Multiphysics Numerical Simulations Research Articles

Steady-state and transient simulations of the MTR research reactor using the high-fidelity N-TH coupling method

The tremendous advancement in computer technology makes it feasible to perform high-fidelity and multi-physics numerical simulations of reactors, allowing for a more accurate and comprehensive description of the phenomena that occur within nuclear reactors. The application of the high-fidelity coupled neutronic and thermal-hydraulic codes to execute the safety analysis and assessment of the fresh highly enriched uranium core of the IAEA-10 MW MTR research reactor is presented in this research. A three-dimensional coupled neutronic and thermal-hydraulic code is realized using the open-source C++ library OpenFOAM. The coupled code used in this study utilizes an internal coupling approach, where the identical program and grid system are utilized to solve both the neutron physical and thermal-hydraulic fields. The neutron physical field in the reactor is solved by the three-dimensional multi-group time-dependent neutron diffusion equations, and the CFD methodology is applied in solving the thermal-hydraulic governing equations. The global and local steady-state calculation results of MTR are obtained, which are in accordance with the results issued from the IAEA TECDOC and other previous relevant studies. The unprotected loss of flow accident transient condition is considered to conduct the simulation and analysis of the MTR research reactor. The evolutions of temporal and spatial distributions of important parameters of the MTR research reactor are acquired under the ULOFA transient condition, revealing the responses of the reactor core during the transient accident without SCRAM. The results obtained show that the high-fidelity N-TH coupling code can be utilized to analyze the safety of the MTR research reactor.

COMPUTATIONAL FLUID–STRUCTURE INTERACTION SIMULATION OF HEMODYNAMICS OF ARTERIOVENOUS FISTULA

The objective of this study is to show how important a compliant wall technique is in simulating non-Newtonian and pulsatile blood flows in arteriovenous fistula (AVF). The three-dimensional idealized geometry is used to investigate the local hemodynamics in the end-to-side, radio-cephalic AVF using computational fluid-stricture interaction (FSI) simulation. The third-order Yeoh law is used to model the behavior of the hyperelatic vessel walls. Hemodynamic parameters such as velocity, wall shear stress (WSS), oscillatory shear index (OSI), vorticity, and venous outflow rate are calculated. The results extracted for WSS on comparison of rigid and compliance wall, rigid wall WSS are 45–48.5% larger values than the compliance wall. The difference between compliance wall and rigid wall OSI is 11.5% and the rigid wall is a 10.86% decrease in compliance wall. The difference between the rigid wall AVF vorticity and the compliance wall vorticity is 18.34%, and the vorticity in the compliance wall is a 20.2% increase over the rigid wall. Maximum principle stress occurs at anastomosis (1335[Formula: see text]Pa) and 507[Formula: see text]Pa, 93[Formula: see text]Pa on vein and artery, respectively. Finally, we conclude that the artery bed and heel of AVF are prone areas of Intimal Hyperplasia. The structural result shows the tendency of the inflammation pattern of the vein required for AVF maturation. ANSYS is a highly effective commercial tool for modeling real-world problems and performing multi-physics numerical simulations.

Kirigami Makes a Soft Magnetic Sheet Crawl.

Limbless crawling on land requires breaking symmetry of the friction with the ground and exploiting an actuation mechanism to generate propulsive forces. Here, kirigami cuts are introduced into a soft magnetic sheet that allow to achieve effective crawling of untethered soft robots upon application of a rotating magnetic field. Bidirectional locomotion is achieved under clockwise and counterclockwise rotating magnetic fields with distinct locomotion patterns and crawling speed in forward and backward propulsions. The crawling and deformation profiles of the robot are experimentally characterized and combined with detailed multiphysics numerical simulations to extract locomotion mechanisms in both directions. It is shown that by changing the shape of the cuts and orientation of the magnet the robot can be steered, and if combined with translational motion of the magnet, complex crawling paths are programed. The proposed magnetic kirigami robot offers a simple approach to developing untethered soft robots with programmable motion.

