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Transport of particles in flows is often modeled in a combined Eulerian–Lagrangian framework. The flow is evaluated on an Eulerian grid, while particles are modeled as Lagrangian points whose positions and velocities are evolved in time, resulting in particle trajectories embedded in the time-dependent flow field. The method essentially resolves the flow field in complex geometries in detail but uses a closure model for the particle dynamics aimed at including the essential particle–fluid interactions at the cost of detailed small-scale physics. Rarefaction effects are usually included through the phenomenological Cunningham correction on the drag force experienced by the particles. In this Lagrangian point-particle approach, any explicit reference to the finite size and the shape of the particles, and their local orientation in the flow field, is typically ignored. In this work we aim to address this gap by deriving, from fully-resolved Direct Simulation Monte Carlo (DSMC) studies, heuristic or closure models for the drag force acting on prolate and oblate spheroidal particles with different aspect ratios, and a fixed orientation, in uniform ambient rarefied flows covering the transition regime between the continuum and free-molecular limits. These closure models predict the drag in the transition regime for all considered parameter settings (validated with DSMC data). The continuum limit is enforced a priori and we retrieve the free-molecular limit with reasonable accuracy (based on comparisons with literature data). We also include in the models the capability to predict effects related to basic gas-surface interactions via the tangential momentum accommodation coefficient. We furthermore assess the validity of the proposed closure model for particle dynamics in proximity to solid walls. This investigation extends our previous work, which focused on small aspect ratio spheroids with exclusively diffusive gas-surface interactions [see Livi et al. (2022)]. The derived models are obtained for isothermal, subsonic flows relevant for particle contamination control in semiconductor manufacturing. • DSMC-enhanced drag force closures for particle tracking under rarefied gas conditions. • The drag force on finite-size spheroids in the transition regime of rarefied flows. • Towards algorithms for particle contamination control in semiconductor manufacturing. • Covering the device-to-nanoparticle scale separation in high-tech equipment. [ABSTRACT FROM AUTHOR]

Physics-based numerical models often depend on several parameters to close. Some of them can be expressed using established theoretical or empirical closure formulations. However, some others aggregate complex physical processes and are hence left as tuneable parameters, and can only be calibrated by trial and error. Yet, calibration data are not always available to do so, which prevents these models from being applied to wide ranges of laboratory or river flows. We hence propose a machine learning-based methodology to close any group of unclosed and correlated parameters, applied here to a two-phase/two-layer (2P2L) morphodynamical model. The methodology combines a numerical experiment with a known theoretical solution and machine learning. It is applied to the considered model to close two friction parameters for which generalizable and vastly acknowledged closure formulations lack in the literature. The resulting hybrid model, combining the original 2P2L model and the closure models, is tested against two laboratory dam break test cases. Despite excessive smoothness and underestimation of the concentration in sediment, the hybrid model performed similarly to other models from the literature requiring trial and error calibration and showed high stability and accuracy regarding the estimation of the water-sediment mixture's inertia. HIGHLIGHTS A methodology to close tuneable parameters applicable to many morphodynamical models.; Methodology based on machine learning and self-generated data from a numerical experiment with theoretical solution.; Presentation of a two-phase/two-layer model for unsteady flows highly laden with sediment, used as a proof of concept of the methodology.; New model's performance is similar to evenly complex models requiring trial and error to calibrate tuneable parameters.;

Neural closure models have recently been proposed as a method for efficiently approximating small scales in multiscale systems with neural networks. The choice of loss function and associated training procedure has a large effect on the accuracy and stability of the resulting neural closure model. In this work, we systematically compare three distinct procedures: "derivative fitting", "trajectory fitting" with discretise-then-optimise, and "trajectory fitting" with optimise-then-discretise. Derivative fitting is conceptually the simplest and computationally the most efficient approach and is found to perform reasonably well on one of the test problems (Kuramoto-Sivashinsky) but poorly on the other (Burgers). Trajectory fitting is computationally more expensive but is more robust and is therefore the preferred approach. Of the two trajectory fitting procedures, the discretise-then-optimise approach produces more accurate models than the optimise-then-discretise approach. While the optimise-then-discretise approach can still produce accurate models, care must be taken in choosing the length of the trajectories used for training, in order to train the models on long-term behaviour while still producing reasonably accurate gradients during training. Two existing theorems are interpreted in a novel way that gives insight into the long-term accuracy of a neural closure model based on how accurate it is in the short term. • Analysis of neural closure models with "derivative fitting" and "trajectory fitting". • Trajectory fitting with discretise-then-optimise gives the best results. • Theoretical support is given by two theorems that bound the long-term error. • Two test problems are investigated: Burgers' and Kuramoto-Sivashinsky equations. [ABSTRACT FROM AUTHOR]

