Dimensional Numerical Simulations Research Articles

Nonplanar effects in simulations of laser-driven ejecta microjet experiments

Recent experiments of laser-driven ejecta microjets performed at OMEGA 60 reveal tortuous jets whereby the jets appear to deviate from their initial trajectory as they travel in vacuum. To understand these data, we perform two dimensional numerical simulations, considering different target thicknesses, pressures, and models of the drive conditions. In particular, modeling the finite laser spot size appears essential in reproducing qualitatively the non-planar shock observed in the experiment. Simulations capture jet deflection by accounting for a slight misalignment of the laser pointing with respect to the groove axis along with spatial variation of the laser pulse intensity. The principal physical mechanism appears to be that lateral momentum is imparted by release waves arising from the non-planar drive. The induced off-axis velocity is small in comparison to the jet axial velocity but integrates into a pronounced deflection over the course of the experiment. The analysis of jet axial and lateral mass distributions is found to be reproduced reasonably by the simulations. Simulated radiographs are in qualitative agreement with the experiments, though their differences point to potential shortcomings in modeling strictly three-dimensional experiments using two-dimensional hydrodynamic simulations. The simple analysis is able to explain part of the observed discrepancy in simulated vs experimental jet masses.

Thermal enhancement of microchannel heat sink using pin-fin configurations and geometric optimization

The microchannel heat sink (MCHS) is a robust cooling technique that ensures the efficiency and reliability of compact electronic devices by dissipating a large amount of heat because of its high surface area-to-volume ratio. This study proposes a novel modification of the pin-fins geometry in MCHS, and geometric optimization using response surface methodology (RSM) to build a low thermal resistant MCHS with enhanced heat transfer efficiency with low-pressure drop. Three dimensional numerical simulations using ANSYS FLUENT 2021 R2 are performed on three pin-fins configurations, i.e., MC-BW (pins mounted transversely to the bottom wall), MC-SW (pins mounted transversely to the side wall), and MC-Mixed (pins mounted transversely to the bottom and side wall). The thermal and flow characteristics are investigated using a laminar conjugate heat transfer model at Reynolds numbers 100–1000. Results show that introducing pin-fins significantly enhances heat dissipation as it continuously breaks the boundary layer and generates flow separation downstream of the pin-fins, which enhances fluid mixing and increases heat transfer augmentation inside MCHS. Among different configurations, the MC-Mixed gives the highest improvement of 50% in the convective heat transfer coefficient at Re = 1000. The highest thermal enhancement factor of η = 1.4 is obtained for the MC-Mixed configuration at Re = 600. For the base wall pin fin configuration RSM yields optimized values of 2.50 mm, 0.25 mm, and 0.045 mm for transverse pitch, longitudinal pitch, and diameter of pin respectively, and for the mixed pin fin configuration it gives 1.0 mm, 0.150 mm, 0.035 mm and 1.250 mm values for transverse pitch, longitudinal pitch, diameter of pin and pitch of side wall pins respectively for the maximum heat transfer and minimum pressure drop.

Implications of bioturbation induced by Procambarus clarkii on seepage processes in channel levees

River levees are subject to bioturbation by various animals which can actively excavate into earthen structures producing an internal erosion that, during the passage of a flood, can grow in time making the levee unstable. This phenomenon can lead to river levee breaching and, as a consequence, collapse, even for relatively minor flood events. A well-known animal burrower is represented by the North American crayfish Procambarus clarkii (P. clarkii), an invasive species in Europe, mainly introduced for commercial purposes, causing a decline in biodiversity and profound habitat changes. The physical damages caused by P. clarkii on levees and banks, such as in rice fields, irrigation ditches, and small channels, have not been fully studied and behavioral components underlying this impact are mostly occasional. To understand the impact of burrowing activity on the seepage process, a field survey was done in a drainage channel in Tuscany, Italy, to evaluate the density and geometry of the internal burrows that were excavated by the crayfish. Based on these observations and some previous laboratory experiments, three dimensional (3D) numerical simulations of the seepage processes were done inside burrowed levees. Numerical results allowed the increase in the hydraulic vulnerability of levees to the process of internal seepage to be disclosed. In particular, for a given river water level, the reduction of the time scale for the phreatic line to reach the levee field side appears to be a function of a quantity here defined as the burrow hydraulic gradient. This quantity is here defined as the ratio between the hydraulic head inside the burrow and the horizontal distance from its end to the field side of the levee. Moreover, a comparison between the 3D with the analogous more common two dimensional (2D) numerical simulations illustrated the schematization which is better suited for describing the seepage processes when animal burrows, not only by crayfish, are present.

