Search

A reduced kinetic method (RKM) with a first-principles collision operator is introduced in a 1D2V planar geometry and implemented in a computationally inexpensive code to investigate non-local ion heat transport in multi-species plasmas. The RKM successfully reproduces local results for multi-species ion systems and the important features expected to arise due to non-local effects on the heat flux are captured. In addition to this, novel features associated with multi-species, as opposed to single species, cases are found. Effects of non-locality on the heat flux are investigated in mass and charge symmetric and asymmetric ion mixtures with temperature, pressure, and concentration gradients. In particular, the enthalpy flux associated with diffusion is found to be insensitive to sharp pressure and concentration gradients, increasing its significance in comparison to the conductive heat flux driven by temperature gradients in non-local scenarios. The RKM code can be used for investigating other kinetic and non-local effects in a broader plasma physics context. Due to its relatively low computational cost it can also serve as a practical non-local ion heat flux closure in hydrodynamic simulations or as a training tool for machine learning surrogates. [ABSTRACT FROM AUTHOR]

In cold pulse experiments in J-TEXT, the ion transport shows similar non-local response as the electron transport channel. Very fast ion temperature decreases are observed in the edge, while the ion temperature in the core promptly begin to rise after the injection of cold pulse. Moreover, the cutoff density is also found for the ion non-local effect. The experimental observed density fluctuation in a high frequency ranging from 500 kHz to 2 MHz is obviously reduced in the whole plasma region during non-local transport phase.

Models of solar and space plasmas require an accurate model for thermal transport. The simplest such model is to assume that the fluid approach is valid and that local transport models can be used. These local transport coefficients are derived under the assumption that the electron mean-free path is “small” compared to the temperature scale length. When this approximation breaks down, non-local transport models or thermal flux limiters must be used to maintain a physically realistic model. This article will review the background theory of how small is “small” for the mean-free path and what options there are for including non-local transport within the fluid framework. Much of this recent work has been motivated by laser–plasma theory, where mean-free paths can be large and the Spitzer–Harm approach is never used.

A double-distribution-function lattice Boltzmann model for large-eddy simulations of a passive scalar field in a neutrally stratified turbulent flow is described. In simulations of the scalar turbulence within and above a homogeneous plant canopy, the model's performance is found to be comparable with that of a conventional large-eddy simulation model based on the Navier–Stokes equations and a scalar advection–diffusion equation in terms of the mean turbulence statistics, budgets of the second moments, power spectra, and spatial two-point correlation functions. For a top-down scalar, for which the plant canopy serves as a distributed sink, the variance and flux of the scalar near the canopy top are predominantly determined by sweep motions originating far above the canopy. These sweep motions, which have spatial scales much larger than the canopy height, penetrate deep inside the canopy and cause scalar sweep events near the canopy floor. By contrast, scalar ejection events near the canopy floor are induced by coherent eddies generated near the canopy top. The generation of such eddies is triggered by the downward approach of massive sweep motions to existing wide regions of weak ejective motions from inside to above the canopy. The non-local transport of scalars from above the canopy to the canopy floor, and vice versa, is driven by these eddies of different origins. Such non-local transport has significant implications for the scalar variance and flux budgets within and above the canopy, as well as the transport of scalars emitted from the underlying soils to the atmosphere. [ABSTRACT FROM AUTHOR]

Spin insulatronics covers efforts to generate, detect, control, and utilize high-fidelity pure spin currents and excitations inside magnetic insulators. Ultimately, the new findings may open doors for pure spin-based information and communication technologies. The aim is to replace moving charges with dynamical entities that utilize low-dissipation coherent and incoherent spin excitations in antiferromagnetic and ferromagnetic insulators. The ambition is that the new pure spin-based system will suffer reduced energy losses and operate at high frequencies. In magnetic insulators, there are no mobile charge carriers that can dissipate energy. Integration with conventional electronics is possible via interface exchange interactions and spin–orbit couplings. In this way, the free electrons in the metals couple to the localized spins in the magnetic insulators. In turn, these links facilitate spin-transfer torques and spin–orbit torques across metal–insulator interfaces and the associated phenomena of spin-pumping and charge-pumping. The interface couplings also connect the electron motion inside the metals with the spin fluctuations inside the magnetic insulators. These features imply that the system can enable unprecedented control of correlations resulting from the electron–magnon interactions. We review recent developments to realize electric and thermal generation, manipulation, detection, and control of pure spin information in insulators. [ABSTRACT FROM AUTHOR]

