Compact Front Research Articles

Mesoscale simulation of granular materials under weak shock compaction–pore size distribution effects

This research established a systematic method to generate various pore-size distributions (PSDs) and studied the effect of PSDs on the shock compaction response of granular materials using two-dimensional mesoscale simulations under identical porosity. Simulations utilized various PSDs for three particle shapes (circle, ellipse, and square). Contacting particle configurations using three PSDs, characterized by spatially uniform distributed pores to heterogeneous distributed pores, and non-contacting particle configurations under a single case of PSD were tested. The PSD of generated particle sets was characterized using coordination number, mean diameter, and bimodality coefficient as statistical metrics. Mesoscale simulations showed that regardless of the conditions of pore distributions, shock compaction of granular materials consistently demonstrates a precursor, shock compaction front, and end. However, the shock compaction velocity of contacting particles was dependent on the PSDs despite the constant initial porosity. The compaction velocity was faster in particle configurations with relatively uniform pore distributions than in heterogeneous pore distributions, which our study demonstrated can be attributed to particle rearrangement during compaction. Circular-shaped particles had high sensitivity in shock compaction response to the various PSDs. Furthermore, a contacting particle configuration tended to propagate the shock compaction wave relatively faster than particles that were in a non-contact configuration. This study established the relative importance of considering PSD as a metric over the coordination number in studies of the shock compaction response of granular materials. Further, insights are provided on the evolving shock substructure to characterize the shock compaction response of granular materials.

Analyzing Foam Injection Dynamics in Porous Media: A Combined Computational and Experimental Approach

The study of foam injection dynamics into porous media holds numerous applications, with its primary use being the optimization of the oil extraction process. Additionally, it finds utility in various other domains, including soil decontamination techniques. This study focuses on evaluating foam injection through low-cost experiments and computational models. The model integrates a classic two-phase immiscible fluid flow in a porous media framework, augmented by a mobility reduction factor to simulate foam’s impact on gas phase mobility. Simple experiments were conducted to assess water displacement using both gas and gas-foam injection methods. Results revealed that while gas injection tends to form preferential paths rapidly, leading to gas disruption, foam injection generates a more compact front, enhancing sweep efficiency by mitigating preferential path formation. The computational model successfully reproduced the experimental findings qualitatively. This research introduces a cost-effective experiment suitable for didactic purposes, illustrating complex concepts and underscoring the complementary nature of experimental and computational methodologies in advancing engineering techniques.

Colloidal droplet desiccation on a electrowetting-on-dielectric (EWOD) platform

The physics of the effects of electric field on the desiccation of colloidal droplets, comprising of dispersed negatively charged nanoparticles [2 μl, 1(w/w. %)], are studied in a standard electrowetting-on-a-dielectric configuration. The extent of contact line pinning during evaporation is found to be a function of the magnitude of the applied voltage and quantified in terms of the dimensionless electrowetting number (η). The pinned contact line led to higher particle compaction as evidenced by the characterization of dried colloidal film thicknesses. Crack formation and their dynamics have been analyzed in detail to elicit the interplay of forces near the contact line region and on the compaction front. These aspects of crack formation are elucidated in the light of magnitude and polarity of the applied electric field. It is found to influence the crack front initiation velocity, the geometry, the number of cracks, and an attempt is made to explain the same via first principle-based approaches. Therefore, this study indicates the possibility of using electrowetting as a technique to fine-tune the crack formation behavior in thin colloidal films.

Weak shock compaction on granular salt

This study conducted integrated experiments and computational modeling to investigate the speeds of a developing shock within granular salt and analyzed the effect of various impact velocities up to 245 m/s. Experiments were conducted on table salt utilizing a novel setup with a considerable bore length for the sample, enabling visualization of a moving shock wave. Experimental analysis using particle image velocimetry enabled the characterization of shock velocity and particle velocity histories. Mesoscale simulations further enabled advanced analysis of the shock wave’s substructure. In simulations, the shock front’s precursor was shown to have a heterogeneous nature, which is usually modeled as uniform in continuum analyses. The presence of force chains results in a spread out of the shock precursor over a greater ramp distance. With increasing impact velocity, the shock front thickness reduces, and the precursor of the shock front becomes less heterogeneous. Furthermore, mesoscale modeling suggests the formation of force chains behind the shock front, even under the conditions of weak shock. This study presents novel mesoscale simulation results on salt corroborated with data from experiments, thereby characterizing the compaction front speeds in the weak shock regime.