Self-similarity and vanishing diffusion in fluvial landscapes.

Complex topographies exhibit universal properties when fluvial erosion dominates landscape evolution over other geomorphological processes. Similarly, we show that the solutions of a minimalist landscape evolution model display invariant behavior as the impact of soil diffusion diminishes compared to fluvial erosion at the landscape scale, yielding complete self-similarity with respect to a dimensionless channelization index. Approaching its zero limit, soil diffusion becomes confined to a region of vanishing area and large concavity or convexity, corresponding to the locus of the ridge and valley network. We demonstrate these results using one dimensional analytical solutions and two dimensional numerical simulations, supported by real-world topographic observations. Our findings on the landscape self-similarity and the localized diffusion resemble the self-similarity of turbulent flows and the role of viscous dissipation. Topographic singularities in the vanishing diffusion limit are suggestive of shock waves and singularities observed in nonlinear complex systems.

Early in vivo detection of denervation-induced atrophy by luminescence transient nanothermometry.

Denervation induces skeletal muscle atrophy due to the loss of control and feedback with the nervous system. Unfortunately, muscle atrophy only becomes evident days after the denervation event when it could be irreversible. Alternative diagnosis tools for early detection of denervation-induced muscle atrophy are, thus, required. In this work, we demonstrate how the combination of transient thermometry, a technique already used for early diagnosis of tumors, and infrared-emitting nanothermometers makes possible the in vivo detection of the onset of muscle atrophy at short (<1 day) times after a denervation event. The physiological reasons behind these experimental results have been explored by performing three dimensional numerical simulations based on the Pennes' bioheat equation. It is concluded that the alterations in muscle thermal dynamics at the onset of muscle atrophy are consequence of the skin perfusion increment caused by the alteration of peripheral nervous autonomous system. This work demonstrates the potential of infrared luminescence thermometry for early detection of diseases of the nervous system opening the venue toward the development of new diagnosis tools.

Flux exchange between fracture and matrix dictates late-time tracer tailing

Understanding and quantifying the solute tailing process in fractured rocks is critical for various subsurface applications but remains challenging. Particularly, the effects of flux exchange between fracture and matrix on the tailing process are considered to be important but have not been explicitly investigated. To fill this knowledge gap, we estimate the power-law scaling of late-time tracer tailing, n, based on three dimensional direct numerical simulations that consider both solute diffusion and advection across fracture-matrix interface. The dependence of solute decay rate n on fracture-matrix interaction is elucidated by simulating cases over a wide range of matrix permeability (km). In the case with matrix diffusion but without advection between fracture and matrix, we find that matrix diffusion significantly enhances tracer tailing. Then, the consideration of advection does not show significant effect on n up to the matrix permeability km of 10−12 m2. When k is greater than 10−12 m2, n declines rapidly down to 1.0 at k = 10−9 m2. We finally demonstrate that the upscaled model of continuous time random work can capture the late-time tailing behavior. This study provides a mechanistic explanation for the heavy tailing process commonly observed in fractured rocks.

