Convective Flow Patterns Research Articles

Effect of hydrodynamic conditions on the Cu(In,Ga)Se2 thin film growth by electrodeposition

Abstract In this paper, an experimental study about CIGS thin film growth due to hydrodynamic conditions variations on the working electrode surface is reported. Local hydrodynamic conditions were produced by the interaction of the natural convection flow with a non-conducting hemisphere placed on the working electrode surface. The electrodeposition process was made by a single bath with copper, indium, gallium and selenium ions to obtain CIGS thin films. The electrodes in the electrolytic cell were placed vertically. The as-electrodeposited CIGS thin film morphology near the hemisphere had a characteristic growth as a consequence of the local hydrodynamic conditions. CIGS thin film morphology and composition variations due to the wake and boundary layer were identified. With the CIGS film morphology obtained due to the presence of the wake was possible to represent the natural convection flow pattern on the Mo electrode surface. The CIGS thin film composition, inside and outside of the wake and throughout the boundary layer, was characterized. The gallium incorporation to the CIGS film was principally affected. As a consequence of this study, it was found that the atomic percentage of gallium in the CIGS film is related to deposition current density.

Computational modeling of convective seepage flow in fluid-saturated heterogeneous rocks: Steady-state approach

Convective seepage flow plays a key role not only in naturally forming mineral deposits and oil reservoirs, but also in the carbon-dioxide sequestration in fluid-saturated rocks. This paper presents a steady-state numerical solver, which is based on the finite element method (FEM) and progressive asymptotic approach procedure (PAAP), to solve steady-state convective seepage flow problems in fluid-saturated heterogeneous rocks. Although this kind of flow problem is commonly solved by using transient-state numerical solvers, the use of the proposed steady-state numerical solver, which is the main characteristic of this study, can have certain advantages. After the proposed steady-state numerical solver is verified by a benchmark testing problem, for which analytical solutions are available for comparison, it is further applied to simulate steady-state convective seepage flow in the Australia North West Shelf basin, which is composed of naturally-formed heterogeneous porous rocks. Through the simulated numerical results, it can be found that: (1) compared with the analytical solutions of the benchmark testing problem, the use of the steady-state numerical solver can produce correct and accurate numerical simulation results for dealing with convective seepage flow problems in fluid-saturated heterogeneous rocks. (2) The main advantage of using the steady-state numerical solver is to avoid the need of considering initial conditions of the problem, which are commonly difficult to be determined because the convective seepage flow within the upper crust is a past event in geology. (3) In the Australia North West Shelf basin, the convective seepage flow and temperature distribution patterns depend strongly on both the permeability contrast of the faults and the basin geometry (including the layer structures and fault locations) in the computational model.

An analytical finite-strain parametrization for texture evolution in deforming olivine polycrystals

Progressive deformation of upper mantle rocks via dislocation creep causes their constituent crystals to take on a non-random orientation distribution (crystal preferred orientation or CPO). The resulting anisotropy of the rock's elastic properties can be observed by seismic techniques, and provides a means to constrain mantle convective flow patterns. Existing methods for calculating the evolution of CPO in deforming olivine-dominated rocks rely on unwieldy discrete representations of the crystal orientation distribution in terms of a large number (103-104) of individual grains. Here we propose a more efficient method in which CPO is represented using three continuous analytical functions (structured basis functions or SBFs), each of which represents a virtual CPO produced by the action of just one of the three dominant slip systems of olivine. The SBFs are then combined using an appropriate weighting scheme to represent a realistic CPO that results from the simultaneous activity of all three slip systems. We assume that olivine CPO is a unique function of the finite strain experienced by the aggregate, which implies that the weights of the SBFs depend only on the two ratios of the lengths of the axes of the finite strain ellipsoid (FSE) and the two ratios of the strengths (critical resolved shear stresses) of the slip systems. Our preferred set of weighting coefficients is obtained by least-squares fitting of the SBF expansion to the predictions of a kinematic model (solved by the method of characteristics) in which the amplitudes of the crystallographic spins do not increase with strain. Calculation of CPO using this model is ≈107 times faster than full homogenization approaches such as the second-order self-consistent model, and the result fits the characteristics-based solution with a variance reduction ≥ 88.6 per cent for equivalent strains up to 0.9. Finally, we propose a simple modification of the FSE that prevents the CPO from becoming singular at large strains.