A proposal of risk indicators for pathological development from hemodynamic simulation: application to aortic dissection

Cardiovascular diseases are the leading cause of mortality in the industrialized world. Among these diseases, aortic dissection affects the aorta wall and is a surgical emergency with a low survival rate. This pathology occurs when an injury leads to a localized tear of the innermost layer of the aorta. It allows blood to flow between the layers of the aortic wall, forcing the layers apart and creating a false lumen. Endovascular treatment seeks to obliterate the entrances to the false lumen with a covered stent. There are very few studies on the postoperative demonstration of blood flow phenomena in the aortic dissection endovascular treatment. It is crucial to study the hemodynamics of blood in the aorta after an intervention because the new geometrical configuration of the aorta with a stent leads to modifications in blood flow. For the surgeons, the procedure can only be performed empirically, using MRI-4D images to view the postoperative flow of the patient’s blood in the aorta with the stent. This paper aims to present a numerical tool developed from the open-source software FOAM- Extend®, allowing for multiphysics numerical simulations. Using MRI data, a bio-faithful model of the patient-specific case was built. Numerical simulations were performed to predict preoperative and postoperative (endovascular treatment) hemodynamics. The modifications of the flow in the aorta were analyzed focusing on the postoperative perfusions. The results were compared with the corresponding MRI data and have a good qualitative agreement. Biomarkers are calculated to localize possible zones of post-operative pathological developments and recommendations may be suggested to the surgeons.

Mass loss, sublimation, and surface damage of lanthanum hexaboride in an arc jet plasma

An experimental and simulation-based approach is used to determine the effects of an arc jet plasma on the refractory ceramic material lanthanum hexaboride (LaB6). Experiments are performed at the High Energy Flux Test facilitY (HEFTY) located at UCLA. An SG-100 plasma jet generates an argon plasma into a vacuum chamber and imparts a maximum heat flux of 19.5 MW/m2 onto LaB6 disks. Heat flux results are calibrated using a combination of thermocouple data as well as multiphysics numerical simulations in COMSOL, which aim to replicate the testing environment. Moreover, material characterization tools including scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction, and optical profilometry are used to better understand the mechanisms by which LaB6 loses mass through evaporation, sublimation, and surface damage during an arc jet exposure. It is determined that a minimum energy fluence of 200–300 MJ/m2 produces a consistent LaB6 melt pool and that an incident heat flux of 19.5 MW/m2 results in a 0.11 mm/s surface recession rate.

An acoustic signature of extreme failure on model granular materials

Unexpectedly, granular materials can fail, the structure even destroyed, spontaneously in simple isotropic compression with stick-slip-like frictional behaviour. This extreme behaviour is conceptually impossible for saturated two-phase assembly in classical granular physics. Furthermore, the triggering mechanisms of these laboratory events remain mysterious, as in natural earthquakes. Here, we report a new interpretation of these failures in under-explored isotropic compression using the time-frequency analysis of Cauchy continuous wavelet transform of acoustic emissions and multiphysics numerical simulations. Wavelet transformation techniques can give insights into the temporal evolution of the state of granular materials en route to failure and offer a plausible explanation of the distinctive hearing sound of the stick-slip phenomenon. We also extend the traditional statistical seismic Gutenberg–Richter power-law behaviour for hypothetical biggest earthquakes based on the mechanisms of stick-slip frictional instability, using very large artificial isotropic labquakes and the ultimate unpredictable liquefaction failure.

Multi-Physics Analysis of a Magnetorheological Valve Train with Experimental Validation

Magnetorheological (MR) fluid devices are widely used in active automotive control applications. However, MR fluid-based valve actuators are not in the limelight. This paper proposes a new flexible valve train with an MR fluid control system; the valve train can enhance the performance of internal combustion engines. A major component of this valve train is the magnetic plate block filled with MR fluid and surrounded by a magnetic coil. This plate block controls the magnetic field in this MR fluid and eventually facilitates flexible valve lifts and valve opening timings. This study overviewed the conceptual design, two-way coupled multi-physics numerical simulations, manufactured an MR valve prototype, and conducted experimental tests on a test bench to understand the real-time performance of the MR valve train. First, computer simulations were performed using a coupled magnetic and thermal multiphysics model to consider the Joule-heating effect of the magnetic coil in the MR magnetic plate block. The simulation results indicated that although the temperature of the MR fluid increased noticeably, it did not exceed the prescribed operating limits. The dimensions of the MR magnetic plate block were optimized. After computer simulations and optimization, a prototype of the proposed MR valve was fabricated and tested to understand its performance in real time. The experimental test results indicated the reliability of the proposed MR valve train in practical scenarios.

