Steady-state Transport Research Articles

Multi-species transport CFD simulation and experimental verification for material flow properties in dissimilar friction stir welding

A thorough understanding of thermal history and material flow behaviour associated with the tool–material interface substantially eradicates defects from the dissimilar weld joints. Therefore, the authors performed a three-dimensional coupled thermal-fluid flow analysis based on computational fluid mechanics (CFD) for dissimilar friction stir welding of 304 stainless steel (304 SS) and 6061-T6 aluminum alloy (Al6061). A steady-state multi-species transport model coupled with a mixture model was established for the first time, where the CFD solver transported species mass fractions of the materials. The developed model can capture the transversal/horizontal material features and embedded steel fragments/strip in the weld joints, which were also detected in macrographs from experiments. The calculated temperature fields were also fairly agreed with experimentally measured temperatures. The highest strain rate and flow velocity were attained on the outside shoulder edge caused by the lowest viscosity on the Al6061 side. The rotational speed of 300 rpm could not extrude the steel from base 304 SS due to insufficient flow velocity, whereas the steel fragments were intermixed with the Al6061 matrix at 600 rpm. At 875 rpm and 1200 rpm, the continuous steel strips were extruded from 304 SS and deposited in Al6061 due to relatively higher heat generation and flow velocity. Furthermore, the velocity vectors revealed the discontinuous material flow at 300 rpm and 600 rpm, and it was turned into almost uniform at 875 rpm and 1200 rpm. The rotational speed of 875 rpm produced the sound weld with a maximum joint efficiency of 75.6%.

Molecular dynamics analysis of water flow through a multiply connected carbon nanotube channel

The filling process of nanoconduits is an active research topic. In this study, we use molecular dynamics simulations to identify the filling process of water molecules in a multiply connected carbon nanotube (MCCNT). For water permeation, a local change in the channel cross-section affects the water filling of MCCNTs because it may lead to irregularities in the permeation profile. A decrease in hydrogen bonds at the junctions of the structure characterizes the permeability of MCCNTs. In contrast to pristine CNTs, the complex nanochannel exhibits a different imbibition profile due to the energy changes at the junction. Next, we examine the local water density and velocity patterns in MCCNT channels to understand how junction regions affect steady-state water transport. We find that there is congestion and irregularities in steady water flow density and velocity profiles. Through this study, we expect to develop effective channels with more complex geometries for water purification and drug delivery.

A hybrid Monte Carlo-discrete ordinates method for phonon transport in micro/nanosystems with rough interfaces

In recent years, the rapid development of micro/nanosystems raises the demand of thermal optimization of the new designs, and thus the accurate and efficient prediction of phonon transport in mesoscopic systems with complex interfaces is required. This work proposes a hybrid Monte Carlo-discrete ordinates method (MC-DOM) for steady-state phonon transport in such systems by developing an algorithm to exchange information between MC and DOM subdomains. Based on the phonon Boltzmann transport equation which is physically compatible with the system scale, our formulation exploits the computational efficiency of DOM in the bulk region with flexibility of MC near complex interfaces. The cross-plane phonon transport through the thin films is considered first as the benchmark for method verification, whose results agree well with those calculated by numerical and analytical solutions. It is noteworthy that compared to MC as a pure particle method, the hybrid method runs faster over a hundred of times for the temperature calculation when high-precision results are needed. The hybrid method is then applied to simulate the in-plane phonon transport through the double-layer thin films with rough interfaces. It is found that for rectangular interfaces with different roughness, the interfaces perpendicular to the heat flux will cause a larger thermal resistance than the parallel ones. Our hybrid method enriches numerical tools for mesoscopic phonon transport simulation, and is helpful to reveal the mechanisms behind and optimize the heat transport in micro/nanosystems with complex geometries.

