Showing papers on "Fluid dynamics published in 2003"

Quadrature method of moments for aggregation-breakage processes.

Abstract: Investigation of particulate systems often requires the solution of a population balance, which is a continuity statement written in terms of the number density function. In turn, the number density function is defined in terms of an internal coordinate (e.g., particle length, particle volume) and it generates integral and derivative terms. Different methods exist for numerically solving the population balance equation. For many processes of industrial significance, due to the strong coupling between particle interactions and fluid dynamics, the population balance must be solved as part of a computational fluid dynamics (CFD) simulation. Such an approach requires the addition of a large number of scalars and the associated transport equations. This increases the CPU time required for the simulation, and thus it is clear that it is very important to use as few scalars as possible. In this work the quadrature method of moments (QMOM) is used. The QMOM has already been validated for crystal growth and aggregation; here the method is extended to include breakage. QMOM performance is tested for 10 different cases in which the competition between aggregation and breakage leads to asymptotic solutions.

Fluid flow in synthetic rough‐walled fractures: Navier‐Stokes, Stokes, and local cubic law simulations

Abstract: [1] The results of three-dimensional Navier-Stokes (NS) and Stokes simulations and two-dimensional local cubic law (LCL) simulations of fluid flow through single rough-walled fractures are presented Synthetic rough-walled fractures were created by combining random fields of aperture and the mean wall topography or midsurface, which quantifies undulation about the fracture plane A finite volume formulation of the LCL that incorporates geometric corrections for fracture undulation is presented Simulations of fluid flow through planar fractures with sinusoidal variation in aperture were compared to published results The rough-walled fracture simulations demonstrated that the total flow rates predicted by the corrected LCL were within 10% of those predicted by the Stokes equations for all the fractures examined in this work Differences between the NS and Stokes simulations clearly demonstrated that inertial forces can significantly influence the internal flow field within a fracture and the total flow rate across a fracture By limiting the total flow rate differences between the NS and Stokes simulations, constraints for three kinematic parameters were determined For all the fractures presented in this work, the corrected LCL was determined to be an acceptable approximation to the NS equations, provided that the kinematic and geometric constraints were met

Ion concentrations and velocity profiles in nanochannel electroosmotic flows

Abstract: Ion distributions and velocity profiles for electroosmotic flow in nanochannels of different widths are studied in this paper using molecular dynamics and continuum theory. For the various channel widths studied in this paper, the ion distribution near the channel wall is strongly influenced by the finite size of the ions and the discreteness of the solvent molecules. The classical Poisson–Boltzmann equation fails to predict the ion distribution near the channel wall as it does not account for the molecular aspects of the ion–wall and ion–solvent interactions. A modified Poisson–Boltzmann equation based on electrochemical potential correction is introduced to account for ion–wall and ion–solvent interactions. The electrochemical potential correction term is extracted from the ion distribution in a smaller channel using molecular dynamics. Using the electrochemical potential correction term extracted from molecular dynamics (MD) simulation of electroosmotic flow in a 2.22 nm channel, the modified Poisson–Boltzmann equation predicts the ion distribution in larger channel widths (e.g., 3.49 and 10.00 nm) with good accuracy. Detailed studies on the velocity profile in electro-osmotic flow indicate that the continuum flow theory can be used to predict bulk fluid flow in channels as small as 2.22 nm provided that the viscosity variation near the channel wall is taken into account. We propose a technique to embed the velocity near the channel wall obtained from MD simulation of electroosmotic flow in a narrow channel (e.g., 2.22 nm wide channel) into simulation of electroosmotic flow in larger channels. Simulation results indicate that such an approach can predict the velocity profile in larger channels (e.g., 3.49 and 10.00 nm) very well. Finally, simulation of electroosmotic flow in a 0.95 nm channel indicates that viscosity cannot be described by a local, linear constitutive relationship that the continuum flow theory is built upon and thus the continuum flow theory is not applicable for electroosmotic flow in such small channels.