Abstract We present a novel non-local damage transport model for hydraulic fracture in porous media. The new model is sought to provide an enhanced description for the long-range damage and transport interactions in the fracture process zone. The non-local model is derived from thermodynamic principles, where a new thermodynamic potential expression for non-local transport is introduced. The thermodynamically consistent model is reduced to a gradient non-local permeability relationship that simultaneously provides a non-local transport description and regularized damage growth via a permeability-stress relationship. The analogy of the proposed model to Darcy-Brinkman fluid flow is demonstrated and discussed. A monolithic 3-field (u − P − κ ˜ ) mixed finite element framework is then used to discretize and solve the nonlinear coupled physics poromechanics problem. The hydraulic fracture continuum models previously presented in the literature rely on using different fluid flow laws inside and outside the fracture zone, which introduces a discontinuity in the continuum model. Our model prescribes Darcy type fluid flow all over the domain with a non-linear permeability constitutive law that allows for elevated fluid velocity in the crack zone, which preserves the continuity of all quantities within the domain. The numerical examples confirm the capability of the proposed model in capturing the essential features of hydraulic fracture simulation. The significance of the non-local transport modeling is demonstrated through modeling hydraulic fracturing in materials with pre-existing high permeability zones. The incorporation of non-local transport effects in hydraulic fracture modeling is proved to uncover possible flow paths through pre-existing high permeability zones. The proposed model can act as a platform for quantifying the fluid leak-off from hydraulic fracture and can be further used to analyze the hydraulic fracture interaction with neighboring geological material. [ABSTRACT FROM AUTHOR]

The non‐local expression proposed by Luciani‐Mora‐Virmont is implemented in a one dimensional fluid model for the scrape‐off layer. Analytical solutions for heat equation are discussed as well as the impact of sheath boundary conditions on the continuity of the temperature profile. The non‐local heat flux is compared to the Spitzer‐Härm heat flux for different collisionality. [ABSTRACT FROM AUTHOR]

The last few years have seen an explosion of interest in hydrodynamic effects in interacting electron systems in ultra-pure materials. In this paper we briefly review the recent advances, both theoretical and experimental, in the hydrodynamic approach to electronic transport in graphene, focusing on viscous phenomena, Coulomb drag, non-local transport measurements, and possibilities for observing nonlinear effects. [ABSTRACT FROM AUTHOR]

We present a novel damage-poroelastic model for analyzing the failure response of porous media in geomechanics applications. In this new approach, a gradient non-local permeability that leads to non-local transport and consequently non local damage, is introduced. Damage evolution is a function of an equivalent strain measure that is computed from non-local permeability using an inverse permeability–strain constitutive relation. A monolithic, mixed finite element method is proposed to solve the coupled system with a displacement–pressure–regularized permeability ( u − p − κ ̃ ) element formulation. The system is linearized and solved using Newton’s method and a backward Euler scheme is used to evolve the system in time. A consistent Jacobian matrix and residual vector are derived analytically and a bilinear damage model is used to evolve the damage. Numerical examples considering hydraulic fracture problems in 1-d and 2-d and damage enhanced consolidation are presented and discussed. The proposed non-local model results are compared with local damage–permeability models. While the local models are shown to suffer from mesh dependence and non-physical spurious oscillations in strain, permeability and fluid pressure evolution, the proposed model is reliable and seems to overcome all these limitations. [ABSTRACT FROM AUTHOR]

