Macroscale Flow Research Articles

Time-of-Flight Distributions and Breakthrough Curves in Heterogeneous Porous Media Using a Pore-Scale Streamline Tracing Algorithm

X-ray microtomography is routinely used to image the three-dimensional pore space of sedimentary rocks. Flow and transport properties can then be simulated directly in such images. Advective transport in porous media is frequently simulated using streamlines. We present a novel streamline tracing algorithm based on a substantial development of the most widely used method (the Pollock algorithm) employed for macroscale (Darcy) flow, making it consistent with solutions of the Navier-Stokes equation with no flow at solid boundaries. We use this new algorithm to calculate breakthrough curves and time-of-flight distributions for advection-dominated transport in two three-dimensional images of sedimentary rocks containing up to $$10^9$$ voxels: a sandstone and a carbonate. We show that our approach provides a more accurate description of flow, particularly when only a few image voxels span each pore. Therefore, it is better suited to capture anomalous (non-Fickian) transport behaviour than the standard Pollock method.

Computational simulation of mixing flow of shear thinning non-Newtonian fluids with various impellers in a stirred tank

The main features of the flow generated by three axial flow impellers installed on the side of cylindrical tanks filled with pseudoplastic solutions exhibiting yield stress were exposed using computational fluid dynamics (CFD). The numerical results were evaluated using velocity vector maps obtained from particle image velocimetry (PIV) experiments. The models were able to predict macroscale flow structures and global mixing parameters under different operating conditions. However, limitations of the model to predict the symmetric flow observed during the experiments were identified at high rotational speeds. The operating conditions included angular speeds from 327rpm to 684rpm, and yield stresses and viscosity levels provided by three carbopol solutions (0.075, 0.09 and 0.1w/w%). These conditions create mainly laminar flow inside the mixing domain. The results indicated that the operating conditions and the blade angle establish the structure of the impeller discharge, which in turn defines the shear rate distribution, some physical cavern properties, and the formation of segregated regions.

Emergence of grain-size effects in nanocrystalline metals from statistical activation of discrete dislocation sources

Nanocrystalline (NC) metals are exceptionally strong because they contain an unusually high density of grain boundaries (GBs) that act as sources and sinks for dislocations and significantly modify dislocation motion. In this study, we seek to understand the relationship between discrete dislocation emissions from GBs and grain size effects seen in the strength of NC metals. We propose a statistical GB dislocation source model responsible for discrete dislocation slip events. We find that the grain size limitation on dislocation source sizes gives rise to grain size effects on the statistical distribution for the critical resolved shear stress (CRSS) for discrete slip events. To establish its impact on mechanical behavior, this GB source model is integrated into a 3D crystal plasticity finite element model for NC Ni. We observe that a Hall–Petch scaling in yield strength emerges from the calculations. Further the predictions achieve quantitative agreement with experimental data from several studies across a wide range of average nanograin sizes. It is revealed that statistical dispersion in the CRSS for discrete slip events causes (a) strain hardening in the macroscale flow stress–strain response and (b) the fraction of grains that accommodate the applied strain and the fraction of active grains that undergo multi-slip to strongly depend on grain size. It is also found to lead to an unusual texture effect on slip activity that is significant in the finer nanograins but weak in larger grains (>100nm). In addition, it causes increases in heterogeneity in strain concentrations with decreasing grain size, suggesting that plastic instabilities are more likely to form as nano-scale grain sizes decrease.

Multi-scale modelling of flow in periodic solid structures through spatial averaging

This paper presents spatially averaged Navier–Stokes equations for modelling macro-scale flow in devices with periodic solid structures such as fin and tube arrays. The properties of steady and unsteady periodically developed flow are investigated to assess different strategies for determining the closure terms in the macro-scale flow equations. It is shown that the spatial averaging technique requires an appropriate weighting function to ensure that the closure terms are spatially constant for periodically developed flow. Moreover, through an appropriate choice of the weighting function, the closure terms can be obtained by solving a local closure problem on a unit cell of the periodic structures. The theoretical framework of this paper is applied as multi-scale modelling technique for flow through a cylindrical tube array. This case study illustrates the advantages of the weighted spatial averaging technique over the volume averaging technique.