Liquid flow in microchannels: experimental observations and computational analyses of microfluidics effects

Abstract: Experimental observations of liquid microchannel flows are reviewed and results of computer experiments concerning channel entrance, wall slip, non-Newtonian fluid, surface roughness, viscous dissipation and turbulence effects on the friction factor are discussed. The experimental findings are classified into three groups. Group I emphasizes 'flow instabilities' and group II points out 'viscosity changes' as the causes of deviations from the conventional flow theory for macrochannels. Group III caters to studies that did not detect any measurable differences between micro- and macroscale fluid flow behaviors. Based on numerical friction factor analyses, the entrance effect should be taken into account for any microfluidic system. It is a function of channel length, aspect ratio and the Reynolds number. Non-Newtonian fluid flow effects are expected to be important for polymeric liquids and particle suspension flows. The wall slip effect is negligible for liquid flows in microconduits. Significant surface roughness effects are a function of the Darcy number, the Reynolds number and cross-sectional configurations. For relatively low Reynolds numbers, Re < 2000, onset to turbulence has to be considered important because of possible geometric non-uniformities, e.g., a contraction and/or bend at the inlet to the microchannel. Channel-size effect on viscous dissipation turns out to be important for conduits with Dh < 100 µm.

Effect of Surface Roughness on Heat Transfer and Fluid Flow Characteristics at Low Reynolds Numbers in Small Diameter Tubes

Abstract: The effect of surface roughness on pressure drop and heat transfer in circular tubes has been extensively studied in literature. The pioneering work of Nikuradse [1] established the sand grain roughness as a major parameter in defining the friction factor during laminar and turbulent flows. Recent studies have indicated a transition to turbulent flows at Reynolds number values much below 2300 during single-phase flow in channels with small hydraulic diameters. In the present work, a detailed experimental study is undertaken to investigate the roughness effects in small diameter tubes. The roughness of the inside tube surface is changed by etching it with an acid solution. Two tubes of 1.032 mm and 0.62 mm inner diameter are treated with acid solutions to provide three different roughness values for each tube. The Reynolds number range for the tests is 500-2600 for 1.067 mm tube and 900-3000 for 0.62 mm tube.

Transport in microchannels - a critical review

Abstract: The tremendous enhancement in heat transport obtained by employing microchannels has provided an e ective alternative to conventional methods of heat dissipation, especially in applications related to cooling of microelectronics. A number of theoretical and experimental studies have been reported on the fluid flow and heat transfer mechanisms in mini and microchannels as well as microtubes. Anomalies and deviations from the behavior expected for conventional channels, both in terms of the frictional and heat transfer characteristics have been noticed in microchannels under specific flow conditions and flow regimes. The present work compiles and analyzes the results of the important investigations on fluid flow and heat transfer in microchannels and microtubes.

Pumping of liquids with ac voltages applied to asymmetric pairs of microelectrodes.

Abstract: The net flow of electrolyte induced by an ac electric potential applied to an array of asymmetric pairs of microelectrodes has recently been reported. The interaction between the oscillating electric field and the oscillating induced charge at the diffuse double layer on the electrodes results in a steady electro-osmotic velocity distribution on top of the electrodes. This slip velocity distribution is anisotropic and produces a net flow of fluid. This paper presents a theoretical analysis of the pumping phenomena based upon an electro-osmotic model in ac fields. The electrical equations are solved numerically using the charge simulation method. The bulk flow generated by the electro-osmotic slip velocity is calculated. The dependence of the fluid flow on voltage and frequency is described and compared to experiments.

A new model for viscous dissipation in porous media across a range of permeability values

Abstract: In this paper a unified mathematical theory for the viscous dissipation term in the governing Brinkman equation is derived. This term has, unlike other models, the correct asymptotic behaviour in both the fully Darcy and Newtonian fluid flow limits.

Turbulent flow : analysis, measurement, and prediction

Abstract: Preface. Acknowledgments. 1. Preliminaries. 2. Overview of Turbulent Flow Physics and Equations. 3. Experimental and Numerical Methods. 4. Properties of Bounded Turbulent Flows. 5. Properties of Turbulent Free Shear Flows. 6. Turbulent Transport. 7. Theory of Idealized Turbulent Flows. 8. Turbulence Modeling. 9. Applications of Turbulence Modeling. 10. Large Eddy Simulations. 11. Analysis of Turbulent Scalar Fields. 12. Turbulence Theory. Author Index. Subject Index.