Steady-state radiochemical transport model of the molten salt reactor experiment

Steady-state radiochemical transport model of the molten salt reactor experiment

Hybrid parallel strategies for the neutron transport code PANDAS-MOC

PANDAS-MOC (Purdue Advanced Neutronics Design and Analysis System with Methods of Characteristics) is being developed to find high fidelity 3D solutions for steady state and transient neutron transport analysis. However, solving such transport problems in a large reactor core could be extremely computationally intensive and memory demanding. Because parallel computing is capable of improving computing efficiency and decreasing memory requirements, three parallel models of PANDAS-MOC are designed using the distributed memory and share memory architectures in this article: a pure message passing interface (MPI) parallel model (PMPI), a segment OpenMP threading hybrid model (SGP), and a whole-code OpenMP threading hybrid model (WCP). Their parallel performances are examined by the C5G7 3D core. For the measured speedup, PMPI model > WCP model > SGP model. The memory consumed by the WCP model is about 60% of that consumed by the PMPI model. This study also demonstrated that the performance of WCP parallelism is limited by the hybrid reduction in the CMFD calculation and omp atomic clause in the MOC sweep. Once they are optimized, the WCP model can outperform the PMPI model.

Time-integral iteration method for two-dimensional anomalous transport.

A methodology is developed to describe time-dependent phenomena associated with nonlocal transport in complex, two-dimensional geometries. It is an extension of the ''iterative method" introduced previously to solve steady-state transport problems [Maggs and Morales, Phys. Rev. E 99, 013307 (2019)10.1103/PhysRevE.99.013307], and it is based on the ''jumping particle" concepts associated with the continuous-time random walk (CTRW) model. The method presented explicitly evaluates the time integral contained in the CTRW master equation. A modified version of the Mittag-Leffler function is used for the waiting-time probability distributions to incorporate memory effects. Calculations of the propagation of ''anomalous transport waves" in various systems, with and without memory, illustrate the technique.

Steady-state optoelectronic measurements of halide perovskites on a selective contact: a path to in-depth comprehension of their photovoltaic activity

Most of the charge transport properties in halide perovskite (HaP) absorbers are measured by transient measurements with pulsed excitations; however, most solar cells in real life function in steady-state conditions. In contrast to working devices that include selective contacts, steady-state measurements need as high as possible photoconductivity (σph), which is typically restricted to the absorber alone. In this paper, we enabled steady-state charge transport measurement using atomic layer deposition (ALD) to grow a conformal, ultra-thin (∼4 nm) ZnO electron transport layer that is laterally insulating due to its thickness. Due to the highly alkaline behavior of the ZnO surfaces, it readily reacts with halide Perovskites. ALD process was used to form an Aluminum oxynitride (AlON) thin (∼2 nm) layer that passivates the ZnO-HaP interface. We show that the presence of the AlON layer prevents HaP degradation caused by the interaction with the ZnO layer, improves the HaP σph, and doubles the HaP carrier diffusion lengths.

Environment-Assisted Modulation of Heat Flux in a Bio-Inspired System Based on Collision Model.

The high energy transfer efficiency of photosynthetic complexes has been a topic of research across many disciplines. Several attempts have been made in order to explain this energy transfer enhancement in terms of quantum mechanical resources such as energetic and vibration coherence and constructive effects of environmental noise. The developments in this line of research have inspired various biomimetic works aiming to use the underlying mechanisms in biological light harvesting complexes for the improvement of synthetic systems. In this article, we explore the effect of an auxiliary hierarchically structured environment interacting with a system on the steady-state heat transport across the system. The cold and hot baths are modeled by a series of identically prepared qubits in their respective thermal states, and we use a collision model to simulate the open quantum dynamics of the system. We investigate the effects of system-environment, inter-environment couplings and coherence of the structured environment on the steady state heat flux and find that such a coupling enhances the energy transfer. Our calculations reveal that there exists a non-monotonic and non-trivial relationship between the steady-state heat flux and the mentioned parameters.

The Source Permeator System and Tritium Transport in the TEX PbLi Loop

Permeation is investigated for the introduction of hydrogen isotopes into lead lithium (PbLi) for the Tritium Extraction eXperiment (TEX). TEX is a forced-convection PbLi loop under construction at Idaho National Laboratory that will test the vacuum permeator (VP) method of tritium extraction from PbLi. The source permeator (SP) delivers atomic hydrogen (H, D, and T) from a gas-phase reservoir into the PbLi via a permeable dense metal membrane. A modular system and a fixed SP system are presented. In the modular design, PbLi flows through the inside of a tubular membrane, and gas-phase hydrogen is introduced on the outside of the membrane. Atomic hydrogen permeates radially inward through the membrane into the PbLi. In the fixed design, PbLi flows into an expansion chamber with closed-ended tubular membranes inserted. Gas-phase hydrogen is introduced on the inside of the closed-ended membranes, and atomic hydrogen permeates radially outward into the flowing PbLi. Hydrogen transport models based on steady-state mass transport through PbLi and permeation through the metal membrane were developed to assess the operation of the SP relative to experimental variables and to allow understanding of uncertain parameter effects, such as PbLi hydrogen transport properties and the effective hydrogen permeability of the VP. This modeling effort considers iron as the SP material and vanadium as the VP material.