Nuclear thermal-hydraulic system analysis codes have been developed to simulate nuclear reactor systems, which solve simplified governing equations by replacing source terms with constitutive relations for simulating entire reactor systems with low computational resources. For half a century, many efforts have been made for wider versatility and higher accuracy of system codes, but various factors can affect the code analysis results, and it was difficult to isolate these factors and interpret them individually. In this study, two system codes, RELAP5 and TRACE, which have many users and are highly reliable, are selected to analyze only the effects of constitutive relations. The influence of constitutive relations is analyzed using in-house platforms that replicate constitute relations of RELAP5 and TRACE equally to exclude factors that may affect analysis results, such as governing equation solvers and user effects. Among the various constitutive relations, the analysis is performed on the wall variables expected to have the most influence on the analysis results. Part 1 paper presents the methodology and wall friction model comparison, while Part 2 paper shows wall heat transfer comparison of the two selected codes.

Nuclear thermal-hydraulic system analysis codes have been developed to comprehensively model nuclear reactor systems to evaluate the safety of a nuclear reactor system. For analyzing complex systems with finite computational resources, system codes usually solve simplified fluid equations for coarsely discretized control volumes with one-dimensional assumptions and replace source terms in the governing equations with constitutive relations. Wall boiling heat transfer models are regarded as essential models in nuclear safety evaluation among many constitutive relations. The wall boiling heat transfer models of two widely used nuclear system codes, RELAP5 and TRACE, are analyzed in this study. It is first described how wall heat transfer models are composed in the two codes. By utilizing the same method described in Part 1 paper, heat fluxes from the two codes are compared under the same thermal-hydraulic conditions. The significant factors for the differences are identified as well as at which conditions the non-negligible difference occurs. Steady-state simulations with both codes are also conducted to confirm how the difference in wall heat transfer models impacts the simulation results.

The magnetohydrodynamic (MHD) control has great potential in the applications of hypersonic vehicles. Typically, the MHD flowfield in these applications is low magnetic Reynolds (Rem) compressible turbulent flow, which is different from that without magnetic field and within the high Rem range. This paper investigated the low Rem compressible MHD isotropic turbulence in different Taylor-scale Reynolds numbers and interaction parameters via the Direct Numerical Simulation (DNS), and the influence of the magnetic field on the turbulence is researched. Results indicate that the normal Reynolds stress decreases in the direction perpendicular to the magnetic field. Moreover, the eddies are stretched in the direction parallel to the magnetic field. Besides, the Joule dissipation quicken the decay of the turbulent kinetic energy, and occurs mainly in large scale vortex. Based on the DNS data, the correlation between fluctuating electric intensity and velocity is studied. Results indicate that the electric intensity agrees well with the first-order approximate solution of the electric potential equation. Accordingly, an improved closure model of the correlation terms is established, where a coefficient related to the local turbulent Mach number is proposed. Comparisons with the DNS results reveal that the proposed model could correctly predict the correlation terms. [ABSTRACT FROM AUTHOR]

In this paper, we consider modeling missing dynamics with a nonparametric non-Markovian model, constructed using the theory of kernel embedding of conditional distributions on appropriate reproducing kernel Hilbert spaces (RKHS), equipped with orthonormal basis functions. Depending on the choice of the basis functions, the resulting closure model from this nonparametric modeling formulation is in the form of parametric model. This suggests that the success of various parametric modeling approaches that were proposed in various domains of applications can be understood through the RKHS representations. When the missing dynamical terms evolve faster than the relevant observable of interest, the proposed approach is consistent with the effective dynamics derived from the classical averaging theory. In the linear Gaussian case without the time-scale gap, we will show that the proposed non-Markovian model with a very long memory yields an accurate estimation of the nontrivial autocovariance function for the relevant variable of the full dynamics. The supporting numerical results on instructive nonlinear dynamics show that the proposed approach is able to replicate high-dimensional missing dynamical terms on problems with and without the separation of temporal scales. [ABSTRACT FROM AUTHOR]