A Passive Micromixer with Koch Snowflakes Fractal Obstacle in Microchannel

The passive micromixer is one of the essential devices that can be integrated into the Lab on Chip (LoC) system. Micromixer is needed to increase mixing efficiency. In this paper, two Koch fractal obstacle-based micromixer models of Secondary Snowflakes Fractal Micromixer (SSFM) and Tertiary Snowflakes Fractal Micromixer (TSFM) were designed. The effect of the Koch fractal resistance angles (15o, 30o, 45o, 315o, 330o, 345o) and the influence of the inlet (T and T-vortex) were studied in this paper using COMSOL Multiphysics numerical simulations. The results showed that the TSFM structure with a 30o angle on the T-vortex inlet is optimal. The deflection phenomena generated by the TSFM obstacle enhance the contact area between the two fluids and chaotic convection can be increased at Reynolds Number (Re) 0.05 and Re 100. This paper examines concentration curves along the channel ranging from 1 mol/L to 5 mol/L. This clearly shows that the fluid flow direction changes within the microchannel. This work provides a new design for the micromixer.

Mass and current uniformity for planar solid oxide fuel cells with discrete landing structured flow fields: A three-dimensional numerical analysis

The rational design of flow fields is of vital importance for the internal distribution uniformity and overall power output of solid oxide fuel cells (SOFCs). This study reports the design of discrete cylindrical landing flow fields for planar SOFCs to tackle the in-plane unevenness problems. The effect of key geometric parameters on the internal mass and current distribution and the overall power output of SOFCs has been investigated using three-dimensional (3D) multi-physics numerical simulations. It is found that the cylindrical landing flow fields significantly reduce the local variation of mass distribution and improve the uniformity of current distribution compared with the parallel landing flow fields. The overall output performance of the cell is improved by the cylindrical landing flow field (D2.0-S1.0) due to the synergetic effect of increased pressure drop, enhanced gas transport and reduced ohmic loss. The results of this study demonstrate the effective of the cylindrical landing flow fields for improving the distribution uniformity of planar SOFCs and provide theoretical insights for further development and optimization of the cylindrical landing flow fields for related applications.

Evolutions of the electromagnetic signatures induced by the propagating wake behind a submerged body

The in situ electromagnetic signatures induced by the movement of conductive seawater in the submarine wake subject to the geomagnetic ambience will provide crucial information for non-acoustic detection. However, several complex multi-physical field coupling processes, for instance, ions concentration variations, turbulent vortex structures, and violent heat and mass transfer have not been systematically investigated, which jointly influence the induced electromagnetic signatures in the propagating stratified wake behind a submerged body. To reveal the interaction mechanisms among flow, heat transfer, ions concentration variations, and electromagnetic fields, we construct a novel multidisciplinary model integrating the mass, momentum, energy, ions concentration, and Maxwell's equations. The pronounced range and intensity of the electromagnetic signatures in the near-field wake are obtained by multi-physics field coupling numerical simulations. The instantaneous visualizations of the near-field wake manifest the distinct fluctuations features, revealing that the magnitude of the induced magnetic intensity can be maintained in the order of 10−10 T. 3D spatial evolutions of the electromagnetic variations are further scrutinized by some quantitative profiles, which indicate that the magnitudes of the wake perturbations fall within the current state of the art instrumentation capabilities. Subsequently, further pertinent analyses about the electromagnetic distributions with different navigational velocities are also explored and reported to elucidate the effects of velocity. These scientific findings provide a conducive insight for lucubrating the evolutions of the induced electromagnetic wake, and assist in further promoting the development of the submersible non-acoustic detection.

A computational fluid dynamic study of the filling of a gaseous hydrogen tank under two contrasted scenarios

The filling of a horizontal hydrogen tank designed for light duty vehicles is investigated by means of multi-physics numerical simulations. The simulation approach, implemented in OpenFOAM, includes compressible Reynolds-Averaged Navier-Stokes (RANS) modeling of the fluid flow and heat transfer in the solid parts. The simulations are carried out for 2D-axisymmetric and 3D configurations. Two filling scenarios of the tank, leading to two distinct thermal behaviors, i.e. homogeneous versus heterogeneous, are simulated and compared to the experimental data issued from the HyTransfer project. In the homogeneous case, where no thermal stratification occurs, the 2D and 3D simulation results are close to the experimental ones. A phenomenon of jet flapping is identified via the 3D simulation. In the heterogeneous case, where thermal stratification occurs, the 3D simulation captures an averaged temperature close to the experimental one, as well as the instant at which the thermal gradients appear. It also captures the deflection of the jet, which is a central element in the emergence of the thermal gradients.