VITAS: A multi-purpose simulation code for the solution of neutron transport problems based on variational nodal methods

Existing neutronic whole-core simulation codes are problem-oriented. With the advancement of computer technology and the development of complicated micro-nuclear reactors, there is a pressing need to create solution methods with broad applicability and the ability to accommodate more complex, high-fidelity, unstructured geometries. This necessity motivates the development of a neutron transport code that can adapt to different types of nuclear reactors. This work describes the code design and theoretical model of VITAS, a multi-purpose simulation code for handling steady-state and time-dependent neutron transport problems. This code incorporates algorithms and models inside a unified variational nodal framework based on the second-order neutron transport equation. The most recent version of VITAS employs the isotropic scattering approximation and permits arbitrary spatial and angular refinement. VITAS is compatible with Cartesian geometry, hexagonal geometry, triangular geometry, and Cartesian geometry with finite element subdivisions in the radial direction (Cartesian-FE). The three-dimensional modeling can be performed by axially extruding the two-dimensional mesh. Users can choose the mesh type and expansion order on-demand for optimal computational efficiency.In this paper, VITAS is validated using benchmark problems, i.e., the TAKEDA3 benchmark and the transient LMW benchmark with Cartesian homogenized geometry, the C5G7 and C5G7-TD exercises with Cartesian-FE geometry, the TAKEDA4 benchmark and the Buckner and Stewart benchmark with hexagonal-z homogenized geometry, and the Dodds benchmark with cylindrical-z geometry described by triangular-z unstructured meshes. The space-angle truncation is extensively examined to demonstrate the asymptotic convergence behavior with p- and h-refinement. The results indicate that VITAS has the potential to become a versatile toolset for solving a variety of nuclear reactor problems.

Hydromagnetic CuO-H2O nanofluid transportation through irreversibility analysis

The current investigation aimed for the exploration of steady-state, incompressible flow and energy transportation of water-based copper oxide nanofluid past a linear stretching sheet through suction/injection possessions. Furthermore, the impact of magnetohydrodynamic, radiation, and entropy generation are taken in the problem development. The nonlinear equations are reframed into single independent variable equations by the implication of appropriate similarity transformations. The Fehlberg scheme is developed for the numerical computations of re-structured non-linear expressions. The variations of influential constraints on the dimensionless physical quantities are investigated and reported in detail. The prominent features of the investigation include the higher velocity and weaker temperature is achieved against the incrementing stretching constraint values. The irreversibility ratio is reduced against the emerging stretching strength constraint. The Bejan number is decaying against the higher magnetic constraint. The blowing and suction have opposite impacts on the skin-friction factors.

Counting statistics of energy transport across squeezed thermal reservoirs

A general formalism for computing the full counting statistics of energy exchanged between 'N' squeezed thermal photon reservoirs weakly coupled to a cavity with 'M' photon modes is presented. The formalism is based on the two-point measurement scheme and is applied to two simple special cases, the relaxation dynamics of a single mode cavity in contact with a single squeezed thermal photon reservoir and the steady-state energy transport between two squeezed thermal photon reservoirs coupled to a single cavity mode. Using analytical results, it is found that the short time statistics is significantly affected by noncommutivity of the initial energy measurements with the reservoirs squeezed states, and may lead to negative probabilities if not accounted properly. Furthermore, it is found that for the single reservoir setup, generically there is no transient or steady-state fluctuation theorems for energy transport. In contrast, for the two reservoir case, although there is no generic transient fluctuation theorem, steady-state fluctuation theorem with a non-universal affinity is found to be valid. Statistics of energy currents are further discussed.

Unravelling the sex-specific diversity and functions of adrenal gland macrophages.

