections on polymer solutions and polymer melts, colloidal dispersions, colloid-polymer mixtures, lyotropic liquid crystals, colloidal dynamics, and on clustering and gelation.

MHD three-dimensional Casson fluid flow past a porous linearly stretching sheet

Abstract: In this paper, magnetohydrodynamic (MHD) Casson fluid flow in two lateral directions past a porous linear stretching sheet is investigated. Self-similar solutions are obtained and compared with the available data for special cases. It is found that the present results are in an excellent agreement with the available data. The dimensionless velocities and shear stresses are obtained in both directions. Pertinent results are presented graphically and discussed quantitatively with respect to variation in Casson flow parameter as well as other fluid flow parameters.

Heat transfer and fluid flow analysis of solar air heater: A review of CFD approach

Abstract: The objective of this article is to present a detailed review of the literature that deals with the application of CFD in the design of solar air heater. Solar air heater is one of the basic equipment through which solar energy is converted into thermal energy. CFD is a simulation tool which uses powerful computer and applied mathematics, to model fluid flow situations for the prediction of heat, mass and momentum transfer and optimal design in various heat transfer and fluid flow processes. The quality of the solutions obtained from CFD simulations are largely within the acceptable range proving that CFD is an effective tool for predicting the behavior and performance of a solar air heater. One of the great challenges in the design of a solar air heater using CFD approach is the selection of appropriate turbulence model. The decision about a suitable turbulence model chosen in a CFD computation is not easy. In this article a CFD investigation is also carried out to select best turbulence model for the design of a solar air heater. A modern CFD code ANSYS FLUENT v12.1 is used to simulate fluid flow through a conventional solar air heater. A two-dimensional flow is assumed. The influences of the five different turbulence models on the quality of the obtained results are tested. It appears from the performed calculations that the Renormalization-group k–e model yields the best results for two-dimensional flow through conventional solar air heaters.

A simple SPH algorithm for multi-fluid flow with high density ratios

Abstract: SUMMARY In this paper, we describe an SPH algorithm for multi-fluid flow, which is efficient, simple and robust. We derive the inviscid equations of motion from a Lagrangian together with the constraint provided by the continuity equation. The viscous flow equations then follow by adding a viscous term. Rigid boundaries are simulated using boundary force particles in a manner similar to the immersed boundary method. Each fluid is approximated as weakly compressible with a speed of sound sufficiently large to guarantee that the relative density variations are typically 1%. When the SPH force interaction is between two particles of different fluids, we increase the pressure terms. This simple procedure stabilizes the interface between the fluids. The equations of motion are integrated using a time stepping rule based on a second-order symplectic integrator. When linear and angular momentum should be conserved exactly, they are conserved to within round-off errors. We test the algorithm by simulating a variety of problems involving fluids with a density ratio in the range 1–1000. The first of these is a free surface problem with no rigid boundaries. It involves the flow of an elliptical distribution with one fluid inside the other. We show that the simulations converge as the particle spacing decreases, and the results are in good agreement with the exact inviscid, incompressible theory. The second test is similar to the first but involves the nonlinear oscillation of the fluids. As in the first test, the agreement with theory is very good, and the method converges. The third test is the simulation of waves at the interface between two fluids. The method is shown to converge, and the agreement with theory is satisfactory. The fourth test is the Rayleigh–Taylor instability for a configuration considered by other authors. Key parameters are shown to converge, and the agreement with other authors is good. The fifth and final test is how well the SPH method simulates gravity currents with density ratios in the range 2–30. The results of these simulations are in very good agreement with those of other authors and in satisfactory agreement with experimental results.Copyright © 2012 John Wiley & Sons, Ltd.

Lattice Boltzmann Method for Simulation of Shale Gas Transport in Kerogen

Abstract: Fluid mechanics of natural gas in organic-rich shale involves nanoscale phenomena that could lead to potential non-Darcy effects during gas production. In general, these are low-Reynoldsnumber and noncontinuum effects and, more importantly, porewall-dominated multiscale effects. In this study, we introduce a new lattice Boltzmann method (LBM) to investigate these effects numerically in simple pore geometries. The standard method was developed in the 1980s to overcome the weaknesses of lattice gas cellular automata and has emerged recently as a powerful tool to solve fluid dynamics problems, in particular in the areas of microand nanofluidics. The new approach takes into account molecularlevel interactions by use of adsorptive/cohesive forces among the fluid particles and defining a Langmuir-slip boundary condition at the organic pore walls. The model allows us to partition mass transport by the walls into two components: slippage of free gas molecules and hopping (or surface transport) of the adsorbed gas molecules. By use of the standard 2D D2Q9 lattice, lowReynolds-number gas dynamics is simulated in a 100-nm model organic capillary and later in a bundle of smaller-sized organic nanotubes. The results point to the existence of a critical Knudsennumber value for the onset of laminar gas flow under typical shale-gas-reservoir pressure conditions. Beyond this number, the predicted velocity profile shows that the mechanisms of slippage and surface transport could lead to molecular streaming by the pore walls, which enhances the gas transport in the organic nanopores. The work is important for development of new-generation shale-gas-reservoir flow simulators, and it can be used in the laboratory for organic-rich-shale characterization.