A volume of fluid method for three dimensional direct numerical simulations of immiscible droplet collisions

This paper presents an advanced Volume of Fluid (VOF) method that enables performant three dimensional Direct Numerical Simulations (DNS) of the interaction of two immiscible fluids in a gaseous environment with large topology changes, e.g., binary droplet collisions. One of the challenges associated with the introduction of a third immiscible phase into the VOF method is the reconstruction of the phase boundaries near the triple line in arbitrary arrangements. For this purpose, an efficient method based on a Piecewise Linear Interface Calculation (PLIC) is shown. Moreover, the surface force modeling with the robust Continuous Surface Stress (CSS) model was enhanced to treat such three-phase situations with large topology changes and thin films. A consistent scaling of the fluid properties at the interfaces ensures energy conservation.The implementation of these methods in the multi-phase flow solver Free Surface 3D (FS3D) allowed a successful validation. A qualitative comparison of the morphology in binary collisions of immiscible droplets as well as a quantitative comparison regarding the threshold velocities that distinguish different collision regimes shows excellent agreement with experimental results.These simulations enable the evaluation of experimentally inaccessible data like the contributions of kinetic, surface and dissipative energy of both immiscible liquids during the collision process. Furthermore, the comparison against binary collisions of the same liquids highlights similarities and differences between immiscible and equal droplet collisions. Both can support the modeling of the immiscible liquid interaction in the future.

Synthesis of solid NMC622 particles by spray drying, post-annealing and lithiation

Layered lithium-nickel-manganese-cobalt oxide (NMC) cathode materials are widely used in Li-ion batteries that require high energy densities, such as those used in plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). Here we studied the synthesis of NMC622 particles by spray pyrolysis, which is a simple one-step process for production of spherical particles. However, synthesising NMC powder using spray pyrolysis has a tendency to produce hollow NMC particles. To gain insight into the mechanism behind the formation of the hollow particles, one dimensional numerical simulation of the physical and chemical phenomena taking place during spray drying were carried out. The effects of several process parameters, including drying air temperature, drying air mass flow rate, and liquid feed mass flow rate, on the evaporation and particle formation process were studied. The increased evaporation rate at higher temperatures was found to result in crust formation on the droplet surface during the particle formation, and thus, in lower solid volume fractions in the dried particles. However, by optimizing the process parameters production of solid NMC622 sulphate particles by spray drying was achieved. The produced NMC622 sulphate particles were then oxidised and lithiated in air at 850 °C via the conventional thermal treatment process. Four lithium precursors, LiOH, Li2CO3, Li2SO4 and LiNO3, were tested for the lithiation of the oxidized NMC particles. The degree of lithiation and the crystalline phase of the powders were determined using ICP-OES and XRD, respectively.

Formula omitted]-mode oscillations of compact stars with realistic equations of state in dynamical spacetime

In this study, we perform full three dimensional numerical relativity simulations of non-rotating general relativistic stars. Extending the studies for polytropic equation of state, we investigate the accuracy and robustness of numerical scheme on measuring fundamental (f)-mode frequency for realistic equations of state (EoS). We use various EoS with varying range of stiffness and numerically evolve perturbed stellar models for several mass configurations (in the range of 1.2−2.0M⊙) for each of these EoS. Using the gravitational waveform obtained from the simulations we extract the f-modes of the stars. The obtained results are tested against the pre-existing perturbation methods and find good agreement. Validity and deviation of universal relations have been carried out and are compared with earlier results under Cowling approximation as well as perturbative approaches. We also show that even using perturbed single star simulations can provide good agreement with f-modes extracted from inspiral phase of binary neutron star simulations which are computationally more expensive.