Study of a periodically forced magnetohydrodynamic system using Floquet analysis and nonlinear Galerkin modelling

We present a detailed study of Rayleigh–Benard magnetoconvection with periodic gravity modulation and uniform vertical magnetic field. Linear stability analysis is carried out using Floquet theory to construct the stability boundaries in order to estimate the magnitude of forcing amplitude $$\epsilon $$ required for having convection in the system for a fixed Rayleigh number Ra, wave number k and modulating frequency $$\Omega $$ . The effects of varying Prandtl number Pr and Chandrasekhar number Q on the threshold of convection are also investigated. A higher Pr value reduces the value of the threshold, whereas a higher Q value increases it. Bicritical states are also observed at which the minimum forcing amplitude needed for convection to begin occurs at two different k values in harmonic and sub-harmonic regions, respectively. We also construct a nonlinear Galerkin model and compare the results with those obtained from linear stability analysis. Two-dimensional (2D) oscillatory convection is observed at the onset, while quasiperiodic and chaotic behaviours are found at higher Ra values. 2D as well as nonlinear convective flow patterns are observed for primary and higher-order instabilities, respectively. Bifurcation diagrams with respect to different parameters such as $$\epsilon $$ , Ra and Q are provided for thorough understanding of the forced nonlinear system.

Synthesis of nanoparticles in carbon arc: measurements and modeling

Abstract

Natural convection flow around heated disk in cubical enclosure

In this study, a numerical analysis of the natural convection characteristics around a circular disk in a cubical enclosure was carried out. The disk and the enclosure were used to simulate a wafer placed in a front-opening unified pod. The disk position, size, and temperature were considered as the main variables, and the effects of each variable on the isotherm, streamline, and local Nusselt number were examined. The results confirmed that the disk position and size in an enclosure had greater effects on the natural convection flow pattern than the disk temperature. Because air flowed in from the edge of the disk, the local Nusselt number was the highest at the edge of the disk. It was also found that a large disk size generated a plume and formed a new thermal boundary layer.

Hot Solvent-Assisted Gravity Drainage in Naturally Fractured Heavy Oil Reservoirs: A New Model and Approach to Determine Optimal Solvent Injection Temperature

Hot solvent-assisted oil recovery is a low emission-intensity oil recovery method from heavy oil resources. This method is particularly promising for fractured reservoirs where the application of current thermal methods may involve challenges associated with heat loss and early breakthrough. In this study a new model of heat and mass transfer for oil recovery from a single matrix block of a naturally fractured reservoir using a hot miscible solvent is developed. Because of the difference in magnitude between thermal and mass diffusivities, heat diffuses beyond the solvent–oil interface and there is no significant convective heat transfer. This results in a reduction of oil viscosity in the center of the matrix block and a vertical convective flow pattern instead of a pattern parallel to the oil–solvent interface observed during cold solvent injection. Using this model, optimization graphs are developed to perform a fast qualitative assessment of the applicability of a hot solvent-assisted gravity drainage...

Analyses of Three-Dimensional Convective Flow Pattern of Vibrating Powder

Analyses of Three-Dimensional Convective Flow Pattern of Vibrating Powder

Feed flow patterns of combined Rayleigh-Bénard convection and membrane permeation

In micro- and nano-scale filtration processes, concentration polarization and fouling phenomena degrade the overall performance of porous membranes. To reduce these phenomena we aim to induce hydrodynamic instabilities near the membrane surface, which promote local turbulence in the vicinity of the membrane-solution interface. In this work, we utilize thermal instabilities related to buoyancy-driven Rayleigh-Bénard convection by heating a novel electrically conductive membrane. The heated membrane induces a local temperature gradient and thus a convective flow perpendicular to the membrane. We investigate the evolution of the mixed convection flow patterns arising from combining Rayleigh-Bénard convection with pressure-driven membrane permeation. The flow patterns are analyzed experimentally by particle image velocimetry and computed with CFD simulations. We study the contribution of the individual convection modes on the resulting mixed convection using the Richardson (Ri) number. By inducing natural convection on the steady flow pattern from dead-end permeation, a large convective vortex forms with significant shear velocities near the membrane surface. These shear velocities increase with increasing impact of natural convection. The resulting shear flows show good potential in enhancing the mixing of the bulk and to enable convective mass transport of accumulated solutes away from the membrane and thus reduce concentration polarization and membrane fouling.