Exploratory data analysis of the Large Scale Gas Injection Test (Lasgit) dataset, focusing on ‘second-order’ events around macro-scale gas flows

Abstract The Large Scale Gas Injection Test (Lasgit) is a field-scale experiment designed to study the impact of gas build-up and subsequent migration through an engineered barrier system (EBS). Lasgit has a substantial experimental dataset containing in excess of 26 million datum points. The dataset is anticipated to contain a wealth of information, ranging from long-term trends and system behaviours to small-scale or ‘second-order’ features. In order to interrogate the Lasgit dataset, a bespoke computational toolkit, designed to expose and quantify difficult to observe phenomena in large, non-uniform datasets, has been developed and applied. Presented results focus on the investigation and interpretation of second-order events occurring in close proximity (temporally and spatially) to a known macro-scale gas flow event that occurred during the second gas injection test. The similarity of the investigated event to dilatant flow observed in laboratory experiments is noted, as is the evidence for localized flow pathways in the bentonite EBS. The sensitivity of the toolkit's ability to highlight second-order events is also evaluated.

Integrated Localized MicroCooling Using High-Displacement Piezoelectric Actuators

RF communication and radar systems present an extreme thermal management challenge. These systems are comprised of tightly packaged, high-power components requiring a controlled temperature to meet performance and reliability parameters. In these systems, there is little space available for macro-scale air flow or other traditional cooling methods. In addition, heat generating components are typically microscale semiconductor devices fabricated on integrated circuit substrates buried deeply within the system. Localized cooling using integrated microsystems may provide a solution to these thermal management issues. One approach is the integration of miniature synthetic air jets near the heat generating components. Synthetic jets are generated using oscillating piezoelectric actuators that force air through a small nozzle at high flow rates and in close proximity to the heat generating component. The resulting jets provide vorticity within the fluid, leading to enhanced mixing with the surrounding lower temperature environment and a subsequent increase in the heat transfer coefficient. The US Army AMRDEC has been developing and operating microactuator test beds, utilizing high-displacement actuator architectures, including THUNDER and Lightweight Piezo-Composite Actuator (LIPCA) approaches, to reduce actuator size while maintaining sufficient flow rates and vorticity. This investigation has modeled and fabricated potential high-displacement miniature actuators, as well as predicted and characterized resulting jet parameters and heat transfer coefficients utilizing these actuators. This paper will present actuator designs, fabrication process, and cooling results using these high-displacement piezoelectric actuators as the drive element.

Analytical Model of Water Flow in Coal with Active Matrix

Abstract This paper presents new analytical model of gas-water flow in coal seams in one dimension with emphasis on interactions between water flowing in cleats and coal matrix. Coal as a flowing system, can be viewed as a solid organic material consisting of two flow subsystems: a microporous matrix and a system of interconnected macropores and fractures. Most of gas is accumulated in the microporous matrix, where the primary flow mechanism is diffusion. Fractures and cleats existing in coal play an important role as a transportation system for macro scale flow of water and gas governed by Darcy’s law. The coal matrix can imbibe water under capillary forces leading to exchange of mass between fractures and coal matrix. In this paper new partial differential equation for water saturation in fractures has been formulated, respecting mass exchange between coal matrix and fractures. Exact analytical solution has been obtained using the method of characteristics. The final solution has very simple form that may be useful for practical engineering calculations. It was observed that the rate of exchange of mass between the fractures and the coal matrix is governed by an expression which is analogous to the Newton cooling law known from theory of heat exchange, but in present case the mass transfer coefficient depends not only on coal and fluid properties but also on time and position. The constant term of mass transfer coefficient depends on relation between micro porosity and macro porosity of coal, capillary forces, and microporous structure of coal matrix. This term can be expressed theoretically or obtained experimentally.

Effect of surface tension on filling flow of carboxymethyl cellulose solution in microchannel

The effect of surface tension on filling flow is neglected generally in macroscale flow. However, it may become a significant factor to impact the filling flow in microchannel. In this paper, the effect of surface tension on filling flow was investigated by a visualisation device to facilitate the observation of flow behaviour. Capillaries in the diameter range of 0·1–0·5 mm were chosen as microchannels and 3% solutions of carboxymethyl cellulose polymer were studied. The results indicate that additional pressure accounting for surface tension influences the filling flow in microchannel slightly. The error of theoretical velocity considering the surface tension reaches 8% in capillary of 0·1 mm in diameter. The surface tension can be neglected when feature size of flow channel is >0·1 mm. Meanwhile, wall slip exists during the filling flow into capillaries and slip velocity increases as decreasing feature size. Therefore, some other factors such as wall slip and size effect could impact filling flow together in microchannel if the feature size of channel is <0·1 mm.

