Showing papers on "Fluid dynamics published in 2018"

Relativistic Fluid Dynamics Far From Local Equilibrium.

Abstract: Fluid dynamics is traditionally thought to apply only to systems near local equilibrium. In this case, the effective theory of fluid dynamics can be constructed as a gradient series. Recent applications of resurgence suggest that this gradient series diverges, but can be Borel resummed, giving rise to a hydrodynamic attractor solution which is well defined even for large gradients. Arbitrary initial data quickly approaches this attractor via nonhydrodynamic mode decay. This suggests the existence of a new theory of far-from-equilibrium fluid dynamics. In this Letter, the framework of fluid dynamics far from local equilibrium for a conformal system is introduced, and the hydrodynamic attractor solutions for resummed Baier-Romatschke-Son-Starinets-Stephanov theory, kinetic theory in the relaxation time approximation, and strongly coupled $N=4$ super Yang-Mills theory are identified for a system undergoing Bjorken flow.

Numerical modeling of the strand deposition flow in extrusion-based additive manufacturing

Abstract: We propose a numerical model to simulate the extrusion of a strand of semi-molten material on a moving substrate, within the computation fluid dynamics paradigm. According to the literature, the deposition flow of the strands has an impact on the inter-layer bond formation in extrusion-based additive manufacturing, as well as the surface roughness of the fabricated part. Under the assumptions of an isothermal Newtonian fluid and a creeping laminar flow, the deposition flow is controlled by two parameters: the gap distance between the extrusion nozzle and the substrate, and the velocity ratio of the substrate to the average velocity of the flow inside the nozzle. The numerical simulation fully resolves the deposition flow and provides the cross-section of the printed strand. For the first time, we have quantified the effect of the gap distance and the velocity ratio on the size and the shape of the strand. The cross-section of the strand ranges from being almost cylindrical (for a fast printing and with a large gap) to a flat cuboid with rounded edges (for a slow printing and with a small gap), which substantially differs from the idealized cross-section typically assumed in the literature. Finally, we found that the printing force applied by the extruded material on the substrate has a negative linear relationship with the velocity ratio, for a constant gap.

Review on Modeling and Simulation of Continuous Casting

Abstract: Continuous casting is a mature, sophisticated technological process, used to produce most of the world’s steel, so is worthy of fundamentally-based computational modeling. It involves many interacting phenomena including heat transfer, solidification, multiphase turbulent flow, clogging, electromagnetic effects, complex interfacial behavior, particle entrapment, thermal-mechanical distortion, stress, cracks, segregation, and microstructure formation. Furthermore, these phenomena are transient, three-dimensional, and operate over wide length and time scales. This paper reviews the current state of the art in modeling these phenomena, focusing on practical applications to the formation of defects. It emphasizes model verification and validation of model predictions. The models reviewed range from fast and simple for implementation into online model-based control systems to sophisticated multiphysics simulations that incorporate many coupled phenomena. Both the accomplishments and remaining challenges are discussed.

The effect of attack angle of triangular ribs on heat transfer of nanofluids in a microchannel

Abstract: In the present study, the effect of attack angle of triangular ribs, by using finite volume method, has been numerically studied in a two-dimensional microchannel. The cooling fluid is water/Ag nanofluid with volume fractions of 0–4% of nanoparticles, and nanoparticle diameters are 25, 50 and 75 nm. The nanofluid flow has been considered as laminar with Reynolds numbers of 5, 100 and 500. Also, the attack angles have been studied at the range of 30°–60°. In this study, the effects of variations in attack angles on triangular ribs, volume fraction of nanoparticles, nanoparticles diameter and Reynolds number have been investigated. The results indicate that using nanoparticles with smaller diameter improves heat transfer rate. Moreover, it is shown that the friction coefficient and pumping power are almost independent of nanoparticle diameter. However, increasing Reynolds number, pumping power enhancement becomes more important by increasing the volume fraction of nanoparticles. In low Reynolds numbers, the influence of ribs is approximately insignificant on the streamlines; it is very effective in high Reynolds numbers. The existence of rib on the direction of fluid motion causes asymmetrical velocity profile in the top section of the rib. Using triangular rib with higher attack angle can improve heat transfer significantly due to the high-velocity gradients and better mixing of fluid flow.