Fault Zone Architecture and Fluid Flow: Insights from Field Data and Numerical Modeling

Abstract: Fault zones in the upper crust are typically composed of complex fracture networks and discrete zones of comminuted and geochemically altered fault rocks. Determining the patterns and rates of fluid flow in these distinct structural discontinuities is a three-dimensional problem. A series of numerical simulations of fluid flow in a set of three-dimensional discrete fracture network models aids in identifying the primary controlling parameters of fault-related fluid flow, and their interactions, throughout episodic deformation. Four idealized, but geologically realistic, fault zone architectural models are based on fracture data collected along exposures of the Stillwater Fault Zone in Dixie Valley, Nevada and geometric data from a series of normal fault zones in east Greenland. The models are also constrained by an Andersonian model for mechanically compatible fracture networks associated with normal faulting. Fluid flow in individual fault zone components, such as a fault core and damage zone, and full outcrop scale model domains are simulated using a finite element routine. Permeability contrasts between components and permeability anisotropy within components are identified as the major controlling factors in fault-related fluid flow. Additionally, the structural and hydraulic variations in these components are also major controls of flow at the scale of the full model domains. The four models can also be viewed as a set of snapshots in the mechanical evolution of a single fault zone. Changes in the hydraulic parameters within the models mimic the evolution of the permeability structure of each model through a single deformation cycle. The model results demonstrate that small changes in the architecture and hydraulic parameters of individual fault zone components can have very large impacts, up to five orders of magnitude, on the permeability structure of the full model domains. Closure of fracture apertures in each fault zone magnifies the magnitude and orientation of permeability anisotropy in ways that are closely linked to the implicitly modeled deformation. Changes in fault zone architecture can cause major changes in permeability structure that, in turn, significantly impact the magnitude and patterns of fluid flux and solute transport both within and near the fault zone. Inferences derived from the model results are discussed in the context of the mechanical strength of an evolving fault zone, fault zone sealing mechanisms which control the conduit-barrier systematics of a fault zone as a flow system, and how these processes are related to fluid flow in natural fault zones.

Casson fluid flow and heat transfer over a nonlinearly stretching surface

Abstract: A boundary layer analysis is presented for non-Newtonian fluid flow and heat transfer over a nonlinearly stretching surface. The Casson fluid model is used to characterize the non-Newtonian fluid behavior. By using suitable transformations, the governing partial differential equations corresponding to the momentum and energy equations are converted into non-linear ordinary differential equations. Numerical solutions of these equations are obtained with the shooting method. The effect of increasing Casson parameter is to suppress the velocity field. However the temperature is enhanced with the increasing Casson parameter.

A new divergence free synthetic eddy method for the reproduction of inlet flow conditions for les

Abstract: This paper describes a recent development of the Synthetic Eddy Method (SEM) proposed by Jarrin et al. (Int J Heat Fluid Flow 30(3):435–442, 2009) for generation of synthetic turbulence. The present scheme is designed to produce a divergence-free turbulence field that can reproduce almost all possible states of Reynolds stress anisotropy. This improved representation, when used to provide inlet conditions for an LES, leads to reduced near-inlet pressure fluctuations in the LES and to a reduced development length, both of which lead to lower computer resource requirements. An advantage of this method with respect to forcing approaches (which require an iterative approach) is the suitability for direct usage with embedded LES. Results for a turbulent channel flow are reported here and compared to those from the original SEM, and other direct approaches such as the VORTEX method of Sergent (2002) and the Synthesized Turbulence approach of Davidson and Billson (Int J Heat Fluid Flow 27(6):1028–1042, 2006), showing overall improved performance and a more accurate representation of turbulence structures immediately downstream of the inlet.

Numerical solutions of Magnetohydrodynamic boundary layer flow of tangent hyperbolic fluid towards a stretching sheet

Abstract: In the present article, we have studied the two dimensional tangent hyperbolic fluid flow towards a stretching sheet with a magnetic field. Governing equations for the proposed model are modelled and then simplified using boundary layer approach and similarity transformations. Simplified governing equations are then solved numerically with the help of fourth and fifth order Runge–Kutta–Fehlberg method. Physical features of the involved parameters are presented and discussed.