Contaminants in groundwater may experience a broad spectrum of velocities and multiple rates of mass transfer between mobile and immobile zones during transport. These conditions may lead to non-Fickian plume evolution which is not well described by the advection–dispersion equation (ADE). Simultaneously, many groundwater contaminants are degraded by processes that may be modeled as first-order decay. It is now known that non-Fickian transport and reaction are intimately coupled, with reaction affecting the transport operator. However, closed-form solutions for these important scenarios have not been published for use in applications. In this paper, we present four new Green's function analytic solutions in the uncoupled, uncorrelated continuous time random walk (CTRW) framework for reactive non-Fickian transport, corresponding to the quartet of conservative tracer solutions presented by Kreft and Zuber (1978) for Fickian transport. These consider pulse injection for both resident and flux concentration combined with detection in both resident and flux concentration. A pair of solutions for resident concentration temporal pulses with detection in both flux and resident concentration is also presented. We also derive the relationship between flux and resident concentration for non-Fickian transport with first-order reaction for this CTRW formulation. An explicit discussion of employment of the new solutions to model transport with arbitrary upgradient boundary conditions as well as mobile–immobile mass transfer is then presented. Using the new solutions, we show that first-order reaction has no effect on the anomalous spatial spreading rate of concentration profiles, but produces breakthrough curves at fixed locations that appear to have been generated by Fickian transport. Under the assumption of a Pareto CTRW transition distribution, we present a variety of numerical simulations including results showing coherence of our analytic solutions and CTRW particle tracking. [ABSTRACT FROM AUTHOR]

The turbulence structure investigation is essential for understanding the physics of flow-thermodynamic processes in Heating, Ventilation and Air conditioning (HVAC) systems. Analysis and calculation of these systems are based on the knowledge of turbulent transfer mechanism and its modeling. In this regard, the paper analyzes the phenomena of non-local momentum transfer and non-local transfer of energy by heat in curved channels, asymmetric jets, wakes and swirling flow. The physics of nongradient turbulent diffusion and negative production in velocity and temperature fields are analyzed. There was a certain analogy between the turbulent processes in these classes of flow. By means of numerical processing of experimental results, modeling of non-local transport in turbulent swirling flow was conducted.

This study formulates and analyzes continuous time random walk (CTRW) models in radial flow geometries for the quantification of non-local solute transport induced by heterogeneous flow distributions and by mobile–immobile mass transfer processes. To this end we derive a general CTRW framework in radial coordinates starting from the random walk equations for radial particle positions and times. The particle density, or solute concentration is governed by a non-local radial advection–dispersion equation (ADE). Unlike in CTRWs for uniform flow scenarios, particle transition times here depend on the radial particle position, which renders the CTRW non-stationary. As a consequence, the memory kernel characterizing the non-local ADE, is radially dependent. Based on this general formulation, we derive radial CTRW implementations that (i) emulate non-local radial transport due to heterogeneous advection, (ii) model multirate mass transfer (MRMT) between mobile and immobile continua, and (iii) quantify both heterogeneous advection in a mobile region and mass transfer between mobile and immobile regions. The expected solute breakthrough behavior is studied using numerical random walk particle tracking simulations. This behavior is analyzed by explicit analytical expressions for the asymptotic solute breakthrough curves. We observe clear power-law tails of the solute breakthrough for broad (power-law) distributions of particle transit times (heterogeneous advection) and particle trapping times (MRMT model). The combined model displays two distinct time regimes. An intermediate regime, in which the solute breakthrough is dominated by the particle transit times in the mobile zones, and a late time regime that is governed by the distribution of particle trapping times in immobile zones. These radial CTRW formulations allow for the identification of heterogeneous advection and mobile-immobile processes as drivers of anomalous transport, under conditions relevant for field tracer tests. [ABSTRACT FROM AUTHOR]