Continuum-particle hybrid coupling for mass, momentum, and energy transfers in unsteady fluid flow

Abstract: The aim of hybrid methods in simulations is to communicate regions with disparate time and length scales. Here, a fluid described at the atomistic level within an inner region P is coupled to an outer region C described by continuum fluid dynamics. The matching of both descriptions of matter is made across an overlapping region and, in general, consists of a two-way coupling scheme (C-->P and P-->C) that conveys mass, momentum, and energy fluxes. The contribution of the hybrid scheme hereby presented is twofold. First, it treats unsteady flows and, more importantly, it handles energy exchange between both C and P regions. The implementation of the C-->P coupling is tested here using steady and unsteady flows with different rates of mass, momentum and energy exchange. In particular, relaxing flows described by linear hydrodynamics (transversal and longitudinal waves) are most enlightening as they comprise the whole set of hydrodynamic modes. Applying the hybrid coupling scheme after the onset of an initial perturbation, the cell-averaged Fourier components of the flow variables in the P region (velocity, density, internal energy, temperature, and pressure) evolve in excellent agreement with the hydrodynamic trends. It is also shown that the scheme preserves the correct rate of entropy production. We discuss some general requirements on the coarse-grained length and time scales arising from both the characteristic microscopic and hydrodynamic scales.

Stochastic rotation dynamics. I. Formalism, Galilean invariance, and Green-Kubo relations

Abstract: A detailed analytical and numerical analysis of a recently introduced stochastic model for fluid dynamics with continuous velocities and efficient multi-particle collisions is presented. It is shown how full Galilean invariance can be achieved for arbitrary Mach numbers and how other low temperature anomalies can be removed. The relaxation towards thermal equilibrium is investigated numerically, and the relaxation time is measured. Equations of motions for the correlation functions of coarse-grained hydrodynamic variables are derived using a discrete-time projection operator technique, and the Green-Kubo relations for all relevant transport coefficients are given. In the following paper (Part 2), analytic expressions for the transport coefficients are derived and compared with simulation results. Long-time tails in the velocity and stress autocorrelation functions are measured and shown to be in good agreement with previous mode-coupling theories for continuous systems.

Transport coefficients of a mesoscopic fluid dynamics model

Abstract: We investigate the properties of stochastic rotation dynamics, a mesoscopic model used for simulating fluctuating hydrodynamics. Analytical results are given for the shear viscosity and the friction exerted on a massive solute particle moving within the fluid. We discuss an efficient way of measuring the shear viscosity and viscous friction, and obtain excellent agreement between the theoretical and numerical calculations.

Electroactive polymer devices for controlling fluid flow

Abstract: The invention describes devices for controlling fluid flow, such as valves. The devices may include one or more electroactive polymer transducers with an electroactive polymer that deflects in response to an application of an electric field. The electroactive polymer may be in contact with a fluid where the deflection of the electroactive polymer may be used to change a characteristic of the fluid. Some of the characteristic of the fluid that may be changed include but are not limited to 1) a flow rate, 2) a flow direction, 3) a flow vorticity, 4) a flow momentum, 5) a flow mixing rate, 6) a flow turbulence rate, 7) a flow energy, 8) a flow thermodynamic property. The electroactive polymer may be a portion of a surface of a structure that is immersed in an external fluid flow, such as the surface of an airplane wing or the electroactive polymer may be a portion of a surface of a structure used in an internal flow, such as a bounding surface of a fluid conduit.

Particle-scale modelling of gas-solid flow in fluidisation †

Abstract: Various approaches have been proposed to model the gas-solid two-phase flow at different time and length scales, including the so-called two-fluid model (TFM), direct numerical simulation (DNS) and combined continuum and discrete model (CCDM). This paper briefly discusses the key features of these models and their relative merit with special reference to modelling gas fluidisation. Focus is then given to CCDM in which the motion of individual particles is obtained by solving Newton's second law of motion and fluid flow by the Navier-Stokes equation based on the concept of local average. The applicability of CCDM is highlighted by its successful simulation of complicated phenomena associated with the transition between fluid-like and solid-like behaviour in raceway formation and bed expansion. At the same time, the usefulness of the resulting particle-scale information is demonstrated in elucidating the fundamentals governing the gas-solid flow. Finally, areas for future development are discussed. # 2003 Society of Chemical Industry

Transition to turbulence in particulate pipe flow.