Multiscale modeling and simulation of dynamic wetting

In this paper, we present a multiscale analysis on dynamic wetting and liquid droplet spreading on solid substrates. In the proposed multiscale dynamic wetting model, we couple molecular scale adhesive interaction (the van der Waals type force) and the macroscale flow – that is: we combine a coarse-grain adhesive contact model with a modified Gurtin–Murdoch surface elasto-dynamics theory to formulate a multiscale moving contact line theory to simulate dynamic wetting. The advantage of adopting the coarse grain adhesive contact model in the moving contact line theory is that it can levitate and separate the liquid droplet with the solid substrate, so that the proposed multiscale moving contact line theory avoids imposing the non-slip condition, and then it removes the subsequent singularity problem, which allows the surface energy difference and surface stress propelling droplet spreading naturally.By employing the proposed method, we have successfully simulated droplet spreading over various elastic substrates. The obtained numerical simulation results compare well with the experimental and molecular dynamics results reported in the literature.

Interfacial velocities and capillary pressure gradients during Haines jumps

Drainage is typically understood as a process where the pore space is invaded by a nonwetting phase pore-by-pore, the controlling parameters of which are represented by capillary number and mobility ratio. However, what is less understood and where experimental data are lacking is direct knowledge of the dynamics of pore drainage and the associated intrinsic time scales since the rate dependencies often observed with displacement processes are potentially dependent on these time scales. Herein, we study pore drainage events with a high speed camera in a micromodel system and analyze the dependency of interfacial velocity on bulk flow rate and spatial fluid configurations. We find that pore drainage events are cooperative, meaning that capillary pressure differences which extend over multiple pores directly affect fluid topology and menisci dynamics. Results suggest that not only viscous forces but also capillarity acts in a nonlocal way. Lastly, the existence of a pore morphological parameter where pore drainage transitions from capillary to inertial and/or viscous dominated is discussed followed by a discussion on capillary dispersion and time scale dependencies. We show that the displacement front is disperse when volumetric flow rate is less than the intrinsic time scale for a pore drainage event and becomes sharp when the flow rate is greater than the intrinsic time scale (i.e., overruns the pore drainage event), which clearly shows how pore-scale parameters influence macroscale flow behavior.

Effect of Permeable Biofilm on Micro- And Macro-Scale Flow and Transport in Bioclogged Pores

Simulations of coupled flow around and inside biofilms in pores were conducted to study the effect of porous biofilm on micro- and macro-scale flow and transport. The simulations solved the Navier-Stokes equations coupled with the Brinkman equation representing flow in the pore space and biofilm, respectively, and the advection-diffusion equation. Biofilm structure and distribution were obtained from confocal microscope images. The bulk permeability (k) of bioclogged porous media depends on biofilm permeability (kbr) following a sigmoidal curve on a log-log scale. The upper and lower limits of the curve are the k of biofilm-free media and of bioclogged media with impermeable biofilms, respectively. On the basis of this, a model is developed that predicts k based solely on kbr and biofilm volume ratio. The simulations show that kbr has a significant impact on the shear stress distribution, and thus potentially affects biofilm erosion and detachment. The sensitivity of flow fields to kbr directly translated to effects on the transport fields by affecting the relative distribution of where advection and diffusion dominated. Both kbr and biofilm volume ratio affect the shape of breakthrough curves.

Flow pattern characterization for R-245fa in minichannels: Optical measurement technique and experimental results

An optical measurement method using image processing for two-phase flow pattern characterization in minichannel is developed. The bubble frequency, the percentage of small bubbles as well as their velocity are measured. A high-speed high-definition video camera is used to measure these parameters and to identify the flow regimes and their transitions. The tests are performed in a 3.0mm glass channel using saturated R-245fa at 60°C (4.6bar). The mass velocity is ranging from 100 to 1500kg/m2s, the heat flux is varying from 10 to 90kW/m2 and the inlet vapor quality from 0 to 1. Four flow patterns (bubbly flow, bubbly–slug flow, slug flow and annular flow) are recognized. The comparison between the present experimental intermittent/annular transition lines and five transition lines from macroscale and microscale flow pattern maps available in the literature is presented. Finally, the influence of the flow pattern on the heat transfer coefficient is highlighted.

