Parallel Plate Model Research Articles

3D characterization of microbially induced carbonate precipitation in rock fracture and the resulted permeability reduction

A new seepage control method for fractured rock is biogrouting through a microbially induced calcite precipitation (MICP) process. A study on the spatial distribution of biogrout in a rock fracture and its effect on permeability reduction is presented in this paper. A series of experiments together with 3D scanning and 3D flow simulation were performed on rock fractures with various initial apertures treated by bio-grouting. A lognormal distribution of MICP precipitates along the flow direction in a fracture was observed. The 3D flow simulation of biogrouted fracture has revealed that the routinely adopted parallel plate model (cubic law) for estimating permeability of channel flow is no longer applicable when the fracture aperture is less than the critical value of 0.7 mm based on this study. This is because partially clogging will occur when the fracture aperture is less than the critical value, resulting in a transition of the flow type from surface flow to channel flow. A semi-empirical equation which can account for the effect of flow type has been proposed for estimating the permeability reduction due to bio-grouting for rock fractures.

Controllability of magnetorheological shock absorber: I. Insights, modeling and simulation**Part of work was done while Xian-Xu ‘Frank’ Bai was a joint PhD student of Chongqing University and University of Maryland, College Park.

The smart fluids—magnetorheological (MR) fluids show the potentials for high-speed shock mitigation applications because of their fast response and large controllable damping range. In order to match the high-speed applications of MR fluids, the structural design and controllability evaluation of MR actuators are of great significance. Controllability of MR actuators are defined to be evaluated by the mechanical behaviors (i.e. damping force controllability), including the controllable damping force range, dynamic range and constant stroking load velocity range, and response time performance (i.e. real-time controllability). We proposed an internal bypass MR shock absorber (MRSA) (Bai et al 2013a IEEE Trans. Mag. 49 3422–5), which presents the possibility in vibration control and shock mitigation applications. In the internal bypass MRSA, the gap formed by the coaxially assembled inner and outer cylinders is the flow channel for MR fluids. The electromagnetic coil windings evenly wound on the outer wall of the inner cylinder and the active rings contribute to the MR effect, and continuous adjustable damping is realized through applying current into the electromagnetic coil windings. In Part I of this paper, the principle and the mechanical behaviors of the internal bypass MRSA is further in-deeply investigated via theoretical modeling and simulations. Based on the fluid dynamics—parallel plate models and the finite element analysis software ANSYS/Fluent, the mathematical model and the fluidic entities of the internal bypass MRSA are established, respectively. Response time model of the internal bypass MRSA under a voltage driver is established. The static magnetic field distributions of the internal bypass MRSA under different current excitations are obtained via ANSYS/Workbench. The response times of the internal bypass MRSA under transient impulse current excitation and transient impulse voltage excitation are obtained by transient electromagnetic simulation. The damping force performance of the conventional unicoil MRSA with an electromagnetic coil winding in positon, multicoil MRSA with five electromagnetic coil windings in positon and the internal bypass MRSA are compared and analyzed. As compared with the conventional unicoil and multicoil MRSAs with coupled structures of the piston and the electromagnetic coil windings, the internal bypass MRSA with decoupled arrangement shows much better damping force controllability. Experimental test and controllability evaluation of the internal bypass MRSA will be conducted and presented in Part II of this paper.

Low‐frequency dielectric dispersion in polymer‐derived amorphous silicon carbonitride ceramics

AbstractThe dielectric response of polymer‐derived amorphous silicon carbonitride (PDC‐SiCN) material was studied over a wide frequency range from 1 mHz to 10 MHz. Aside from presenting a relaxation process over 105 to 0.03 Hz frequency range, the material showed a strong low‐frequency dielectric dispersion (LFDD), which resulted in extremely high permittivities (up to 105) at frequencies lower than 0.03 Hz. It is identified that this LFDD resulted from relaxation process was not induced by conductivity. Therefore, PDC‐SiCN material showed two relaxation processes. Based on its microstructure, two polarization mechanisms were proposed for PDC‐SiCN: a mechanism based on polarons generated by unpaired electrons present in the material and an interfacial polarization mechanism as a result of different properties of SiCxN4−x matrix and free‐carbon clusters. Interfacial polarization process was simulated based on a parallel plate model involving two different materials. The result indicated that the relaxation time of an interfacial polarization can be distributed over a broad time range in the material.