Causality and existence of solutions of relativistic viscous fluid dynamics with gravity

Abstract: A new approach is described to help improve the foundations of relativistic viscous fluid dynamics and its coupling to general relativity. Focusing on neutral conformal fluids constructed solely in terms of hydrodynamic variables, we derive the most general viscous energy-momentum tensor yielding equations of motion of second order in the derivatives, which is shown to provide a novel type of generalization of the relativistic Navier-Stokes equations for which causality holds. We show how this energy-momentum tensor may be derived from conformal kinetic theory. We rigorously prove local existence, uniqueness, and causality of solutions of this theory (in the full nonlinear regime) both in a Minkowski background and also when the fluid is dynamically coupled to Einstein's equations. Linearized disturbances around equilibrium in Minkowski spacetime are stable in this causal theory. A numerical study reveals the presence of an out-of-equilibrium hydrodynamic attractor for a rapidly expanding fluid. Further properties are also studied, and a brief discussion of how this approach can be generalized to nonconformal fluids is presented.

Two-Phase Bubble Columns: A Comprehensive Review

Abstract: We present a comprehensive literature review on the two-phase bubble column; in this review we deeply analyze the flow regimes, the flow regime transitions, the local and global fluid dynamics parameters, and the mass transfer phenomena. First, we discuss the flow regimes, the flow regime transitions, the local and global fluid dynamics parameters, and the mass transfer. We also discuss how the operating parameters (i.e., pressure, temperature, and gas and liquid flow rates), the operating modes (i.e., the co-current, the counter-current and the batch modes), the liquid and gas phase properties, and the design parameters (i.e., gas sparger design, column diameter and aspect ratio) influence the flow regime transitions and the fluid dynamics parameters. Secondly, we present the experimental techniques for studying the global and local fluid dynamic properties. Finally, we present the modeling approaches to study the global and local bubble column fluid dynamics, and we outline the major issues to be solved in future studies.

Turbulent natural convection heat transfer in rectangular enclosures using experimental and numerical approaches: A review

Abstract: Natural convection is one of the most important modes of fluid flow and heat transfer. This paper presents a review of the research on turbulent natural convection in rectangular cavities using numerical and experimental techniques. In this review we have attempted to summarize the published papers on this topic with some interesting and important results. Numerous configurations of the enclosures with different initial and boundary conditions, heat source locations and radiative properties of medium and walls have been considered under the effects of various parameters such as the Rayleigh and Prandtl numbers, surface emissivity, cavity inclination angle, thermal properties, etc. Finally, some suggestions have been provided for future studies in the considered area.

The Influence of Fracturing Fluids on Fracturing Processes: A Comparison Between Water, Oil and SC-CO 2

Abstract: Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. However, experimental observations have shown that different fracturing fluids yield variations in the induced fracture. During the hydraulic fracturing process, fracturing fluids will penetrate into the borehole wall, and the evolution of the fracture(s) then results from the coupled phenomena of fluid flow, solid deformation and damage. To represent this, coupled models of rock damage mechanics and fluid flow for both slightly compressible fluids and CO2 are presented. We investigate the fracturing processes driven by pressurization of three kinds of fluids: water, viscous oil and supercritical CO2. Simulation results indicate that SC-CO2-based fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based and oil-based fracturing. We explore the relation between the breakdown pressure to both the dynamic viscosity and the interfacial tension of the fracturing fluids. Modeling demonstrates an increase in the breakdown pressure with an increase both in the dynamic viscosity and in the interfacial tension, consistent with experimental observations.

Deep Dynamical Modeling and Control of Unsteady Fluid Flows

Abstract: The design of flow control systems remains a challenge due to the nonlinear nature of the equations that govern fluid flow. However, recent advances in computational fluid dynamics (CFD) have enabled the simulation of complex fluid flows with high accuracy, opening the possibility of using learning-based approaches to facilitate controller design. We present a method for learning the forced and unforced dynamics of airflow over a cylinder directly from CFD data. The proposed approach, grounded in Koopman theory, is shown to produce stable dynamical models that can predict the time evolution of the cylinder system over extended time horizons. Finally, by performing model predictive control with the learned dynamical models, we are able to find a straightforward, interpretable control law for suppressing vortex shedding in the wake of the cylinder.

Characteristics of three dimensional stagnation point flow of Hybrid nanofluid past a circular cylinder

Abstract: The characteristics of three-dimensional stagnation point flow of Hybrid nanofluid past a circular cylinder are explored. The fluid flow is entertained in the presence/absence of thermal slip effects. The flow model is controlled through the partial differential equations. Since these equations are highly non-linear in character. So for the order reduction a suitable set of transformation is used. The reduced system is solved by using shooting method. The obtained results are offered through graphs and tables. It is noticed that the heat transfer rate is high in Hybrid nanofluid as compared to nanofluid. The present work is validated by developing comprising with existing literature.