A two‐dimensional analysis of the influence of delamination on the wave propagation in a fiber metal laminate

AbstractStructural Health Monitoring (SHM) of composite materials using Guided Ultrasonic Wave (GUW) represent a central research topic with a large range of application fields. One major concern in this context is the detection of hidden inter‐laminar damage such as delaminations in composites, as these cannot be detected by visual inspection [1]. Fiber Metal Laminate (FML) consist of different layers of metal (e.g. steel) and Fiber Reinforced Polymer (FRP) stacked on top of each other and belong to a new type of material system that has been insufficiently explored in the context of damage detection using GUW. Following the objective of damage detection using GUW, two dimensional numerical simulations of the wave propagation in FML composed of layers made from steel and Carbon Fiber Reinforced Polymer (CFRP) are conducted. In order to properly identify and describe the different phenomena occurring when the GUW encounter a delamination three kind of numerical models were considered: undamaged plate, damaged plate with symmetrically and off‐center placed delamination. As far as the modeling of delaminations, the present work is restricted to a zero‐thickness‐delamination generated by de‐merging nodes at the interface of the layers. Mode‐selective excitation (fundamental symmetric and anti‐symmetric mode wave) of the models was then applied and the B‐scan representations of the in‐plane and out‐of‐plane displacement fields were generated and compared. Effects such as reflection, transmission and mode conversion of the main wave are observed and analyzed. Thus, a singular phenomenon as the trapped mode wave can be clearly observed and interpreted.

The effect of non-minimally coupled scalar field on gravitational waves from first-order vacuum phase transitions

We investigate first-order vacuum phase transitions in the presence of a non-minimally coupled scalar field starting with the coupling effect on the initial dynamics of phase transitions by defining an effective potential for the scalar field and then performing three dimensional numerical simulations to observe any possible distinction in gravitational wave power spectrum. Although we give a description of the model with the expanding background, in this particular paper, we exclude the scale factor contribution since we primarily focus on the immediate impact of the non-minimal coupling at initial phases of the transition. Even in this case we found that this modification has discernible effect on the power spectrum of the gravitational wave energy density.

Numerical study on the propagation characteristics of rotating detonation waves in a ramjet engine

The rotating detonation ramjet engine (RDRE) is a new-generation aero-engine with tremendous potential. In order to reveal the characteristics of the wave system in RDRE and study the propagation characteristics of the rotating detonation wave (RDW), a three dimensional numerical simulation on a simplified model of RDRE was performed with premixed ethylene/air mixture. The results show that an upstream oblique shock wave (UOSW) is induced by the RDW. It then weakens gradually as propagating upstream and merges with the circumferential tilted normal shock wave (NSW). The UOSW near the outer wall propagates faster upstream, whereas the position of the NSW on the inner and outer walls is almost coincident during the process of propagating downstream. The periodic fluctuations of the pressure peaks and the propagation speed of the RDW were observed, and the irregular pre-combustion occurs near the contact surface. The results also indicate that a sustainable dynamic balance of the periodic fluctuation in the intensity of RDW is formed in the RDRE. The irregular pre-combustion causes the irregular fluctuation of the pressure peaks near the contact surface. The UOSW propagates upstream much faster than the NSW propagating downstream, and the estimated ratio of the circumferential range effected by UOSW to the circumferential range effected by NSW is about 1: 5.

Investigation of a Dissipating Mesospheric Bore Using Airglow Imager and Direct Numerical Simulation

AbstractAtmospheric gravity waves play an important role in driving the dynamics of the Mesosphere and Lower Thermosphere and the basic structure of this region is determined by momentum deposition of these waves. Mesospheric bores are a type of non‐linear response that cause the amplification of gravity wave, due to trapping, that is characterized by a propagating step‐like jump followed by undulating waves. They require a stable layer or duct to travel horizontally with little attenuation thereby capable of transporting wave energy and momentum over larger distances. We present a prominent bright undular bore event observed in the mesospheric O(1S), O2, and OH emission layers on 16 March 2021 over Germany. A striking feature of this observation is the capture of bore's rapid dissipation around the center of the imager's field of view. The vertical temperature profile obtained from the satellite data indicates the presence of temperature inversion layer which acted as a thermal duct for the bore propagation. In addition, we have performed idealized two dimensional direct numerical simulations (DNS) of Navier‐Stokes equations under Boussinesq approximation. The DNS results reproduce many important characteristics of the observed airglow event like the nonlinear wave‐steepening, number of trailing waves, and its dissipation by implementing a thermal duct and a wave‐like perturbation. Furthermore, the DNS results also indicate that the duct width and amplitude of the initial perturbation have a considerable effect on the bore morphology.