Simulation of coupled hydro-mechanical interactions during grouting process in fractured media based on the combined finite-discrete element method

Fracture grouting is one of the most widely used methodologies to reduce the hydraulic conductivity and enhance the strength of rock masses in geotechnical engineering (e.g., mining, civil engineering and hydraulic engineering). Understanding and controlling the migration process of grout slurry are critical to optimize the grouting process. In this paper, a grouting process simulator is generated for hydro-mechanical (HM) coupling grouting problems, linking the combined finite-discrete element method (FDEM) and a grouting flow simulator, called as FDEM-grouting. The initiation and propagation of fractures as well as the deformation of solid mass are solved by the FDEM. Meanwhile, the grout flow in fractures is modeled based on the principle of the parallel-plate model. This FDEM-grouting can consider the HM coupling effect, the stress-induced fracture aperture variation, the fracture networks evolution and the hardening of grout during the grouting process, which are rarely considered in previous studies. The present FDEM-grouting is validated by intensive numerical tests. Then, several influence factors of grout penetration are discussed. Lastly, we investigate a fracture grouting process with considering the effect of in-situ stress and the interference between grouting holes. Results show that this FDEM-grouting is potentially useful to optimize the fracture grouting process.

Experimental and numerical investigations on the effect of fracture geometry and fracture aperture distribution on flow and solute transport in natural fractures

The impact of fracture geometry and aperture distribution on fluid movement and on non-reactive solute transport was investigated experimentally and numerically in single fractures. For this purpose a hydrothermally altered and an unaltered granite drill core with axial fractures were investigated. Using three injection and three extraction locations at top and bottom of the fractured cores, different dipole flow fields were examined. The conservative tracer (Amino-G) breakthrough curves were measured using fluorescence spectroscopy. Based on 3-D digital data obtained by micro-computed tomography 2.5-D numerical models were generated for both fractures by mapping the measured aperture distributions to the 2-D fracture geometries (x-y plane). Fluid flow and tracer transport were simulated using COMSOL Multiphysics®.By means of numerical simulations and tomographic imaging experimentally observed breakthrough curves can be understood and qualitatively reproduced. The experiments and simulations suggest that fluid flow in the altered fracture is governed by the 2-D fracture geometry in the x-y plane, while fluid flow in the unaltered fracture seems to be controlled by the aperture distribution. Moreover, we demonstrate that in our case simplified parallel-plate models fail to describe the experimental findings and that pronounced tailings can be attributed to complex internal heterogeneities. The results presented, implicate the necessity to incorporate complex domain geometries governing fluid flow and mass transport into transport modeling.

Parallel-plate waveguide model adopted to perform inset-fed rectangular microstrip patch antenna for Wi-Fi application

ABSTRACTInset-fed rectangular microstrip patch antenna is performed using parallel-plate waveguide model. Quasi-Transverse Electromagnetic (TEM) modes approximation is presented and validated to establish the electromagnetic field distribution susceptible to be supported by the guided structure modelling the considered antenna. Input impedance calculation based on the electromagnetic theory in travelling-wave in conjunction with the field distribution elaborated in quasi-TEM mode approximation is carried out. Magnetostatic approach solution is used to analyse electric field radiation properties of the proposed antenna. Some relevant antenna performances such as impedance bandwidth of , gain of and half power beam-width of have been achieved and good agreement with the theoretic predictions has been illustrated.