The Coupled-Eulerian-Lagrangian (CEL) method is an established numerical approach for the simulation of geotechnical boundary value problems involving soil–structure interaction and large deformations. A consequence of this approach is the existence of two overlapping numerical meshes, one Lagrangian and one Eulerian. The structure is usually modeled by the Lagrangian part while the soil is represented by the Eulerian part. The material within the Eulerian part of the mesh overlap may be considered a kind of 'virtual' or 'ghost' material, as it only exists due to the numerical approach and has no counterpart in reality. Up to now it has never been evaluated how much this ghost material influences the simulation results. In this study, two geotechnical applications of CEL are analyzed which contain ghost material in a zone of soil–structure-interaction. Each example implements several model variants by applying different material combinations and contact formulations. Additional simulations with a pure Arbitrary-Lagrangian-Eulerian (ALE) approach, in which no ghost material exists, are presented to give further insights. [ABSTRACT FROM AUTHOR]

Previous modeling studies have shown that observed micro-scale (mm) variability of nutrients and phytoplankton biomass can strongly impact the large-scale mean growth response of phytoplankton in ways that cannot be represented by typical models based on the mean field approximation. Also, models accounting for the flexible eco-physiology of phytoplankton predict quite different responses to changing environmental conditions compared to most current (inflexible) models. Combining these two ideas for nutrient-phytoplankton systems we have developed a new “Flexible NP closure model” to represent competition among the three typically observed phytoplankton size classes: pico-, nano-, and micro-phytoplankton. Both micro-scale variability and flexible eco-physiology are expected to impact the competition among these size classes. With this work we begin to address how both these factors determine the size structure and size diversity of phytoplankton in the ocean. Under eutrophic conditions, variability does not impact the modeled growth rate of any size class. On the other hand, under oligotrophic conditions, variability preferentially enhances the biomass of the largest typically observed micro-size class, and reduces the biomass of the smallest nano- and intermediate pico-size classes.

Carboniferous volcanic rocks are widely distributed in the Junggar Basin and may provide a unique insight into the tectonic history of this basin. In this paper, we focus on the tectonic evolution of the northern Junggar Basin (NJB) using geochronological and geochemical data to establish the diachronous closure model for the Keramaili oceanic basin. The Lower Carboniferous strata in the eastern part (well WC‐1) and the Upper Carboniferous strata in the western part (well Y‐2) of the NJB are composed of a mix of compositional and rock‐type forms. These volcanic rocks are mostly high‐K assemblages characterized by high K (1.73–6.48%), high alkali (Na2O + K2O = 5.04–7.45%), and low TiO2 (0.57–1.06%). They also exhibit various REE concentrations (68.44–168.19 ppm), an enrichment of LREE (LaN/YbN = 4.54–8.09), and small negative Eu anomalies (δEu = 0.74–0.90). The depletion of HFSE and relevant tectonic setting discrimination diagrams demonstrate that these two volcanic associations were formed in a subduction‐related environment. The data also indicate that the high‐K volcanic rocks are derived from a mix of melted subducted sediments and mantle metasomatized by silica‐rich fluids. Although these two sets of volcanic rocks formed in Early and Late Carboniferous, the similar petrographic and geochemical natures suggest similar magma source and tectonic setting. Along with different regional tectonic settings in the NJB during the Carboniferous, we propose an asynchronous subduction of Keramaili oceanic basin, resulting in the inconsistent temporal and spatial distribution of Upper Carboniferous volcanic rocks in the NJB. [ABSTRACT FROM AUTHOR]