Effect of nonlinear lensing on the coupling of ultrafast laser pulses to hollow-core waveguides.

Gas-filled hollow-core fibers are a flexible platform for the manipulation of ultrafast laser pulses through a variety of nonlinear optical effects. Efficient high-fidelity coupling of the initial pulses is very important for system performance. Here we study the effect of self-focusing in gas-cell windows on the coupling of ultrafast laser pulses into hollow-core fibers using (2+1)-dimensional numerical simulations. As expected, we find that the coupling efficiency is degraded and the duration of the coupled pulses changed when the entrance window is too close to the fiber entrance. The interplay of nonlinear spatio-temporal reshaping and the linear dispersion of the window create different results depending on the window material, pulse duration, and pulse wavelength, with longer-wavelength beams more tolerant of high intensity in the window. While shifting the nominal focus to compensate can restore some of the lost coupling efficiency, it improves the pulse duration only marginally. From our simulations we derive a simple expression for the minimum distance between the window and the HCF entrance facet. Our results have implications for the often space-constrained design of hollow-core-fiber systems, especially where the input energy is not constant.

Numerical investigation on heat transfer and fluid flow in a multi-minichannel heat sink: Effect of channel configurations

Three dimensional numerical simulation has been performed to explore the hydro-thermal performance of novel configurations of multichannel (MMCHS) heat sink. The study included the effect of three channel configurations matched with chosen inlet/outlet locations of the cooling fluid on the flow distribution between channels. The three new modified channel configurations were considered in this work, namely, arc-shaped, S-shaped and wave-shaped channels. The study is conducted under laminar flow condition with single phase flow using water as a coolant of mass flow rate range (0.002–0.005 kg s−1). The results revealed that the new modified channels are capable of enhancing heat transfer comparative with conventional straight channel. Among the all configurations, the wave-shaped channels showed the highest enhancement in Nusselt number of 30.5% followed by S-shaped channels with 18.7% and arc-shaped channels with 10.8%. Meanwhile, the arc-shaped channels achieved lowest pressure drop with a ratio of 16% less than the traditional channel. Furthermore, it's found that temperature uniformity for new channel configurations is better than traditional channel. The maximum difference in temperature distribution showed a reduction about 2.64 K in wave-shaped channels. The best performance factor of MMCHS has been accomplished by wave-shaped channels with a value of 1.3.

Numerical Simulation of Two-Dimensional Shear Flow

Numerical experiments on finite difference solutions of time dependent two-dimensional Navier-Stokes equations are used to study the transition triggering off in a mixing layer of initial tanh(y) profile. The inflow is excited by sinusoidal waves resulting from the linear theory of hydrodynamic instability. Numerical realizations are compared, through stream wise growth of momentum thickness and vorticity plots, with Winant and Browand experiments.&#x0D; The transition control and triggering off are studied through the finite difference solution of the two-dimensional time dependant Navier-Stokes equations. In a shear flow, the transition results from hydrodynamic amplification of unstable disturbances stimulated by perturbations of diverse origin. In a unidirectional flow, the most unstable disturbances are two-dimensional transverse waves {2, 3} and experiments have shown the area where small disturbances grow exponentially {5,8,9,10}. Thus, a two dimensional numerical simulation is well suited to the study of transition triggering off.