We propose a device modeling theory based on an improved drift-diffusion solution that is suitable for simulation of the efficiency droop effect in GaN LED. Our theory modifies the drift-diffusion transport by adding a non-local carrier transport component that mimics the effect of hot carriers near the multiple quantum well region. The non-local transport model is supported by recent experimental evidence of Auger-induced hot carriers as well as explaining the experimental low turn-on voltage that conventional drift-diffusion theory fails to predict. A surprising finding from the simulation is that the hot-Auger carriers have a positive effect of reducing the junction resistance of the LED and thus help improve the overall wall-plug efficiency. [ABSTRACT FROM AUTHOR]

An analytical model for mean wind profiles in sparse canopies (W. Wang, Boundary-Layer Meteorol 142:383-399, ) has been further developed, with (1) an explicit solution being derived, and (2) a linear term being added to the $$K$$ -closure scheme to improve the shear-stress parametrization when the contribution of non-local transport is significant. Results from large-eddy simulations and from laboratory experiments are used to evaluate the model and adjust model parameters, showing that the model can well simulate canopy wind and stress profiles not only for sparse-canopy scenarios, but also for dense-canopy scenarios. The analytical solution converges exactly to the standard surface-layer logarithmic wind profile in the case of zero canopy density, and tends to an exponential wind profile for a dense canopy. [ABSTRACT FROM AUTHOR]

Abstract: The correct characterization of the effective reactive transport dynamics is an important issue for modeling reactive transport on the Darcy scale, specifically in situations in which reactions are localized, that is when different reactions occur in different portions of the porous medium. Under such conditions the conventional approach of homogenizing only the porous medium chemistry is not appropriate. We consider here reactive transport in a porous medium that is characterized by mass transfer between a mobile and a distribution of immobile regions. Chemical and physical heterogeneities are reflected by distributions of kinetic reaction rate constants and residence times in the immobile zones. We derive an effective reactive transport equation for the mobile solute that is characterized by non-local physical mass transfer and reaction terms. Specifically, chemical heterogeneity is upscaled in terms of a reactive memory function that integrates both chemical and physical heterogeneity. Mass transfer limitations due to physical heterogeneity yield effective kinetic rate coefficients that can be much smaller than the volumetric average of the local scale coefficients. These results help to explain and quantify the often reported discrepancy between observed field reaction rate constants and the ones obtained under well mixed laboratory conditions. Furthermore, these results indicate that transport under physical and chemical heterogeneity cannot be upscaled separately. [ABSTRACT FROM AUTHOR]

This study assessed the effects of food supply on sediment reworking by Nereis diversicolor. We hypothesized that food supply would enhance sediment reworking and that the frequency of food supply would affect the intensity of bioturbation. Mesocosm experiments consisted of four treatments: (1) without worms (control cores), (2) with worms and no food supply, (3) with worms and daily food supply, (4) with worms and weekly food supply. Fluorescent particles, used as tracers, were spread over the sediment surface. Sediment reworking was quantified after 28 days based on the tracer distribution profiles. Results showed that sediment reworking by N. diversicolor was exclusively due to non-local transport processes. Food supply greatly increased non-local transport coefficients (more than 3 times) in comparison with those measured in the absence of a food supply. However, the intensity of sediment bioturbation by these worms was unaffected by the frequency of food supply. This study showed that environmental conditions affecting the quantity of food supply at the water-sediment interface could strongly influence bioturbation process. [ABSTRACT FROM AUTHOR]

A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum. [ABSTRACT FROM AUTHOR]

Large-eddy simulations of a clear convective boundary layer (CBL) and a stratocumulus-topped boundary layer are studied. Bottom-up and a top-down scalars were included in the simulations, and the principle of linear superposition of variables was applied to reconstruct the fields of any arbitrary conserved variable. This approach allows a systematic analysis of countergradient fluxes as a function of the flux ratio, which is defined as the ratio between the entrainment flux and the surface flux of the conserved quantity. In general, the turbulent flux of an arbitrary conserved quantity is counter to the mean vertical gradient if the heights where the vertical flux and the mean vertical gradient change sign do not coincide. The regime where the flux is countergradient is therefore bounded by the so-called zero-flux and zero-gradient heights. Because the vertical flux changes sign only if the entrainment flux has an opposite sign to the surface flux, countergradient fluxes are predominantly found for negative flux ratios. In the CBL the flux ratio for the virtual potential temperature is, to a good approximation, constant, and equal to -0.2. Only if the moisture contribution to the virtual potential temperature is negligibly small will the flux ratio for the potential temperature be equal to this value. Otherwise, the flux ratio for the potential temperature can have any arbitrary (negative) value, and, as a consequence, the fluxes for the potential temperature and the virtual potential temperature will be countergradient at different heights. As a practical application of the results, vertical profiles of the countergradient correction term for different entrainment-to-surface-flux ratios are discussed. [ABSTRACT FROM AUTHOR]