Asymmetric Nanochannel Network-Based Bipolar Ionic Diode for Enhanced Heavy Metal Ion Detection

A higher rectification degree in ionic diodes is required to achieve better performance in applications. Nonetheless, the active geometrical change that is critical for inducing electrical potential asymmetry is difficult to realize in typical ionic diodes because of the intrinsic limitation of the fabrication method. Here, we propose a nanochannel-network-based bipolar diode with a high rectification degree of ∼1600─the highest value realized until now, to the best of our knowledge. Such a high rectification is obtained based on the synergetic effect of the bipolar surface charge and the optimization of the microchannel through experimental studies and multiphysics numerical simulations. It induces ion concentrations at the heterogeneous junction based on the accumulation effect under the forward potential bias. In particular, this proposed molecular concentration occurs in the ohmic region without vortex and instability that is inevitable at the conventional nano-electrokinetic concentration. Combining this accumulation with the horizontally aligned configuration of the nanochannel network membrane (NCNM), a highly sensitive and quantitative mercury ion (Hg2+) sensor based on a fluorescent signal is fabricated that allows direct measurement using a general fluorescent microscope. The detection limit of Hg2+ is 10 pM, which is ∼10 times lower than the best detection limit realized so far (∼100 pM) in fluorescent dye-based detection. This demonstrates the potential of asymmetric NCNM for high-performance ion transport in applications such as energy conversion, based on its design and material flexibility.

Dexterous magnetic manipulation of conductive non-magnetic objects

Dexterous magnetic manipulation of ferromagnetic objects is well established, with three to six degrees of freedom possible depending on object geometry1. There are objects for which non-contact dexterous manipulation is desirable that do not contain an appreciable amount of ferromagnetic material but do contain electrically conductive material. Time-varying magnetic fields generate eddy currents in conductive materials2-4, with resulting forces and torques due to the interaction of the eddy currents with the magnetic field. This phenomenon has previously been used to induce drag to reduce the motion of objects as they pass through a static field5-8, or to apply force on an object in a single direction using a dynamic field9-11, but has not been used to perform the type of dexterous manipulation of conductive objects that has been demonstrated with ferromagnetic objects. Here we show that manipulation, with six degrees of freedom, of conductive objects is possible by using multiple rotating magnetic dipole fields. Using dimensional analysis12, combined with multiphysics numerical simulations and experimental verification, we characterize the forces and torques generated on a conductive sphere in a rotating magnetic dipole field. With the resulting model, we perform dexterous manipulation in simulations and physical experiments.

On the complete interface development of Al/Cu magnetic pulse welding via experimental characterizations and multiphysics numerical simulations

A complex Al/Cu magnetic pulse welding interface is systematically investigated using experimental characterizations and numerical simulations. A Coupled electromagnetic-mechanical simulation is proposed to compute the impact velocity and impact angle along the entire interface. This model allows to further understand the formation mechanism of various interface characteristics during MPW. The results revealed that the impact velocity gradually decreases in conjunction with the gradual increase of the impact angle. These simulations elucidate the experimentally observed successive interface morphologies, i.e., the unwelded zone, vortex zone, intermediate (IM) layers and wavy interface. Microstructural characterizations show that the IM layers are formed by mechanical mixing combined with melting and are characterized by highly heterogeneous porous zone with random sizes and distributions of void. Subsequently, an Eulerian simulation is proposed to investigate the thermomechanical effects during the wave formation which vastly influence the joint quality. The predicted wavy morphologies, temperature distribution and average equivalent plastic strain along the interface provided a better understanding on the formation steps of the experimentally observed wave morphologies. The actual kinematics during wave generation revealed that the wave was formed with repeated deformation of the interface material. The wave amplitude increases with increasing jetting angle; while the wavelength increases with increasing collision point velocity. The ratio between collision point velocity and impact velocity is found to be the most suitable parameter for explaining the influence of the collision parameters on the wavelength. These results indicate that the combination of the Coupled electromagnetic-mechanical simulation and Eulerian simulation can help us investigate the governing mechanisms of complex interface morphologies and further used to optimize the processing parameters during high speed impact welding.

Aspect ratio affects iceberg melting

Iceberg meltwater is a critical freshwater flux from the cryosphere to the oceans. Global climate simulations therefore require simple and accurate parameterisations of iceberg melting. Iceberg shape is an important but often neglected aspect of iceberg melting. Icebergs have an enormous range of shapes and sizes, and distinct processes dominate basal and side melting. We show how different iceberg aspect ratios and relative ambient water velocities affect melting using a combined experimental and numerical study. The experimental results show significant variations in melting between different iceberg faces, as well as within each iceberg face. These findings are reproduced and explained with novel multiphysics numerical simulations. At high relative ambient velocities melting is largest on the side facing the flow, and mixing during vortex generation causes local increases in basal melt rates of over 50%. Double-diffusive buoyancy effects become significant when the relative ambient velocity is low. Existing melting parameterisations do not reproduce our findings. We propose several corrections to capture the influence of aspect ratio on iceberg melting.