SummaryDespite the ubiquitous function of macrophages across the body, the diversity, origin, and function of adrenal gland macrophages remain largely unknown. We define the heterogeneity of adrenal gland immune cells using single-cell RNA sequencing and use genetic models to explore the developmental mechanisms yielding macrophage diversity. We define populations of monocyte-derived and embryonically seeded adrenal gland macrophages and identify a female-specific subset with low major histocompatibility complex (MHC) class II expression. In adulthood, monocyte recruitment dominates adrenal gland macrophage maintenance in female mice. Adrenal gland macrophage sub-tissular distribution follows a sex-dimorphic pattern, with MHC class IIlow macrophages located at the cortico-medullary junction. Macrophage sex dimorphism depends on the presence of the cortical X-zone. Adrenal gland macrophage depletion results in altered tissue homeostasis, modulated lipid metabolism, and decreased local aldosterone production during stress exposure. Overall, these data reveal the heterogeneity of adrenal gland macrophages and point toward sex-restricted distribution and functions of these cells.

Steady-state and transient electronic transport properties of β-(Al x Ga1–x )2O3/Ga2O3 heterostructures: An ensemble Monte Carlo simulation

The steady-state and transient electron transport properties of β-(Al x Ga1−x )2O3/Ga2O3 heterostructures were investigated by Monte Carlo simulation with the classic three-valley model. In particular, the electronic band structures were acquired by first-principles calculations, which could provide precise parameters for calculating the transport properties of the two-dimensional electron gas (2DEG), and the quantization effect was considered in the Γ valley with the five lowest subbands. Wave functions and energy eigenvalues were obtained by iteration of the Schrödinger–Poisson equations to calculate the 2DEG scattering rates with five main scattering mechanisms considered. The simulated low-field electron mobilities agree well with the experimental results, thus confirming the effectiveness of our models. The results show that the room temperature electron mobility of the β-(Al0.188Ga0.812)2O3/Ga2O3 heterostructure at 10 kV⋅cm−1 is approximately 153.669 cm2⋅V−1⋅s−1, and polar optical phonon scattering would have a significant impact on the mobility properties at this time. The region of negative differential mobility, overshoot of the transient electron velocity and negative diffusion coefficients are also observed when the electric field increases to the corresponding threshold value or even exceeds it. This work offers significant parameters for the β-(Al x Ga1−x )2O3/Ga2O3 heterostructure that may benefit the design of high-performance β-(Al x Ga1−x )2O3/Ga2O3 heterostructure-based devices.

Advances in the theory of existence and numerical simulations for the one-dimensional transport equation

The steady-state transport equation with internal sources and semi-reflective boundaries in a participative medium in slab geometry was analytically and numerically solved in this work. The existence theory in Hölder spaces when the sources belong to the space of continuous functions were established, generalizing results of previous works. Also, the Nyström method combines with singularity removal techniques was applied to produce an algorithm for calculating the scalar flux and criticality. The analytical estimates and the numerical methodology were obtained by looking at integral formulation as a Fredholm equation of the second type. Tabulated numerical results show the efficiency of the proposed methodology.

Steady-State Heat Transport and Work With a Single Artificial Atom Coupled to a Waveguide: Emission Without External Driving

We observe the continuous emission of photons into a waveguide from a superconducting qubit without the application of an external drive. To explain this observation, we build a two-bath model where the qubit couples simultaneously to a cold bath (the waveguide) and a hot bath (a secondary environment). Our results show that the thermal-photon occupation of the hot bath is up to 0.14 photons, 35 times larger than the cold waveguide, leading to nonequilibrium heat transport with a power of up to 132 zW, as estimated from the qubit emission spectrum. By adding more isolation between the sample output and the first cold amplifier in the output line, the heat transport is strongly suppressed. Our interpretation is that the hot bath may arise from active two-level systems being excited by noise from the output line. We also apply a coherent drive, and use the waveguide to measure thermodynamic work and heat, suggesting waveguide spectroscopy is a useful means to study quantum heat engines and refrigerators. Finally, based on the theoretical model, we propose how a similar setup can be used as a noise spectrometer which provides a new solution for calibrating the background noise of hybrid quantum systems.

Cover Image, Volume 46, Issue 6

The cover image is based on the Research Article Analytical solution for one-dimensional steady-state contaminant transport through a geomembrane layer (GMBL)/compacted clay layer (CCL)/attenuation layer (AL) composite liner considering consolidation by Huaxiang Yan et al., https://doi.org/10.1002/nag.3334.