Evidence of robust spin-dependent transport in monolayer graphene, deposited on the (0001) surface of the antiferromagnetic (AFM)/magneto-electric oxide chromia (Cr 2 O 3 ), is provided. Measurements performed in the non-local spin-Hall geometry reveal a robust signal that is present at zero external magnetic field and which is significantly larger than any possible ohmic contribution. The spin-related signal persists well beyond the Néel temperature (≈307 K) that defines the transition between the AFM and paramagnetic states, remaining visible at the highest studied temperature of close to 450 K. This robust character is consistent with prior theoretical studies of the graphene/Cr 2 O 3 system, predicting that the lifting of sub-lattice symmetry in the graphene shall induce an effective spin-orbit term of ≈40 meV. Overall, the results indicate that graphene-on-chromia heterostructures are a highly promising framework for the implementation of spintronic devices, capable of operation well beyond room temperature., (© 2022 Wiley-VCH GmbH.)

• Multi-dimensional numerical scheme based on high-order finite elements. • Fully implicit formulation including the self-consistent electromagnetic fields. • Numerical energy conservation proved theoretically and on a test problem. • Non-local transport is captured by the model and demonstrated on a physical problem. Detailed description of the transport processes in plasma is crucial for many disciplines. When the mean-free-path of the electrons is comparable or exceeds a characteristic length scale of the plasma profile, non-local behavior can be observed. Predictions of the diffusion theory are not valid and non-local electric and magnetic fields are generated. Kinetic modeling of these phenomena on time scales several orders of magnitude longer than the electron–electron collision time has proven to be cumbersome due to prohibitive requirements on the time step and violation of the conservation laws in the classical explicit Vlasov–Fokker–Planck methods. Therefore, a multi-dimensional conservative implicit Vlasov–Fokker–Planck–Maxwell method is proposed, where the distribution function is approximated by a truncated Cartesian tensor expansion. The electric and magnetic fields are modeled self-consistently, describing the generation process and emergence of non-locality in detail. Mixed finite elements are employed in space and the velocity dimension is discretized by staggered finite differences. Conservation properties are proved theoretically and the overall features are benchmarked on a series of physically representative problems. The second order convergence in velocity and the spatial order proportional to the polynomial order of the finite elements is shown. Further possible extensions of the method are discussed. [ABSTRACT FROM AUTHOR]

The aim of this study was to assess bioturbation rates in relation to macrozoobenthos and environmental variables in the Svalbard fjords, Barents Sea and Nansen Basin during spring to summer transition. The results showed differences in benthic community structure across sampled area in relation to sediment type and phytopigment content. Fjords, Barents Sea and the shallow parts of Nansen Basin (<400 m) were characterized by high functional groups diversity, and by biodiffusive and non-local rates ranging from 0.05 to 1.75 cm -2  y -1 and from 0.2 to 3.2 y -1 , respectively. The deeper parts of Nansen Basin (>400m), dominated by conveyors species, showed only non-local transport rates (0.1-1 y -1 ). Both coefficients intensity varied with benthic biomass. Non-local transport increased with species richness and density and at stations with mud enriched by fresh phytopigments, whereas biodiffusion varied with sediment type and organic matter quantity. This study quantified for the first time the two modes of sediment mixing in the Arctic, each of which being driven by different environmental and biological situations., (Copyright © 2019 Elsevier Ltd. All rights reserved.)