Impact Analysis of Variation in Geometrical Features on Intrinsic Characteristics of Capacitive Micro-machined Ultrasonic Transducers

Capacitive Micro-machined Ultrasonic Transducers (CMUTs) are the ultrasonic devices which produce better features in contrast to piezoelectric transducers. The intrinsic parameter of CMUT varies with the variation in geometrical dimension of the device. The cavity height and the radius of the CMUT with circular membrane is varied in the lumped parallel plate model for its impact on the parameters. In this paper, analytical model of circular CMUTs is developed and analyzed by using the parallel-plate capacitor equations. The impact of geometrical changes has been discussed with the parametric analysis of deflection, capacitance, pull in voltage and pressure. The results discussed here will be more helpful in deciding the miniaturization limit of the CMUT prior to fabrication.

Numerical modeling of flow and transport in the Bari industrial area by means of rough walled parallel plate and random walk models

Abstract. Modeling fluid flow and solute transport dynamics in fractured karst aquifers is one of the most challenging tasks in hydrogeology. The present study investigates the hotspots of groundwater contamination in the industrial area of Modugno (Bari – southern Italy), where the limestone aquifer has a fractured and karstic nature. A rough walled parallel plate model coupled with a geostatistical analysis to infer the values of the equivalent aperture has been implemented and calibrated on the basis of piezometric data. Using the random walk theory, the steady-state distribution of hypothetical contamination with the source at the hotspot has been carried out, reproducing a pollution scenario which is compatible with the observed one. From an analysis of the flow and transport pattern it is possible to infer that the anticline affecting the Calcare di Bari formation in direction ENE–WSW influences the direction of flow as well as the propagation of the contaminant. The results also show that the presence of nonlinear flow influences advection, in that it leads to a delay in solute transport with respect to the linear flow assumption. This is due to the non-constant distribution of solutes according to different pathways for fractured media which is related to the flow rate.

Scanning Dielectric Constant Microscopy for imaging single biological cells

A method is developed for obtaining nanoscale dielectric constant maps of any material, using dynamic Electrostatic Force Microscopy (EFM) in conjunction with an analytical model by Hudlet et al for the tip-apex force, modified for dielectric material; and is applied for visualizing dielectric constant variation at sub-cellular level. The method is simple and does not rely on high-end electronics required for sub-attofarad resolution for capacitance measurement or resource intensive computation for specific geometry, but can be applied on available commercial Atomic Force Microscope (AFM). The method is not restricted to obtaining dielectric constant at a single point but it provides the determination of lateral variation in dielectric constant in a sample. The dielectric constant maps obtained using the modified Hudlet tip-apex model are compared with those obtained using the parallel plate and partially-filled parallel plate models. The accuracy of the dielectric constant values depends on the validity of the analytical model used. The dielectric constant values obtained using the modified Hudlet tip-apex model are in agreement with the reported values.

Thin-Film Architectures with High Spectral Selectivity for Thermophotovoltaic Cells

Thermophotovoltaic (TPV) systems are a promising technology for distributed conversion of high-temperature heat to electricity. To achieve high conversion efficiency, the transport of sub-bandgap radiation between the thermal emitter and PV cell should be suppressed. This can be achieved by recycling sub-bandgap radiation back to the emitter using a spectrally selective cell. However, conventional TPV cells exhibit limited sub-bandgap reflectance. Here we demonstrate thin-film In0.53Ga0.47As-based structures with high spectral selectivity, including record-high average sub-bandgap reflectance (96%). Selectivity is enabled by short optical paths through a high-quality material fabricated using epitaxial lift-off, high-reflectance back surfaces, and optimized interference. In addition, we use a parallel-plate TPV model to evaluate the impact of specific structural features on performance and to optimize the cell architecture. We show that a dielectric spacer between InGaAs and the Au back surface is an impo...