An efficient computational model for macroscale simulations of moving contact lines

We propose an efficient level-set approach for numerical simulation of moving contact lines. The main purpose is to formulate and test a model wherein the macroscale flow is resolved while the effects of the microscopic region near a contact line are represented using asymptotic theories. The model covers viscous as well as inertial regimes. Test simulations include axisymmetric displacement flow in a tube and droplet spreading on a flat surface. The results show that the present approach leads to excellent convergence properties even with very coarse grids; furthermore, the results agree well with asymptotic analysis, with those obtained with a method for direct numerical simulations (wherein an adaptive grid is used) and also with experiments.

An experimental investigation of flow boiling heat transfer of R-134a in horizontal and vertical mini-channels

Flow boiling experiments with R-134a flowing in horizontal and vertical mini-channels are performed to obtain flow visualization and heat transfer data. A 1.75mm diameter channel with a length of 600mm is used as test section. Two sets of the horizontal flow and vertical upward flow data are presented based on a mass flux range of 200–1000kg/m2s, a heat flux range of 1–80kW/m2 and a saturation pressure range of 7–13bar. Flow visualization and heat transfer results show heat transfer coefficient depending on flow pattern. The results also indicate that flow regime transition and heat transfer coefficient tend to depend on flow direction under the certain experimental conditions. In addition, the comparisons between the experimental data and the predictions recommended for either macro-scale flow boiling or micro-scale flow boiling are presented in this work.

A NOTE ON FOLDING MECHANISMS IN THE CAPE FOLD BELT, SOUTH AFRICA

The relationship between folding and faulting in the Cape Fold Belt has been raised as an enigma. The mineral deformation mechanisms accommodating folding are integral to the relation between faults and folds. Here I discuss field and microstructural observations of folded rocks from the Laingsburg region in the Western Cape, and the underlying mineral deformation mechanisms accommodating strain in these rocks. In competent units, deformation was dominantly accomplished by flexural slip faulting and jointing; in relatively incompetent layers macroscale flow was accommodated by dissolution-precipitation creep and distributed cataclasis. Combined with previous studies suggesting deformation occurred at lowermost greenschist facies temperatures, these observations indicate that folding in the Cape Fold Belt occurred at temperatures and pressures within the normally frictional regime. Folding and thrust fault development therefore generally occurred concurrently, and partitioning between localised and distributed deformation was governed by factors such as fluid pressure conditions, strain rate, and relative viscosity.

Analysis of Microscopic Displacement Mechanisms of a MIOR Process in Porous Media with Different Wettability

The wettability of the reservoir rock has an important effect on the displacement of fluids on a microscopic scale in all EOR processes, especially in the microbial improved oil recovery (MIOR) process. This study describes the effect of wettability on microscopic two-phase flow displacement mechanisms of bacterial flooding. It enables us to get better understanding and prediction capability of macroscale flow behavior of the MIOR process. To achieve the goal, a number of visualization experiments have been carried out in glass micromodels with water wet and oil wet wettability status. Synthetic brine and model oil and an alkane oxidizing bacterium are used to explain the different behavior of microscopic displacement mechanisms in the water wet and oil wet micromodels. The results obtained in the two media are presented and compared. The observational results show the effect of wettability of porous media on remaining oil saturation and oil phase transportation. In water wet model, the oil is remained mostly as isolated drops while in oil wet model, the remaining oil is the continuous phase. The bacteria have the ability to displace the residual oil trapped in the micromodel. It is shown that the bacteria have various performances in the oil wet and water wet systems. The acting mechanisms supporting the displacement process are the interfacial tension reduction, wettability changes, and pore blocking.