Fluid flow in rock fractures: From the Navier-Stokes equations to the cubic law

Abstract: The mathematical analysis of the flow of a single-phase Newtonian fluid through a rough-walled rock fracture is reviewed, starting with the Navier-Stokes equations. By a combination of order-of-magnitude analysis, appeal to available analytical solutions, and reanalysis of some data from the literature, it is shown that the Navier-Stokes equations can be linearized if the Reynolds number is less than about 10. Further analysis shows that the linear Stokes equations can be replaced by the simpler Reynolds lubrication equation if the wavelength of the dominant aperture variations is about three times greater than the mean aperture. However, this criterion does not seem to be strongly obeyed by all fractures. The Reynolds equation (i.e., the local cubic law) may therefore suffice in estimating fracture permeabilities to within a factor of about 2, but more accurate estimates will require solution of the Stokes equations. Similarly, estimates of mean aperture based on inverting transmissivity data may have errors of a factor of two if any version of the local cubic law is used to relate transmissivity to mean aperture.

Fluid dynamics and thermal physics of two-phase flows: Problems and achievements

Abstract: The problem and specific aspects of studying two-phase flows laden with solid particles, droplets, and bubbles are considered. The main characteristics of two-phase flows and methods for their modeling are reported. The results of experimental and computation-theoretical investigations of two-phase flows of different types are described.

Hydro‐mechanical modeling of cohesive crack propagation in multiphase porous media using the extended finite element method

Abstract: SUMMARY In this paper, a numerical model is developed for the fully coupled hydro-mechanical analysis of deformable, progressively fracturing porous media interacting with the flow of two immiscible, compressible wetting and non-wetting pore fluids, in which the coupling between various processes is taken into account. The governing equations involving the coupled solid skeleton deformation and two-phase fluid flow in partially saturated porous media including cohesive cracks are derived within the framework of the generalized Biot theory. The fluid flow within the crack is simulated using the Darcy law in which the permeability variation with porosity because of the cracking of the solid skeleton is accounted. The cohesive crack model is integrated into the numerical modeling by means of which the nonlinear fracture processes occurring along the fracture process zone are simulated. The solid phase displacement, the wetting phase pressure and the capillary pressure are taken as the primary variables of the three-phase formulation. The other variables are incorporated into the model via the experimentally determined functions, which specify the relationship between the hydraulic properties of the fracturing porous medium, that is saturation, permeability and capillary pressure. The spatial discretization is implemented by employing the extended finite element method, and the time domain discretization is performed using the generalized Newmark scheme to derive the final system of fully coupled nonlinear equations of the hydro-mechanical problem. It is illustrated that by allowing for the interaction between various processes, that is the solid skeleton deformation, the wetting and the non-wetting pore fluid flow and the cohesive crack propagation, the effect of the presence of the geomechanical discontinuity can be completely captured. Copyright © 2012 John Wiley & Sons, Ltd.

Engineering fluid flow using sequenced microstructures

Abstract: Controlling the shape of fluid streams is important across scales: from industrial processing to control of biomolecular interactions. Previous approaches to control fluid streams have focused mainly on creating chaotic flows to enhance mixing. Here we develop an approach to apply order using sequences of fluid transformations rather than enhancing chaos. We investigate the inertial flow deformations around a library of single cylindrical pillars within a microfluidic channel and assemble these net fluid transformations to engineer fluid streams. As these transformations provide a deterministic mapping of fluid elements from upstream to downstream of a pillar, we can sequentially arrange pillars to apply the associated nested maps and, therefore, create complex fluid structures without additional numerical simulation. To show the range of capabilities, we present sequences that sculpt the cross-sectional shape of a stream into complex geometries, move and split a fluid stream, perform solution exchange and achieve particle separation. A general strategy to engineer fluid streams into a broad class of defined configurations in which the complexity of the nonlinear equations of fluid motion are abstracted from the user is a first step to programming streams of any desired shape, which would be useful for biological, chemical and materials automation.

Fluid dynamics of rotating detonation engines with hydrogen and hydrocarbon fuels

Abstract: Rotating detonation engines (RDE’s) represent a logical step from pulsed detonation engine concepts to a continuous detonation engine concept for obtaining propulsion from the high efficiency detonation cycle. The hydrogen/air and hydrogen/oxygen RDE concepts have been most extensively studied, however, being able to use hydrocarbon fuels is essential for practical RDE’s. The current paper extends our hydrogen/air model to hydrocarbon fuels with both air and pure oxygen as the oxidizer. Before beginning the RDE calculations, several detonation tube results are summarized showing the ability of the code to reproduce the correct detonation velocity and CJ properties. In addition, a calculation capturing the expected irregular detonation cell patterns of ethylene/air is also shown. To do the full range of fuels and oxidizers, we found the use of temperature-dependent thermodynamic properties to be essential, especially for hydrocarbon/oxygen mixtures. The overall results for air-breathing RDE’s with hydrocarbons ranged from 1990 to 2540 s, while in pure oxygen mode the specific impulse varied from 700 to 1070 s. These results were between 85% and 89% of the expected ideal detonation cycle results, and are in line with previous hydrogen/air estimates from our previous work. We conclude from this that hydrocarbon RDE’s are viable and that the basic flow-field patterns and behaviors are very similar to the hydrogen/air cases detailed previously.