Influence of Fracture Heterogeneity Using Linear Congruential Generator (LCG) on the Thermal Front Propagation in a Single Geothermal Fracture-Rock Matrix System

An implicit finite difference numerical model has been developed to investigate the influence of fracture heterogeneity on the propagation of thermal front in a single horizontal fracture-matrix system. Instead of depending on a complex and data-demanding geostatistical method for a precise representation of fracture aperture, a statistical linear congruential generator (LCG) method was applied in the present study to replicate the unpredictable nature of fracture aperture morphology. The results have been compared with the parallel plate model and simple sinusoidal model. Finally, sensitivity analysis of fracture aperture size and fluid flow rate has been carried out to identify the conditions at which fracture heterogeneity is critical. The results indicate that LCG-aperture enhances the heat transfer between fracture and hot rock matrix compared to the parallel and sinusoidal fractures. Further, the temperature profiles in hot rock indicate that there was a greater loss of heat for the case of LCG-aperture (25% loss) compared to sinusoidal (16%) and parallel plate (8%) apertures. It was found that heterogeneity does not play a major role at small fracture aperture size (≤50 μm) and at low flow rates. However, as fracture aperture size increases, the heterogeneity plays a vital part even at low flow rates.

Simulate intersecting 3D hydraulic cracks using a hybrid “FE-Meshfree” method

In this work, a discontinuous hybrid “FE-Meshfree” method is developed to simulate three-dimensional (3D) cracking. Then, an algorithm of Local Adaptive Background Sub-element (LABS-element) is developed in the context of hybrid “FE-Meshfree” method to simulate intersecting cracks. Compared to the extended finite element method (XFEM), the present hybrid “FE-Meshfree” method is not required to increase the size of global stiffness matrix, introduce extra unknowns, and construct special enrichment functions. The present algorithms of LABS-element and “FE-Meshfree” method are validated by intensive numerical tests, which achieves the balance between accuracy and flexibility. Additionally, a hydraulic and mechanical (H-M) coupling model is generated, in which the deformation of rocks and the propagation of cracks are solved by the hybrid “FE-Meshfree” method, meanwhile the fluid flow in cracks is solved by a fluid simulator based on the principle of parallel-plate flow model. This H-M coupling model is then used to investigate the propagation of fluid-driven cracks in rock mass with multiple pre-existing cracks, and the observed fluid pressure feedback is potentially useful during shale oil/gas well simulation.

Imaging Channel Connectivity in Nafion Using Electrostatic Force Microscopy.

Channel connectivity is an important material property that is considered in making higher-performance proton-exchange membranes. Our group has previously demonstrated that nearly 50% of the aqueous surface domains in Nafion films do not have a connected path to the opposite side of the membrane. These so-called "dead-end" channels lead to a loss in the conductance efficiency of the membrane. Understanding the structure of these dead-end channels is an important step in improving the conductance of the membrane. Although conductive atomic force microscopy is able to provide insight into the connected channels, it does directly report on the dead-end channels. To address this, we use electrostatic force microscopy (EFM) to probe channel connectivity in a Nafion thin film (100-300 nm) under ambient conditions. EFM provided an image of the capacitive phase shift, which is influenced by surface charge, dielectric permittivity, and tip-sample geometry. We studied several individual channels and measured the quadratic dependence of the EFM signal with the bias voltage. Applying a simple parallel plate model allowed us to assign differences in the EFM signal to particular channel shapes: connected cylindrical channels, dead-end cylinder channels, and bottleneck channels.