Dynamic characteristics of density-graded cellular materials for impact mitigation

Due to their exceptional impact resistance capabilities, density-graded cellular materials have immense potential in applications where crashworthiness requirements are of prime importance. Under impact loading, the deformation in these materials is characterized by compaction front propagation. Previous studies have utilized numerical techniques to solve the equations governing compaction shock propagation for cellular materials with density gradation. In this study, analytical solutions are formulated using compaction front position as the independent variable. The expressions for the velocity of the impinging rigid mass, energy absorption, incident stress, and transmitted stress are determined. The analytical solutions are shown to be in excellent agreement with the cell-based finite element solutions. The effect of density gradient on energy absorption and stresses is studied. The impact resistance factor is employed to assess different density-graded cellular materials for their effectiveness in impact mitigation. This study demonstrates that density-graded cellular materials can offer superior impact protection to objects at both the incident and transmitted ends. Lower density toward the incident end enhances impact resistance to objects located there, and likewise, lower density at the transmitted end offers more impact resistance to objects at that end.

Drainage explains soil liquefaction beyond the earthquake near-field

Earthquake-induced soil-liquefaction is a devastating phenomenon associated with loss of soil rigidity due to seismic shaking, resulting in catastrophic liquid-like soil deformation. Traditionally, liquefaction is viewed as an effectively undrained process. However, since undrained liquefaction only initiates under high energy density, most earthquake liquefaction events remain unexplained, since they initiate far from the earthquake epicenter, under low energy density. Here we show that liquefaction can occur under drained conditions at remarkably low seismic-energy density, offering a general explanation for earthquake far-field liquefaction. Drained conditions promote interstitial fluid flow across the soil during earthquakes, leading to excess pore pressure gradients and loss of soil strength. Drained liquefaction is triggered rapidly and controlled by a propagating compaction front, whose velocity depends on the seismic-energy injection rate. Our findings highlight the importance of considering soil liquefaction under a spectrum of drainage conditions, with critical implications for liquefaction potential assessments and hazards.

Pressure evolution in shock-compacted granular media

The pressure evolution associated with the transient shock-induced infiltration of gas flow through granular media consisting of mobile particles is numerically investigated using a coupled Eulerian–Lagrangian approach. The coupling between shock compaction and interstitial flow has been revealed. A distinctive two-stage diffusing pressure field with deflection occurring at the tail of the compaction front is found, with corresponding spikes in both gaseous velocity and temperature profiles emerging within the width of the compaction front. The compaction front, together with the deflection pressure, reaches a steady state during the later period. An analytical prediction of the steady deflection pressure that considers the contributions of porosity and the non-isothermal effect is proposed. The isothermal single-phase method we developed, combining the porosity jump condition across the compaction front, shows consistent pressure evolution with the non-isothermal CMP-PIC one under weak shock strength and low column permeability. Lastly, the microscale mechanism governing the formation of not only pressure deflection but also gaseous velocity and temperature spikes within the width of the compaction front has been described. These aforementioned evolutions of the flow field are shown to arise from the nozzling effects associated with the particle-scale variations in the volume fraction.

Compact front module with integrated miniaturised antenna in package for parabolic reflector antenna in satellite ground terminals

A compact front module with integrated miniaturised antenna in package for parabolic reflector antenna is presented for GEO-KOMPSAT-2A (GK-2A) satellite ground terminal application at 1692 MHz. The active feeder antenna module consists of four parts: a pair of miniaturised slot antennas, a pair of differential feeding lines, a metal shielding box, and front circuits, including a low noise amplifier (LNA) and a surface acoustic wave bandpass filter (SAW BPF). To reduce the lateral size of the feeder antennas and obtain required radiation patterns, meandered radiation slots are suggested and fed by a pair of differential coplanar waveguide (CPW) lines. The antenna substrate serves as the module cover and the electromagnetic shielding with a lateral size of 48 × 48 mm2 (0.27 × 0.27 λ 2 ${\lambda }^{2}$ ). Additionally, the feeder antenna module is proven to work with the specified parabolic reflector, both in theory and simulation. The front module with the packaged antenna is fabricated, and the measured output gain exceeds 20 dBi within a desired narrow band. Moreover, the module also achieves excellent performance in the GK-2A satellite signal-receiving experiment when used to feed the parabolic reflector antenna. The entire front module exhibits good package integration and application performance.