Characteristics of heat transfer and fluid flow in microtube and microchannel using conventional fluids and nanofluids: A review

Abstract: Research on convective heat transfer on internal microtube and microchannel has been extensively conducted in the past decade. This review summarizes numerous researches on two topics; the first section focuses on studying the fluid flow and heat transfer behavior of different types of microtubes (MT) and microchannel (MC) at different orientations. The second section concentrates on nanofluids; its preparation, properties, behavior, and many others. The purpose of this article is to get a clear view and detailed summary of the influence of several parameters such as the geometrical specifications, boundary conditions, and type of fluids. The maximum Nusselt number is the main target of such research where correlation equations were developed in experimental and numerical studies are reported. The heat transfer enhancement of nanofluids along with the nanofluids preparation technique, types and shapes of nanoparticles, base fluids and additives, transport mechanisms, and stability of the suspension are also discussed.

Existence of a Weak Solution to a Nonlinear Fluid–Structure Interaction Problem Modeling the Flow of an Incompressible, Viscous Fluid in a Cylinder with Deformable Walls

Abstract: We study a nonlinear, unsteady, moving boundary, fluid–structure interaction (FSI) problem arising in modeling blood flow through elastic and viscoelastic arteries. The fluid flow, which is driven by the time-dependent pressure data, is governed by two-dimensional incompressible Navier–Stokes equations, while the elastodynamics of the cylindrical wall is modeled by the one-dimensional cylindrical Koiter shell model. Two cases are considered: the linearly viscoelastic and the linearly elastic Koiter shell. The fluid and structure are fully coupled (two-way coupling) via the kinematic and dynamic lateral boundary conditions describing continuity of velocity (the no-slip condition), and the balance of contact forces at the fluid–structure interface. We prove the existence of weak solutions to the two FSI problems (the viscoelastic and the elastic case) as long as the cylinder radius is greater than zero. The proof is based on a novel semi-discrete, operator splitting numerical scheme, known as the kinematically coupled scheme, introduced in Guidoboni et al. (J Comput Phys 228(18):6916–6937, 2009) to numerically solve the underlying FSI problems. The backbone of the kinematically coupled scheme is the well-known Marchuk–Yanenko scheme, also known as the Lie splitting scheme. We effectively prove convergence of that numerical scheme to a solution of the corresponding FSI problem.

Spatial fluctuations of fluid velocities in flow through a three-dimensional porous medium.

Abstract: We use confocal microscopy to directly visualize the spatial fluctuations in fluid flow through a three-dimensional porous medium. We find that the velocity magnitudes and the velocity components both along and transverse to the imposed flow direction are exponentially distributed, even with residual trapping of a second immiscible fluid. Moreover, we find pore-scale correlations in the flow that are determined by the geometry of the medium. Our results suggest that despite the considerable complexity of the pore space, fluid flow through it is not completely random.

Systems of Navier-Stokes equations on Cantor sets

Abstract: We present systems of Navier-Stokes equations on Cantor sets, which are described by the local fractional vector calculus. It is shown that the results for Navier-Stokes equations in a fractal bounded domain are efficient and accurate for describing fluid flow in fractal media.

Compressive Sensing Based Machine Learning Strategy For Characterizing The Flow Around A Cylinder With Limited Pressure Measurements

Abstract: Compressive sensing is used to determine the flow characteristics around a cylinder (Reynolds number and pressure/flow field) from a sparse number of pressure measurements on the cylinder. Using a supervised machine learning strategy, library elements encoding the dimensionally reduced dynamics are computed for various Reynolds numbers. Convex L1 optimization is then used with a limited number of pressure measurements on the cylinder to reconstruct, or decode, the full pressure field and the resulting flow field around the cylinder. Aside from the highly turbulent regime (large Reynolds number) where only the Reynolds number can be identified, accurate reconstruction of the pressure field and Reynolds number is achieved. The proposed data-driven strategy thus achieves encoding of the fluid dynamics using the L2 norm, and robust decoding (flow field reconstruction) using the sparsity promoting L1 norm.

Streamwise-localized solutions at the onset of turbulence in pipe flow.

Abstract: Although the equations governing fluid flow are well known, there are no analytical expressions that describe the complexity of turbulent motion. A recent proposition is that in analogy to low dimensional chaotic systems, turbulence is organized around unstable solutions of the governing equations which provide the building blocks of the disordered dynamics. We report the discovery of periodic solutions which just like intermittent turbulence are spatially localized and show that turbulent transients arise from one such solution branch.