Experimental and numerical study on protective effect of RC blast wall against air shock wave

Prototype experiments were carried out on the explosion-proof performance of the RC blast wall. The mass of TNT detonated in the experiments is 5 kg and 20 kg respectively. The shock wave overpressure was tested in different regions. The above experiments were numerically simulated, and the simulated shock wave overpressure waveforms were compared with that tested and given by CONWEP program. The results show that the numerically simulated waveform is slightly different from the test waveform, but similar to CONWEP waveform. Through dimensional analysis and numerical simulation under different working conditions, the equation for the attenuation rate of the diffraction overpressure behind the blast wall was obtained. According to the corresponding standards, the degree of casualties and the damage degree of the brick concrete building at a certain distance behind the wall can be determined when parameters are set. The above results can provide a reference for the design and construction of the reinforced concrete blast wall.

Heat transfer by natural convection and radiation in three dimensional differentially heated tall cavities

Natural convection coupled with radiation is numerically studied in a tall rectangular parallelepiped cavity filled with air and constant differentially heated opposite vertical walls. This type of tall cavity is found in walls built with hollow blocks. Three dimensional steady state numerical simulations are performed. Numerical models for pure natural convection and natural convection coupled with radiation are validated with experimental results. In this work, the tall cavity studied has a 0.06 m width, 0.155 m depth and a 2.56 m height, giving a vertical aspect ratio of A=42.7 and a horizontal aspect ratio of A′=2.6. The cold wall temperature, Tc, is fixed. The hot wall temperature, Th, is varied, obtaining the Rayleigh numbers: RaW, 1.01×104, 1.07×105 and 7.20×105. The emissivity, ϵ, varies from 0.0 to 1.0. The results show that with a given Th, the convective Nusselt number, Nuc, depends on ϵ. For the emissivity value of hollow block materials (ϵ=0.8) the radiative heat transfer represents 83% for the lowest Th. For pure natural convection, a Nuc correlation as a function of RaW is obtained. A simplified calculation of the total heat transfer, Nut, is proposed.

Comment on “Groundwater Affects the Geomorphic and Hydrologic Properties of Coevolved Landscapes” by Litwin et al.

AbstractThe objective of this comment is to correct two sets of statements in Litwin et al. (2022, https://doi.org/10.1029/2021JF006239), which consider our research work (Bonetti et al., 2018, https://doi.org/10.1098/rspa.2017.0693; Bonetti et al., 2020, https://doi.org/10.1073/pnas.1911817117). We clarify here that (a) the specific contributing area is defined in the limit of an infinitesimal contour length instead of the product of a reference contour width (Bonetti et al., 2018, https://doi.org/10.1098/rspa.2017.0693), and (b) not all solutions obtained from the minimalist landscape evolution model of Bonetti et al. (2020, https://doi.org/10.1073/pnas.1911817117) are rescaled copies of each other. We take this opportunity to demonstrate that the boundary conditions impact the obtained solutions, which has not been considered in the dimensional analysis of Litwin et al. (2022, https://doi.org/10.1029/2021JF006239). We clarify this point by using dimensional analysis and numerical simulations for a square domain, where only one horizontal length scale (the side length l) enters the physical law.

Simulation Study on the Effects of Operating Temperature on Cell Electrodes in Solid Oxide Fuel Cells

In this study, a three−dimensional numerical simulation on electrodes in solid oxide fuel cells (SOFCs) is investigated in both regular cell and button cell configurations. The cell unit models with a regular cell with an active area of 5cm × 5cm and with a button cell with an active area of 2.54 cm2 were conducted to investigate the voltage distribution on cell electrodes in the solid oxide fuel cells (SOFCs). The performance characteristics in SOFC cell unit are determined through a numerical simulation method by using a computational fluid dynamic (CFD). The COMSOL Multiphysics software is used to investigate the model. The results show that the cell voltage in both regular cell and button cell with operating temperatures of 650 and 700 °C were lower than those at 750 °C. This means that when the operating temperature increases, the voltage and current density on the solid oxide fuel cell electrodes increases, and the performance of the cell is also improved.