Effect of balanced edge cutting configurations on the aeroacoustics of high-speed turbomachinery

Abstract Flow-noise corresponding to the balanced edge cutting configurations on the compressor wheel of a turbocharger is investigated in this study. The turbocharger is a type of high-speed turbomachinery that rotates at up to 2.0×105 RPM. Microscopic air bubbles are generated inside the compressor wheel due to inaccuracies in the cast introduced in the production process. These bubbles cause a mass-unbalance phenomenon when operated at high rotational speeds, which even leads to breakage in severe cases. To prevent theses problems, cutting configurations are utilized at various locations on the rear of the wheel. However, the cutting shapes rather produce severe vortices from a flow perspective, which has a significant effect on the flow noise. Therefore, after calculating the steady-state shear stress transport k-ω based on the Reynolds-averaged Navier-Stokes equation model, transient large eddy simulation was conducted from initial values using the commercial computational fluid dynamics software, ANSYS FLUENT V20.2. The flow-noise was calculated based on unsteady analysis data with the Ffowcs-Williams and Hawkings equation. The flow-noise of the balanced edge cutting configurations of turbocharger compressor wheels was calculated according to various receiver points, and rotational velocities. Results show that various noise characteristics occur depending on the rotational speeds and the receivers depending on the cutting configurations.

Autocorrelation functions and ergodicity in diffusion with stochastic resetting

Diffusion with stochastic resetting is a paradigm of resetting processes. Standard renewal or master equation approach are typically used to study steady state and other transport properties such as average, mean squared displacement etc. What remains less explored is the two time point correlation functions whose evaluation is often daunting since it requires the implementation of the exact time dependent probability density functions of the resetting processes which are unknown for most of the problems. We adopt a different approach that allows us to write a stochastic solution for a single trajectory undergoing resetting. Moments and the autocorrelation functions between any two times along the trajectory can then be computed directly using the laws of total expectation. Estimation of autocorrelation functions turns out to be pivotal for investigating the ergodic properties of various observables for this canonical model. In particular, we investigate two observables (i) sample mean which is widely used in economics and (ii) time-averaged-mean-squared-displacement (TAMSD) which is of acute interest in physics. We find that both diffusion and drift–diffusion processes with resetting are ergodic at the mean level unlike their reset-free counterparts. In contrast, resetting renders ergodicity breaking in the TAMSD while both the stochastic processes are ergodic when resetting is absent. We quantify these behaviors with detailed analytical study and corroborate with extensive numerical simulations. Our results can be verified in experimental set-ups that can track single particle trajectories and thus have strong implications in understanding the physics of resetting.

A two-layer continuously distributed capillary O2 transport model applied to blood flow regulation in resting skeletal muscle

The microcirculation is the site of direct oxygen (O2) transfer from blood to tissue, and also of O2 delivery control via regulation of local blood flow. In addition, a number of diseases including type II diabetes mellitus (DMII) and sepsis are known to produce microcirculatory dysfunction in their early phases. Given the complexity of microvascular structure and physiology, and the difficulty of measuring tissue oxygenation at the micro-scale, mathematical modelling has been necessary for understanding the physiology and pathophysiology of O2 transport in the microcirculation and for interpreting in vivo experiments. To advance this area, a model of blood-tissue O2 transport in skeletal muscle was recently developed which uses continuously distributed capillaries and includes O2 diffusion, convection, and consumption. The present work extends this model to two adjacent layers of skeletal muscle with different blood flow rates and applies it to study steady-state O2 transport when flow regulation is stimulated using an O2 exchange chamber. To generate a model which may be validated through in vivo experiments, an overlying O2 permeable membrane is included. The model is solved using traditional methods including separation of variables and Fourier decomposition, and to ensure smooth profiles at the muscle-muscle and muscle-membrane interfaces, matching conditions are developed. The study presents qualitative verification for the model, using visualizations of tissue O2 pressure (PO2) distributions for varying capillary density (CD), and presents capillary velocity response values in the near layer for varying chamber PO2 under the assumption that outlet capillary O2 saturation (SO2) is equalized between adjacent layers. These compensatory velocity profiles, along with effective ’no-flux’ chamber PO2 values, are presented for varying CD and tissue O2 consumption values. Insights gained from the two-layer model provide guidance for interpreting and planning future in vivo experiments, and also provide motivation for further development of the model to improve understanding of the interaction between O2 transport and blood flow regulation.