Thermal convection of viscous fluids in a faulted system: 3D benchmark for numerical codes

We propose a new benchmark for the simulation of thermal convection in a 3D faulted system. Linear stability analysis is adopted to estimate the critical viscous-dependent Rayleigh number. These results are used to quantify the reliability of OpenGeoSys-5, Golem and FEFLOW simulators in accounting for the onset conditions and in predicting the long-term behavior of convective flow patterns. By comparing the analytical and numerical results, we can conclude that the proposed methodology and Rayleigh expressions can be applied as benchmark case for any numerical study involving coupled hydrothermal fluid flow in fault zones.

COBRA-SFS Thermal-Hydraulic Analysis Code for Spent-Fuel Storage and Transportation Casks: Models and Methods

COBRA-SFS, a thermal-hydraulic code developed for steady-state and transient analysis of multiassembly spent-fuel storage and transportation systems, has been incorporated into the Used Nuclear Fuel-Storage, Transportation and Disposal Analysis Resource and Data System tool as a module devoted to spent-fuel-package thermal analysis. This paper summarizes the basic formulation of the equations and models used in the COBRA-SFS code, showing that COBRA-SFS fully captures the important physical behavior governing the thermal performance of spent-fuel storage systems, with internal and external natural convection flow patterns, and heat transfer by convection, conduction, and thermal radiation. Of particular significance is the capability for detailed thermal radiation modeling within the fuel rod array.

Computational study of FEM on radiation pattern of convective MHD flow through a Porous Medium.

Computational study of FEM on radiation pattern of convective MHD flow through a Porous Medium.

Recognition and tracking of convective flow patterns using Wollaston shearing interferometry

The GeoFlow experiment on the ISS is designed to study convective flows in a spherical gap under microgravity conditions. The main challenge, however, is the visualization of the fluid flow especially under the safety requirements of the Columbus module. The Wollaston shearing interferometry unit of the Fluid Science Laboratory works by optical means alone and is therefore utilized as measurement device for temperature fluctuations. The resulting interferograms in terms of fringe patterns are the base for the presented advanced post-processing techniques. They are used to identify convective patterns, to track these structures and to reconstruct the inaccessible three-dimensional temperature field. A comparison between experimentally gained results and numerically calculated interferograms is given, too. We show that convective patterns are automatically recognized and tracked accurately in experimental images by means of the generalized structure tensor. Furthermore, generic numerical simulations are used to deduce the internal temperature distribution by comparison with interferograms from the experiment.

The Effect of the Angle of Inclination on the Efficiency in a Medium-Temperature Flat Plate Solar Collector

In this experimental work, the effects of the inclination angle β and the (Ti − Ta)/G on the efficiency and the UL-value were investigated on a medium-temperature flat plate solar collector. The experiments were based on steady-state energy balance, by heat flow calorimetry at indoor conditions and considering the standard American National Standard Institute/American Society of Heating Refrigerating and Air Conditioning Engineers (ANSI/ASHRAE) 93-2010. The solar radiation was emulated by the Joule effect using a proportional integral derivative (PID) control considering two conditions of the absorber temperature, Case 1: (To − Ti) > 0, and Case 2: (To − Ti) = 0. The inclination angles were 0°–90° and the (Ti − Ta)/G were 0.044–0.083 m2·°C/W and 0.124–0.235 for Case 1 and Case 2, respectively. The variations of β and (Ti − Ta)/G cause efficiency changes up to 0.37–0.45 (21.6%) and 0.31–0.45 (45.0%), respectively, for Case 1. Also, the UL(β) reached changes up to 10.1–12.0 W/m2·°C (19.2%) and 8.4–12.0 W/m2·°C (41.7%), respectively, for Case 1. The most significant changes of UL(β)/UL(90°) vs. β were 8.0% at the horizontal position for Case 1, while for Case 2, the maximum change was 1.8% only. Therefore, the changes of the inclination angle cause significant variations of the convective flow patterns within the collector, which leads to considerable variation of the collector efficiency and its UL value.

The Suppression and Promotion of Magnetic Flux Emergence in Fully Convective Stars

AbstractEvidence of surface magnetism is now observed on an increasing number of cool stars. The detailed manner by which dynamo-generated magnetic fields giving rise to starspots traverse the convection zone still remains unclear. Some insight into this flux emergence mechanism has been gained by assuming bundles of magnetic field can be represented by idealized thin flux tubes (TFTs). Weber & Browning (2016) have recently investigated how individual flux tubes might evolve in a 0.3M⊙ M dwarf by effectively embedding TFTs in time-dependent flows representative of a fully convective star. We expand upon this work by initiating flux tubes at various depths in the upper ~50-75% of the star in order to sample the differing convective flow pattern and differential rotation across this region. Specifically, we comment on the role of differential rotation and time-varying flows in both the suppression and promotion of the magnetic flux emergence process.