The importance of fault damage zones for fluid flow in low-permeable carbonate rocks – fault-related compaction fronts in the Danish North Sea

This study documents the timing and driving forces of the formation of fault-related compaction fronts in the Upper Cretaceous to lowermost Paleogene Chalk Group in the southern Danish Central Graben, based on the integration of 3D seismic, petrophysical log and clumped isotope data. The compaction fronts reflect zones in the low-permeable Chalk Group that underwent time-transgressive fault reactivation and pore fluid venting driven by movements of deeper salt and inversion movements. The fault-damage zones formed narrow permeability fairways that facilitated better drainage of compaction-driven pore fluids trapped in the matrix, eventually resulting in preferential mechanical compaction of the chalk. Salt doming during the Paleocene – Early Eocene, possibly linked to regional inversion tectonics, led to an initial phase of fault reactivation, offsetting the entire Chalk Group; pockmarks within this interval indicate release of pressurized fluids on the seafloor. Clumped isotope data from calcite-cemented veins associated with these fault-damage zones indicate precipitation from fluids that likely originated from Lower to Middle Jurassic strata at the root of these faults some 1500 m below the Chalk Group. Local thickening of the Paleocene to Lower Miocene Rogaland and Hordaland Groups matches a 20–50 m thinning of the chalk within the compaction fronts. This indicates that preferential drainage and compaction continued as the chalk became buried with clays, which became affected by polygonal-faulting causing episodic leak-offs. The results indicate that fault damage zones in low-permeability rocks may initially act as permeability fairways, but the improved drainage of formation fluids may over time cause preferential mechanical compaction and calcite precipitation. At present, the fault-related compaction fronts form low-porosity chalk bodies that may have acted as seals and/or re-directed fluid migration. The results have important implications for static and dynamic reservoir models, also in the light of Carbon Capture Storage and geothermal energy extraction and storage.

The Influence of Grain Size Distribution on Mechanical Compaction and Compaction Localization in Porous Rocks

AbstractThe modes of formation of clastic rocks result in a wide variety of microstructures, from poorly‐sorted heterogeneous rocks to well‐sorted and nominally homogeneous rocks. The mechanical behavior and failure mode of clastic rocks is known to vary with microstructural attributes such as porosity and grain size. However, the influence of the grain size distribution, in particular the degree of polydispersivity or modality of the distribution, is not yet fully understood, because it is difficult to study experimentally using natural rocks. To better understand the influence of grain size distribution on the mechanical behavior of porous rocks, we prepared suites of synthetic samples consisting of sintered glass beads with polydisperse grain size distributions. We performed hydrostatic compression experiments and found that, all else being equal, the onset of grain crushing occurs much more progressively and at lower pressure in polydisperse synthetic samples than in monodisperse samples. We conducted triaxial experiments in the regime of shear‐enhanced compaction and found that the stress required to reach inelastic compaction was lower in polydisperse samples compared to monodisperse samples. Further, our microstructural observations show that compaction bands developed in monomodal polydisperse samples while delocalized cataclasis developed in bimodal polydisperse samples, where small grains were systematically crushed while largest grains remained intact. In detail, as the polydispersivity increases, microstructural deformation features appear to transition from localized to delocalized through a hybrid stage where a compaction front with diffuse bands propagates from both ends of the sample toward its center with increasing bulk strain.

Using Evolving Algorithm with Distance Indicator for Solving Different Non-linear Optimization Problems

In this paper, we have relied on the dominant control system as an important tool in building the group of leaders because it allows leaders to contain less dense areas, avoid local areas and produce a more compact and diverse Pareto front. Nine standard nonlinear functions yielded this result. MaBAT/R2 appears to be more efficient than MOEAD, NSGAII, MPSOD, and SPEA2. MATLAB was used to generate all the results of the proposed method and other methods in the same field of work.

Dynamic compaction induced heterogeneity in boron carbide powder

To investigate the impact-induced heterogeneous responses caused by particle rotation and slip in the process of powder compaction, dynamic compression experiments of Boron carbide powders with two packing densities (i.e. 1.22 and 1.11 g/cm3) are conducted by the mini-split Hopkinson pressure bar (mini SHPB) device combining with the simultaneous and in-situ X-ray imaging system. The compaction front, characterized as the impact-induced heterogeneous deformation, is observed and analyzed by the digital image correlation (DIC) method based on the images recorded by the simultaneous X-ray imaging system. The distribution of the contrast ratio of X-ray images is applied to demonstrate the propagation of compaction, and it shows a clear compaction front. The mean rotation degree and the shearing resistance at the compaction band are discussed in detail to describe the particle state during compaction. The results illustrate a relatively stable compaction process in denser sample A and an unstable one in looser sample B, which account for the abnormal evolutions of compaction density and porosity when comparing these two samples. A relaxation-diffusion model is then obtained based on the equivalent shear strain activation mechanism to describe the forming of the compaction front. The shearing diffusion resistance, a generalized driving force for compaction front shifting, is introduced to investigate particles’ movement at the compaction front, which is more powerful for describing the compaction degree of powder. The results might help improve contact extent and sintering of powders.