Fluid flow and heat transfer over a rotating and vertically moving disk

Abstract: The object of this study is the development of the fluid flow and resultant heat transfer caused by a rotating disk moving vertically upward or downward during an unsteady flow motion. The problem is formulated such that the similarity equations governing the physical phenomenon eventually reduced to those reported in the traditional viscous pumping study of von Karman for a vertically motionless but still rotating disk. The non-rotating disk with the upward or downward motion leads to the formation of a two-dimensional flow over the disk. Otherwise, the rotation and vertical action of the disk sets up a three-dimensional flow over the surface. It is observed that the upward and downward motion of the disk exerts an effect similar to that of the injection/suction through the wall, albeit with observable differences. Moreover, the viscous pumping is found to be a jet-like radial velocity as the disk moves upward fast. Although the downward movement of the disk suppresses the velocity field, a growth in the boundary layer thickness is anticipated, contrary to the traditional wall suction. The temperature field is shown to be highly dependent on the form of the wall temperature, which is maintained at a time-varying function. Moreover, the impact of the vertical wall movement is observed to be overwhelmed by high disk rotations.

Hydrodynamic Interactions Among Bubbles, Drops, and Particles in Non-Newtonian Liquids

Abstract: The understanding of hydrodynamic forces around particles, drops, or bubbles moving in Newtonian liquids is modestly mature. It is possible to obtain predictions of the attractive–repulsive interaction for moving ensembles of dispersed particulate objects. There is a certain intuition of what the effects of viscous, inertial, and surface tension forces should be. When the liquid is non-Newtonian, this intuition is gone. In this review, we summarize recent efforts at gaining fundamental understanding of hydrodynamic interactions in non-Newtonian liquids. Due to the complexity of the problem, most investigations rely on experimental observations. However, computations of non-Newtonian fluid flow have made increasingly significant contributions to our understanding of particle, drop, and bubble interactions. We focus on gravity-driven flows: rise or sedimentation of single spheroidal objects, pairs, and dispersions. We identify the effects of two main rheological attributes—viscoelasticity and shear-dependen...

Entropy generation in a second grade magnetohydrodynamic nanofluid flow over a convectively heated stretching sheet with nonlinear thermal radiation and viscous dissipation

Abstract: This study addresses entropy generation in magnetohydrodynamic flow of a second grade nanofluid over a convectively heated stretching sheet with nonlinear thermal radiation and viscous dissipation. The second grade fluid is assumed to be electrically conducting and is permeated by an applied non-uniform magnetic field. We further consider the impact on the fluid properties and the Nusselt number of homogeneous-heterogeneous reactions and a convective boundary condition. The mathematical equations are solved using the spectral local linearization method. Computations for skin-friction coefficient and local Nusselt number are carried out and displayed in a table. It is observed that the effects of the thermophoresis parameter is to increase the temperature distributions throughout the boundary layer. The entropy generation is enhanced by larger magnetic parameters and increasing Reynolds number. The aim of this manuscript is to pay more attention of entropy generation analysis with heat and fluid flow on second grade nanofluids to improve the system performance. Also the fluid velocity and temperature in the boundary layer region rise significantly for increasing the values of the second grade nanofluid parameter.

SPNets: Differentiable Fluid Dynamics for Deep Neural Networks

Abstract: In this paper we introduce Smooth Particle Networks (SPNets), a framework for integrating fluid dynamics with deep networks. SPNets adds two new layers to the neural network toolbox: ConvSP and ConvSDF, which enable computing physical interactions with unordered particle sets. We use these lay- ers in combination with standard neural network layers to directly implement fluid dynamics inside a deep network, where the parameters of the network are the fluid parameters themselves (e.g., viscosity, cohesion, etc.). Because SPNets are imple- mented as a neural network, the resulting fluid dynamics are fully differentiable. We then show how this can be successfully used to learn fluid parameters from data, perform liquid control tasks, and learn policies to manipulate liquids.

Definitions of vortex vector and vortex

Abstract: Although the vortex is ubiquitous in nature, its definition is somewhat ambiguous in the field of fluid dynamics. In this absence of a rigorous mathematical definition, considerable confusion appears to exist in visualizing and understanding the coherent vortical structures in turbulence. Cited in the previous studies, a vortex cannot be fully described by vorticity, and vorticity should be further decomposed into a rotational and a non-rotational part to represent the rotation and the shear, respectively. In this paper, we introduce several new concepts, including local fluid rotation at a point and the direction of the local fluid rotation axis. The direction and the strength of local fluid rotation are examined by investigating the kinematics of the fluid element in two- and three-dimensional flows. A new vector quantity, which is called the vortex vector in this paper, is defined to describe the local fluid rotation and it is the rotational part of the vorticity. This can be understood as that the direction of the vortex vector is equivalent to the direction of the local fluid rotation axis, and the magnitude of vortex vector is the strength of the location fluid rotation. With these new revelations, a vortex is defined as a connected region where the vortex vector is not zero. In addition, through direct numerical simulation (DNS) and large eddy simulation (LES) examples, it is demonstrated that the newly defined vortex vector can fully describe the complex vertical structures of turbulence.