Critical shear stress for incipient motion of a particle on a rough bed

The main objectives of the present study are to obtain improved models of hydrodynamic forces and torque on a particle sitting on a bed and to use these models for the investigation of incipient motion and resuspension of particles. The improved models for force and torque are obtained from numerical simulations of a particle sitting on a bed with a turbulent flow of logarithmic mean velocity profile approaching the particle. Since the mean turbulent velocity profile can depart from the logarithmic profile in case of macroscale rough beds or flow down steep slopes, we have also considered forces and torque on a particle due to both linear and uniform mean flow profiles. The computed drag and lift coefficients and the predicted critical shear stress for incipient particle motion and resuspension are compared against available experimental results. The improved force and torque models are also used to evaluate the effect of turbulent velocity fluctuations on the critical shear stress for incipient motion and resuspension. The present results are of direct relevance to cases where the particle is mostly exposed to the ambient flow. In cases where the particle protrusion is small and is submerged mostly within a pocket of other particles, the above formulation can be used with a redefined area of exposure and flow velocity seen by the particle. However, the drag, lift, and torque coefficients and the resisting forces will be influenced by the partial exposure and the details of pocket geometry, which require further investigation.

A microfluidic device for high throughput bacterial biofilm studies

Bacteria are almost always found in ecological niches as matrix-encased, surface-associated, multi-species communities known as biofilms. It is well established that soluble chemical signals produced by the bacteria influence the organization and structure of the biofilm; therefore, there is significant interest in understanding how different chemical signals are coordinately utilized for community development. Conventional methods for investigating biofilm formation such as macro-scale flow cells are low-throughput, require large volumes, and do not allow spatial and temporal control of biofilm community formation. Here, we describe the development of a PDMS-based two-layer microfluidic flow cell (μFC) device for investigating bacterial biofilm formation and organization in response to different concentrations of soluble signals. The μFC device contains eight separate microchambers for cultivating biofilms exposed to eight different concentrations of signals through a single diffusive mixing-based concentration gradient generator. The presence of pneumatic valves and a separate cell seeding port that is independent from gradient-mixing channels offers complete isolation of the biofilm microchamber from the gradient mixer, and also performs well under continuous, batch or semi-batch conditions. We demonstrate the utility of the μFC by studying the effect of different concentrations of indole-like biofilm signals (7-hydroxyindole and isatin), either individually or in combination, on biofilm development of pathogenic E. coli. This model can be used for developing a fundamental understanding of events leading to bacterial attachment to surfaces that are important in infections and chemicals that influence the biofilm formation or inhibition.

Multi-scale modeling of shock interaction with a cloud of particles using an artificial neural network for model representation

The evolution of a solid-gas mixture under the influence of a shock wave depends on particle-particle and particle-shock interactions; i.e. the macroscopic distribution of particles is determined by physics at the particle (micro)-scale. This work seeks to simulate the macro-scale dynamics of gas-solid mixtures by employing information accumulated from direct numerical simulations (DNS) at the micro- (i.e., particle) scale. Data on the forces experienced by particles in a cloud are collected from DNS using a compressible Eulerian solver and provided to an artificial neural network (ANN); the simulations are performed for a range of control parameters, such as Mach number, particle radii, particle-fluid density ratio, position, and volume fraction. Beginning with a simple single stationary particle case and progressing to moving particle laden clouds, the ANN is trained to evolve and reproduce correlations between the control parameters and particle dynamics. The trained ANN is then used in computing the macro-scale flow behavior in a model of shocked dusty gas advection. The model predicts particle motion and other macro-scale phenomena in agreement with experimental observations.

CO2 Mass Transfer Induced through an Airlift Loop by a Microbubble Cloud Generated by Fluidic Oscillation

Carbon dioxide is one of the most common gases produced from biological processes. Removal of carbon dioxide from these processes can influence the direction of biological reactions as well as the pH of the medium, which affects bacterial metabolism. Kinetics of carbon dioxide transfer mechanisms are investigated by sparging with conventional fine bubbles and microbubbles. The estimate of the concentrations of CO2(aq), H2CO3, HCO3 –, and CO32– from pH measurement in an airlift loop sparged mixer is derived. The canonical estimate of overall mass transfer coefficient of CO2 has been estimated as 0.092 min–1 for a microbubble size of 550 m compared with 0.0712 min–1 for a fine bubble (mean bubble size of 1.3 mm) sparging. It is observed that the efficiency of CO2 removal has increased up to 29% by microbubble sparging compared with fine bubble sparging. Laminar bubbly flow modeling of the airlift loop configuration correctly predicts the trend of the change in overall mass transfer in both gas stripping with nitrogen and gas scrubbing for CO2 exchange, while demonstrating the expected separated flow structure. The models indicate that the macroscale flow structure is transient and pseudoperiodic. This latter feature should be tested by flow visualization, as preferential frequencies in the flow can be exploited for enhanced mixing.