Numerical studies on convective stability and flow pattern in three-dimensional spherical mantle of terrestrial planets

Numerical studies on convective stability and flow pattern in three-dimensional spherical mantle of terrestrial planets

Plasma transport driven by the Rayleigh‐Taylor instability

AbstractTwo important differences between the giant magnetospheres (i.e., Jupiter's and Saturn's magnetospheres) and the terrestrial magnetosphere are the internal plasma sources and the fast planetary rotation. Thus, there must be a radially outward flow to transport the plasma to avoid infinite accumulation of plasma. This radial outflow also carries the magnetic flux away from the inner magnetosphere due to the frozen‐in condition. As such, there also must be a radial inward flow to refill the magnetic flux in the inner magnetosphere. Due to the similarity between Rayleigh‐Taylor (RT) instability and the centrifugal instability, we use a three‐dimensional RT instability to demonstrate that an interchange instability can form a convection flow pattern, locally twisting the magnetic flux, consequently forming a pair of high‐latitude reconnection sites. This process exchanges a part of the flux tube, thereby transporting the plasma radially outward without requiring significant latitudinal convection of magnetic flux in the ionosphere.

Phase-field modelling of β(Ti) solidification in Ti-45at.%Al: columnar dendrite growth at various gravity levels

The effect of solutal convection on the solidification of γ titanium aluminides, specifically on β(Ti) dendrite growth, is not well known. With the aim of supporting directional solidification experiments under hyper-gravity using a large diameter centrifuge, 2D-phase field simulations of β(Ti) dendrite growth have been performed for the binary alloy Ti-45at.%Al and various gravity scenarios. Both, the direction and magnitude of the gravity vector were varied systematically in order to reveal the subtle interplay between the convective flow pattern and mushy zone characteristics. In this presentation, gravity effects are discussed for early dendrite growth. For selected cases the evolution on longer timescales is also analyse of and oscillatory modes leading to dynamically stable steady state growth are outlined. In a dedicated simulation series forced flow is superimposed, as to mimic thermally driven fluid flow expected to establish on the macroscopic scale (sample size) in the centrifugal experiments. Above a certain threshold this flow turns dominant and precludes solutally driven convective effects.

Magneto-convective instabilities in horizontal cavities

A linear stability analysis is performed to investigate the onset of convective motions in a flat cavity filled with liquid metal. A volumetric heat source is uniformly distributed in the fluid and a horizontal magnetic field is imposed. Walls perpendicular to the magnetic field are thermally insulating, and the top wall is isothermal and the bottom adiabatic. When a magnetic field is applied, electromagnetic forces tend to transform 3D convective flow structures into quasi-2D rolls aligned to the magnetic field. By integrating 3D equations along magnetic field lines, a quasi-2D mathematical model has been derived. A dissipation term in the 2D equations accounts for 3D viscous effects in boundary layers at Hartmann walls perpendicular to the magnetic field. The influence of various parameters on flow stability is investigated. The flow is stabilized by increasing the magnetic field intensity or the electric conductance of Hartmann walls and by reducing the aspect ratio of the cavity. Numerical simulations are performed to verify the analytical results and to describe the main convective flow patterns in the non-linear regime.

Convection associated with exclusion zone formation in colloidal suspensions.

The long-range repulsion of colloids from various interfaces has been observed in a wide range of studies from different research disciplines. This so-called exclusion zone (EZ) formation occurs near surfaces such as hydrogels, polymers, or biological tissues. It was recently shown that the underlying physical mechanism leading to this long-range repulsion is a combination of ion-exchange at the interface, diffusion of ions, and diffusiophoresis of colloids in the resulting ion concentration gradients. In this paper, we show that the same ion concentration gradients that lead to exclusion zone formation also imply that diffusioosmosis near the walls of the sample cell must occur. This should lead to convective flow patterns that are directly associated with exclusion zone formation. We use multi-particle tracking to study the dynamics of particles during exclusion zone formation in detail, confirming that indeed two pronounced vortex-like convection rolls occur near the cell walls. These dramatic flow patterns persist for more than 4 hours, with the typical velocity decreasing as a function of time. We find that the flow velocity depends strongly on the surface properties of the sample cell walls, consistent with diffusioosmosis being the main physical mechanism that governs these convective flows.