Effects of fracture surface roughness and shear displacement on geometrical and hydraulic properties of three-dimensional crossed rock fracture models

Abstract While shear-flow behavior through fractured media has been so far studied at single fracture scale, a numerical analysis of the shear effect on the hydraulic response of 3D crossed fracture model is presented. The analysis was based on a series of crossed fracture models, in which the effects of fracture surface roughness and shear displacement were considered. The rough fracture surfaces were generated using the modified successive random additions (SRA) algorithm. The shear displacement was applied on one fracture, and at the same time another fracture shifted along with the upper and lower surfaces of the sheared fracture. The simulation results reveal the development and variation of preferential flow paths through the model during the shear, accompanied by the change of the flow rate ratios between two flow planes at the outlet boundary. The average contact area accounts for approximately 5–27% of the fracture planes during shear, but the actual calculated flow area is about 38–55% of the fracture planes, which is much smaller than the noncontact area. The equivalent permeability will either increase or decrease as shear displacement increases from 0 to 4 mm, depending on the aperture distribution of intersection part between two fractures. When the shear displacement continuously increases by up to 20 mm, the equivalent permeability increases sharply first, and then keeps increasing with a lower gradient. The equivalent permeability of rough fractured model is about 26–80% of that calculated from the parallel plate model, and the equivalent permeability in the direction perpendicular to shear direction is approximately 1.31–3.67 times larger than that in the direction parallel to shear direction. These results can provide a fundamental understanding of fluid flow through crossed fracture model under shear.

Modeling Simultaneous Multiple Fracturing Using the Combined Finite-Discrete Element Method

Simultaneous multiple fracturing is a key technology to facilitate the production of shale oil/gas. When multiple hydraulic fractures propagate simultaneously, there is an interaction effect among these propagating hydraulic fractures, known as the stress-shadow effect, which has a significant impact on the fracture geometry. Understanding and controlling the propagation of simultaneous multiple hydraulic fractures and the interaction effects between multiple fractures are critical to optimizing oil/gas production. In this paper, the FDEM simulator and a fluid simulator are linked, named FDEM-Fluid, to handle hydromechanical-fracture coupling problems and investigate the simultaneous multiple hydraulic fracturing mechanism. The fractures propagation and the deformation of solid phase are solved by FDEM; meanwhile the fluid flow in the fractures is modeled using the principle of parallel-plate flow model. Several tests are carried out to validate the application of FDEM-Fluid in hydraulic fracturing simulation. Then, this FDEM-Fluid is used to investigate simultaneous multiple fractures treatment. Fractures repel each other when multiple fractures propagate from a single horizontal well, while the nearby fractures in different horizontal wells attract each other when multiple fractures propagate from multiple parallel horizontal wells. The in situ stress also has a significant impact on the fracture geometry.

Delayed pull-in transitions in overdamped MEMS devices

We consider the dynamics of overdamped MEMS devices undergoing the pull-in instability. Numerous previous experiments and numerical simulations have shown a significant increase in the pull-in time under DC voltages close to the pull-in voltage. Here the transient dynamics slow down as the device passes through a meta-stable or bottleneck phase, but this slowing down is not well understood quantitatively. Using a lumped parallel-plate model, we perform a detailed analysis of the pull-in dynamics in this regime. We show that the bottleneck phenomenon is a type of critical slowing down arising from the pull-in transition. This allows us to show that the pull-in time obeys an inverse square-root scaling law as the transition is approached; moreover we determine an analytical expression for this pull-in time. We then compare our prediction to a wide range of pull-in time data reported in the literature, showing that the observed slowing down is well captured by our scaling law, which appears to be generic for overdamped pull-in under DC loads. This realization provides a useful design rule with which to tune dynamic response in applications, including state-of-the-art accelerometers and pressure sensors that use pull-in time as a sensing mechanism. We also propose a method to estimate the pull-in voltage based only on data of the pull-in times.