Broadband and tunable terahertz polarization converter based on graphene composite metasurface

In this paper, a broadband and tunable terahertz polarization converter metasurface is proposed, which is composed of an open metal ring embedded with graphenes and a columnar metal on polyimide (PI) dielectric–metal substrate. The polarization conversion rate (PCR) converted to cross-polarized in reflection mode can be regulated from 95% to 20% in a wide band from 2.1 to 3.6 THz by adjusting the Fermi energy of graphene. Simulation results show that broadband polarization conversion results from three resonances in neighboring frequencies, and the physical mechanism underlying tunable function is explained by the distribution of surface current in different states. In addition, the reflection coefficient and reflection phase in orthogonal component are calculated to further reveal basic principle of proposed polarization converter. This study could continue the process of developing compact and active polarization and wave front control application in the THz, and may be applied in various areas such as wireless communication, terahertz sensing and imaging.

Measurement of volume fraction distribution in a drying film by imaging with a digital camera

We propose a method to measure the evolution of the volume fraction distribution in colloidal films during their drying process, using only the transmitted light intensity recorded with a commercial digital camera. From the Lambert-Beer law, the volume fraction of a dispersion film with a certain film thickness can be measured from the transmitted light intensity. A pseudo-absorption capacity was imparted to the transparent dispersion by adding a red dye, and the ratio of green to blue light absorbance was measured for the liquid film. For any film, the volume fraction increased spatially towards a compaction front, and the volume fraction in this region reached approximately 63.4%. The volume fraction inside the liquid region of the film also increased during drying. These trends are similar to previous results obtained using other methods. The magnification in this method can be easily changed by replacing the imaging equipment, and changes in volume fraction distribution near the drying front were observed at high magnification using different lenses. This method, based on captured photographic images, measure the two-dimensional distribution of the volume fraction, making it easy to observe changes in the volume fraction distribution caused by drying at both macroscopic and microscopic levels. It is likely that the proposed method will be difficult to apply to anisotropic particles or to larger particles which cause significant scattering.

The Importance of Fault Damage Zones for Fluid Flow in Low-Permeable Carbonate Rocks – Fault-Bound Compaction Fronts in the Danish North Sea

The Importance of Fault Damage Zones for Fluid Flow in Low-Permeable Carbonate Rocks – Fault-Bound Compaction Fronts in the Danish North Sea

Morphology quantification of three-dimensional fluid invasion patterns

In many situations, patterns of immiscible fluid displacement appear obviously different at first glance, but can hardly be distinguished using the commonly applied quantification by fractal dimension. In this work, we propose the mean finger area of the invading fluid, the average distance of defending fluid elements to the invading fluid as well as a discrete surface area of a coarse grained fluid representation as three alternative methods to characterize fluid displacement patterns in three dimensional permeable media. Applying the proposed methods to X-ray microtomography data of fluid displacement experiments in bead packs of homogeneous and mixed wettability, all of the three methods allow to clearly distinguish between a compact front morphology for wetting invading liquids and a finger-like structure for non-wetting invading liquids. When compared to the fractal dimension of the fluid pattern, all three quantities reveal more details with respect to the structure of the invading liquid. Applying these methods to microtomography data of fluid displacement in heterogeneously wetting bead packs reveal a fingering structure and preferential invasion paths that are controlled by local wettability.

A 300-GHz Transmitter Front End With −4.1-dBm Peak Output Power for Sub-THz Communication Using 130-nm SiGe BiCMOS Technology

This article presents a compact 300-GHz transmitter front end manufactured in a 130-nm SiGe BiCMOS process. The transmitter consists of a 240-GHz amplifier multiplier chain (AMC) and a modified 300-GHz Gilbert mixer. Limited by the space between top two thick metal layers of the SiGe process, the coupling coefficient between the coils, which form a transformer-based balun, is usually small at subterahertz (THz). Therefore, the vertical or horizontal coupling single-turn transformer-based balun will exhibit large insertion loss at around 300 GHz. In this work, a self-shielded Marchand balun (SSMB) with an enhancement to the coupling coefficient is proposed, which is realized by a novel multilayer metal topology with self-shielded coupling (SSC). The AMC is composed of a 120-GHz frequency doubler, a 120-GHz two-stage power amplifier (PA), and a two-way power synthesis balanced frequency doubler. This AMC exhibits a measured peak output power of 5.5 dBm at 252 GHz, with 48-GHz 3-dB bandwidth from 212 to 260 GHz. The transmitter chip achieves a maximum output power of −4.1 dBm at 300 GHz and delivers an output power better than −10 dBm from 270 to 315 GHz. Over the 30-GHz 3-dB bandwidth from 280 to 310 GHz, the transmitter shows a maximum OP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1dB</sub> of −6.5 dBm at 296 GHz, a peak conversion gain of −11.2 dB at 298 GHz, and a local oscillator (LO)-to-RF leakage rejection better than 40 dB, with only 300-mW dc power consumption. Compared with other state of the arts, the transmitter exhibits a comparable output power among silicon-based transmitters near 300 GHz.