Abstract: We investigate experimentally the influence of suspended particles on the transition to turbulence. The particles are monodisperse and neutrally buoyant with the liquid. The role of the particles on the transition depends upon both the pipe to particle diameter ratios and the concentration. For large pipe-to-particle diameter ratios the transition is delayed while it is lowered for small ratios. A scaling is proposed to collapse the departure from the critical Reynolds number for pure fluid as a function of concentration into a single master curve.

Realistic Animation of Fluid with Splash and Foam

Abstract: In this paper we describe a method for modeling and rendering dynamic behavior of fluids withsplashes and foam. A particle system is built into a fluid simulation system to represent an ocean wavecresting and spraying over another object. We use the Cubic Interpolated Propagation (CIP) method asthe fluid solver. The CIP method can solve liquid and gas together in the framework of fluid dynamicsand has high accuracy in the case of relatively coarse grids. This enables us to simulate the fluids in ashort time and describe the motion of splashes in the air that is associated with the liquid motion well.The foam floating on the water also can be described using the particle system. We integrate the rigidbody simulation with the fluid and particle system to create sophisticated scenes including splashes andfoam. We construct state change rules that are used with the particle system. This controls the generation,vanishing and transition rule of splashes and foam. The transition rule makes the seamless connection betweena splash and foam. We employed a fast volume rendering method with scattering effect for particles.One of the important features of our method is the combination of fast simulation and rendering techniques,which provides dynamic and realistic scenes in a short time.

Smoke simulation for large scale phenomena

Abstract: In this paper, we present an efficient method for simulating highly detailed large scale participating media such as the nuclear explosions shown in figure 1. We capture this phenomena by simulating the motion of particles in a fluid dynamics generated velocity field. A novel aspect of this paper is the creation of highly detailed three-dimensional turbulent velocity fields at interactive rates using a low to moderate amount of memory. The key idea is the combination of two-dimensional high resolution physically based flow fields with a moderate sized three-dimensional Kolmogorov velocity field tiled periodically in space.

On analysis of steady flows of fluids with shear-dependent viscosity based on the lipschitz truncation method ∗

Abstract: We deal with a system of partial differential equations describing a steady motion of an incompressible fluid with shear-dependent viscosity and present a new global existence result for $ p>\frac{2d}{d+2} $. Here p is the coercivity parameter of the nonlinear elliptic operator related to the stress tensor and d is the dimension of the space. Lipschitz test functions, a subtle splitting of the level sets of the maximal functions for the velocity gradients, and a decomposition of the pressure are incorporated to obtain almost everywhere convergence of the velocity gradients.

Instabilities in Gravity Driven Flow of Thin Fluid Films

Abstract: This paper presents theoretical, computational, and experimental aspects of the instability development in the flow of thin fluid films. The theoretical part involves basic fluid me- chanics and presents derivation of the thin film equation using lubrication approximation. A simplified version of this equation is then analyzed analytically using linear stability analysis, and also numerically. The results are then compared directly to experiments. The experimental part outlines the setup, as well as data acquisition and analysis. This immediate comparison to experiments is very useful for gaining better insight into the interpretation of various theoretical and computational results.

Extension of the lattice-boltzmann method for direct simulation of suspended particles near contact

Abstract: Computational methods based on the solution of the lattice-Boltzmann equation have been demonstrated to be effective for modeling a variety of fluid flow systems including direct simulation of particles suspended in fluid. Applications to suspended particles, however, have been limited to cases where the gap width between solid particles is much larger than the size of the lattice unit. The present extension of the method removes this limitation and improves the accuracy of the results even when two solid surfaces are near contact. With this extension, the forces on two moving solid particles, suspended in a fluid and almost in contact with each other, are calculated. Results are compared with classical lubrication theory. The accuracy and robustness of this computational method are demonstrated with several test problems.