Edge-Guided Mode Performance and Applications in Nonreciprocal Millimeter-Wave Gyroelectric Components

The analogies between the behavior of gyromagnetic and gyroelectric nonreciprocal structures, the use of the simple transfer matrix approach, and the edge-guided (EG) wave property, supported in a parallel plate model for integrated magnetized semiconductor waveguide, are investigated in those frequency regions, where the effective permittivity is negative or positive. As with their ferrite counterparts, the leakage of the EG waves along the curvilinear semiconductor-metal edge is observed for those frequencies where the effective permittivity is positive. This induces spurious resonances in the semiconductor junctions having a convex–concave geometry. Furthermore, the very small reflection from the open semiconductor edge is observed for those frequencies where the effective permittivity is negative. This provides the unidirectional wave propagation in the parallel plate semiconductor (PPS) guide where one edge is open as well as in the PPS guide inserted into a dc magnetic antiparallel field. Hence, as a practical application, new EG mode nonreciprocal semiconductor devices are proposed as complementary to the already known ferrite versions. The simulated characteristics of the ideal junction and their implementation in finline technology clearly illustrate their broadband, nonreciprocal operation in the millimeter-wave regions (25–250 GHz), something which is difficult to achieve with ferrite devices.

A numerical approach for assessing effects of shear on equivalent permeability and nonlinear flow characteristics of 2-D fracture networks

Abstract Hydro-mechanical properties of rock fractures are core issues for many geoscience and geo-engineering practices. Previous experimental and numerical studies have revealed that shear processes could greatly enhance the permeability of single rock fractures, yet the shear effects on hydraulic properties of fractured rock masses have received little attention. In most previous fracture network models, single fractures are typically presumed to be formed by parallel plates and flow is presumed to obey the cubic law. However, related studies have suggested that the parallel plate model cannot realistically represent the surface characters of natural rock fractures, and the relationship between flow rate and pressure drop will no longer be linear at sufficiently large Reynolds numbers. In the present study, a numerical approach was established to assess the effects of shear on the hydraulic properties of 2-D discrete fracture networks (DFNs) in both linear and nonlinear regimes. DFNs considering fracture surface roughness and variation of aperture in space were generated using an originally developed code DFNGEN. Numerical simulations by solving Navier–Stokes equations were performed to simulate the fluid flow through these DFNs. A fracture that cuts through each model was sheared and by varying the shear and normal displacements, effects of shear on equivalent permeability and nonlinear flow characteristics of DFNs were estimated. The results show that the critical condition of quantifying the transition from a linear flow regime to a nonlinear flow regime is: 10−4 〈 J

Moment fluid equations for ions in weakly ionized plasma.

A one-dimensional fluid model for ions in weakly ionized plasma is proposed. The model differs from the existing ones in two aspects. First, a more accurate approximation of the collision terms in the fluid equations is suggested. For this purpose, the results obtained using the Monte Carlo kinetic model of the ion swarm experiments are considered. Second, the ion energy equation is taken into account. The fluid equations are closed using a simple model of the ion velocity distribution function. The accuracy of the fluid model is examined by comparing with the results of particle-in-cell Monte Carlo simulations. In particular, several test problems are considered using a parallel plate model of the capacitively coupled radio-frequency discharge. It is shown that the results obtained using the proposed fluid model are in good agreement with those obtained from the simulations over a wide range of discharge conditions. An approximation of the ion velocity distribution function for the problem under consideration is also discussed.

Development and Experimental Validation of a Non-Linear, All-Elastomer In-Plane Capacitive Pressure Sensor Model

A large deformation mechanics model is applied to predict capacitance changes in an all-elastomer capacitive tactile sensor, and the predictive model is experimentally validated. The compressive model predicts a non-linear relationship between the contact normal force and resulting capacitance change due to changes in electrode gap and electrode layer thickness. Broad parametric studies demonstrate that higher sensitivities can be achieved with lower modulus materials and smaller electrode gaps. Sensors are fabricated using a reusable silicon mold and experimental results are compared to predictions from the capacitance model. Capacitance-force model predictions yielded by the calibrated capacitance analytical model are shown to be in remarkable agreement with experimental measurements. A fringe effects term included in the capacitance model highlights the limitations of the parallel plate model especially for sensors with large electrode layer gaps.