Gravity effects on oil–water two-phase displacement in homogeneous porous media

Gravity plays an important role in enhanced oil recovery and groundwater hydrology. A two-dimensional visual homogeneous micromodel was used in this study to describe the role of gravity in displacement processes. A theoretical analysis is proposed for three flow modes, i.e., vertical-upward, vertical-downward, and horizontal displacements, in which water and decane are used for the displacing and the displaced phases, respectively. A relatively compact displacement front was obtained at high flow rates in the three displacement modes, and the front gradually became unstable with a decrease in the flow rate. Compared with horizontal displacement, in vertical-upward displacements, gravity can hinder the evenness of the flow and aggravate the front finger formations at the inlet. This process forces the heavier displacing phase to expand horizontally at the midpoint and weakens the front's fingers. In the vertical-downward displacement process, two states occurred at the same low flow rate: stable flow and unstable flow. Unstable flows occurred more frequently with a decrease in the flow rate. To better understand the role of gravity in displacement, we proposed a theoretical prediction model for the flow state transition of the three displacement modes by combining the capillary force, viscous force, and gravity based on pore-filling events. Finally, to predict the final recovery factor for various displacement modes, four dimensionless formulations were produced using the capillary number, the gravity number, the bond number, and the viscosity ratio.

Impact of Wettability and Gravity on Fluid Displacement and Trapping in Representative 2D Micromodels of Porous Media (2D Sand Analogs)

AbstractThe combined effects of gravitational forces and wettability on displacement and trapping have not previously been investigated experimentally in realistic 2D porous media. Here we report the first use of representative 2D micromodels that accurately reproduce characteristic geometric, morphological, and topological properties of 3D sand packs. We use these representative micromodels to compare the wettability dependence of displacement front morphology and trapping processes using selected fluid pairs at capillary numbers between 10−6 and 10−5. The contact angles studied ranged from 30° (normal imbibition) to 150° (strong drainage). The impact of gravity was dramatic: In the horizontal flow case, only one finger developed, that is longitudinal displacement was always favored, whereas gravity created a more compact displacement front with broad lateral extension, and this lateral movement created loops, leading to by‐pass trapping. As a result, the trapping efficiency rose from 7% to 29%. We observed universal scaling of the cluster size distribution demonstrating that our micromodels are statistically representative. The universal 2D‐scaling exponent of 2.05 was obtained with an acceptable error smaller than 5%. Our experimental results obtained by fluorescence microscopy showed (a) precursor corner flow for weak imbibition at θ = 51°, that is above the critical contact angle of αc = 45° (Zhao et al., 2016, https://doi.org/10.1073/pnas.1603387113), and (b) a new wetting transition from corner flow to full duct flow under neutral wettability conditions, that is at θc ≈ 90°; and (c) above this second critical contact angle, that is under strong drainage conditions, we again observed corner flow (specifically, core‐annular flow).

Shaping surfaces and interfaces of 2D materials on mica with intercalating water and ethanol

Interfaces between mica and graphene, as well as the transition metal dichalcogenides (TMDCs) MoS2 and WS2, were wetted with water and ethanol, and investigated employing scanning force microscopy and molecular dynamics (MD) simulations. Below 25% RH, water wets the graphene-mica interface with labyrinthine structures, exhibiting branch widths of about 50 nm for single layers of graphene, increasing to almost an order of magnitude more for four graphene layers. At mica-TMDC interfaces, water films exhibit a transition from labyrinthine to compact morphology upon going from single- to multi-layers of the TMDCs. Ethanol films show a compact morphology at all the interfaces, regardless of the number of 2D material layers on top. The film morphologies are attributed to an equilibrium between electrostatic repulsion of preferentially oriented molecular dipoles, and the line tension of the wetted areas, which is dominated by the deformation of the 2D materials at the edges of the wet areas. The compact front of the water wetting film under multilayers of TMDCs is attributed to a much larger bending stiffness of these materials than of graphene multilayers. The thickness dependent stiffness of the 2D materials may be employed to shape their surfaces from the nano- to the micrometer scale.