Microwave assisted pyrolysis of Kraft lignin in single mode high-Q resonant cavities: Degradation kinetics, product chemical composition, and numerical modeling

Useful fuels and chemicals can be produced from lignin by microwave-assisted pyrolysis, but a dearth of understanding of this process impedes its successful implementation. Continuous mass loss kinetics of the pyrolysis of Kraft lignin pellets were carried out in an innovative reactor system comprised of a high-Q cylindrical microwave resonant cavity and a specially designed quartz reactor, in the temperature range of 300–700 °C. Multiphysics numerical simulations indicated that both absorbed power and resulting temperatures profiles are heavily dependent on position of the sample relative to the electric field. Kraft lignin degradation (5 g samples) was complete in about 40 s, which was much faster than conventionally heated reactors. Activation energies (5–22 kJ/mol) and pre-exponential factors (0.06–0.64 s−1) were indicative that the process is low in energy consumption. At higher temperatures, phenols and phenolics were the major constituents of the bio-oil. A reliable method of obtaining microwave-assisted mass loss kinetics continuously is established.

Zooming in on Individual Star Formation: Low- and High-Mass Stars

Star formation is a multi-scale, multi-physics problem ranging from the size scale of molecular clouds ($\sim$10s pc) down to the size scales of dense prestellar cores ($\sim$0.1 pc) that are the birth sites of stars. Several physical processes like turbulence, magnetic fields and stellar feedback, such as radiation pressure and outflows, are more or less important for different stellar masses and size scales. During the last decade a variety of technological and computing advances have transformed our understanding of star formation through the use of multi-wavelength observations, large scale observational surveys, and multi-physics multi-dimensional numerical simulations. Additionally, the use of synthetic observations of simulations have provided a useful tool to interpret observational data and evaluate the importance of various physical processes on different scales in star formation. Here, we review these recent advancements in both high- ($M \gtrsim 8 \, M_{\rm \odot}$) and low-mass star formation.

Effect of gas impurity on the convective dissolution of CO2 in porous media

Growing needs for energy and the essential role of fossil fuels in energy market require attempts such as carbon dioxide (CO2) sequestration in saline aquifers to stabilize and mitigate atmospheric carbon concentrations. The possibility of co-injection of impurities along with CO2 allows for the direct disposal of flue gas and hence a significant reduction in the cost of CO2 sequestration projects by eliminating the separation process. In this study, the results of series of novel experiments in a high-pressure visual porous cell are reported, which allow for visually and quantitatively examining the dynamics of convective dissolution in brine-saturated porous media in the presence of an overlying impure free gas phase with conditions that reflect more closely the subsurface conditions. We find cases with 10% (respectively, 20%) N2 impurity show higher (respectively, lower) dissolution flux and greater (respectively, smaller) pressure drop at early time as compared to those with pure CO2. To shed light on this non-trivial behavior, image analysis is performed and hydrodynamic dispersion is revealed as the root to this behavior. Robust scaling relations for the obtained dispersion coefficients and transition time to the shut-down dissolution regime are reported based on dimensionless numbers. Additionally, multiphysics numerical simulations are performed and coupled with the captured dispersion quantities. Our findings elucidate how the presence of impurity in gas stream affects the overall efficiency of CO2 convective mixing, and how its essence can be leveraged in practice while improving the numerical models.

WITHDRAWN

The document associated with this DOI has been withdrawn.

Initial study of internally embedded shear-mode piezoelectric transducers for the detection of joint defects in laminate structures

There has been recent interest in embedding sensor networks into bondlines to create intelligent materials that can detect, report, and potentially respond to their state. This article presents an initial, large-scale investigation into embedding shear-mode (d15) lead zirconate titanate piezoelectric transducers into the bondline of laminate structures, near the centerline, for the ultrasonic detection of joint defects. The study included analysis of dispersion curves, multiphysics numerical simulations, and experimental results for an aluminum–epoxy–aluminum laminate structure. Analysis of the time of flight and displacement profiles confirmed that antisymmetric waves were generated and propagated through the structure. Simulations were performed for models containing disbonds, through-thickness cracks, and voids with experimental validation of a specimen containing a void to evaluate the effectiveness of d15 lead zirconate titanates embedded in a bondline as actuators and sensors for damage detection. The results were examined by looking at the cross-sectional deformation in simulations, and the signal changes were evaluated by calculating the root mean square deviation damage index and inspecting attenuation and phase shift in pristine and damaged structures. It was found that the d15 shear-mode lead zirconate titanates actuated and sensed antisymmetric waves that were sensitive to all types of damages considered.