The transition-sensitive, three-equation k - kL - ω eddy-viscosity closure model was used for simulations of three-dimensional, single-element and multi-element wing configurations operating in close proximity to the ground. The aim of the study was to understand whether the model correctly simulated the transitional phenomena that occurred in the low Reynolds number operating conditions and whether it offered an improvement over the classical fully turbulent k-ω shear stress transport model. This was accomplished by comparing the simulation results to experiments conducted in a 2.7 m × 1.7 m closed-return, three-quarter-open-jet wind tunnel. The model was capable of capturing the presence of a laminar separation bubble on the wing and predicted sectional forces and surface-flow structures generated by the wings in wind tunnel testing to within 2.5% in downforce and 4.1% in drag for a multi-element wing. It was found, however, that the model produced insufficient turbulent kinetic energy during shear-layer reattachment, predicted turbulent trailing-edge separation prematurely in areas of large adverse pressure gradients, and was found to be very sensitive to inlet turbulence quantities. Despite these deficiencies, the model gave results that were much closer to wind-tunnel tests than those given by the fully turbulent k-ω shear stress transport model, which tended to underestimate downforce. Significant differences between the transitional and fully turbulent models in terms of pressure field, wake thickness and turbulent kinetic energy production were found and highlighted the importance of using transitional models for wings operating at low Reynolds numbers in ground effect. The k - kL - ω model has been shown to be appropriate for the simulation of separation-induced transition on a three-dimensional wing operating in ground effect at low Reynolds number. [ABSTRACT FROM AUTHOR]

• A new explicit inter-layer mass exchange closure model is rigorously developed. • The new macroscopic model contains curvature, mass exchange and deformation terms • The new model can be used to understand unsaturated flow and transport mechanisms in thin porous media. • The model improves computational speeds allowing users to obtain faster and accurate simulations at lower cost. • The model can be used to solve interesting thin porous media-related problems of common interests. Unsaturated flow through thin layers of porous media are encountered in many industrial applications, including fuel cells and liquid-absorbing hygiene products such as wipes, paper towels, and diapers. Simplified and efficient closure models are very valuable for understanding fluid flow and transport mechanisms in thin porous media and for the development and design of these products. In this study, a new explicit inter-layer mass exchange closure model is rigorously developed and incorporated into the macroscopic mass balance model in order to obtain a complete set of equations which can be used to describe the absorbency processes in thin porous media and to predict the temporal and spatial behaviors of variables such as saturation, porosity, and layer thickness. The developed 2D averaged macroscopic model will enormously improve the computational speed, allowing one to develop a fast and reasonably accurate simulation of the unsaturated flow. Furthermore, the new closure model will account for the mass exchange between the interior adjacent layers and will include the liquid-absorption effect to better understand the transports mechanisms for characterizing absorbent materials and validating models. The developed closure model is validated for the water flooding problem arising in polymer electrolyte membrane fuel cells and can be adapted and used in the future to solve new interesting thin porous media-related problems of common interests. [ABSTRACT FROM AUTHOR]

We present the new discrete optimization-based interface-aware sub-scale dynamics (IA-SSD) closure model for multimaterial cells for Lagrangian cell-centered hydrodynamics. For the multimaterial cell, the kinematic and thermodynamic properties (e.g., velocity, density, pressure and internal energy) will typically vary between the materials. The discrete closure model is responsible for an accurate update of the thermodynamic states of the individual material components in the multimaterial cell, and for determining the nodal forces that move the vertices of the cell. The IA-SSD closure model consists of two stages — a bulk stage followed by a sub-scale stage. During the bulk stage, the total change in the volume of the cell, total force applied to the cell, and total work done on the cell are distributed between the materials to update their volume, velocity and total energy. This distribution is performed using volume fractions of the materials. During the second stage, sub-scale interactions of the materials inside the multimaterial cell are taken into account. At this stage, information about the topology of the materials inside the multimaterial cell is used, allowing the orientations of internal interfaces to be included in the model. Each material interacts in a pair-wise fashion with the materials with which it has a common boundary. The interactions are based on the solution of the acoustic Riemann problem between each pair of materials and are limited using physically justified constraints: positivity of volume, positivity of internal energy, and controlled rate of pressure relaxation. To determine the values of the limiter coefficients, a constrained-optimization framework is employed using a quadratic objective function with linear constraints. It is a first-of-its kind application of constrained optimization to develop discrete closure models in a more rigorous fashion. The pair-wise interaction between materials is essentially one dimensional in the direction that is normal to interface. For this reason, we demonstrate in this paper the performance of our new model on one dimensional numerical examples. [ABSTRACT FROM AUTHOR]

We present a class of efficient parametric closure models for 1D stochastic Burgers equations. Casting it as statistical learning of the flow map, we derive the parametric form by representing the unresolved high wavenumber Fourier modes as functionals of the resolved variable’s trajectory. The reduced models are nonlinear autoregression (NAR) time series models, with coefficients estimated from data by least squares. The NAR models can accurately reproduce the energy spectrum, the invariant densities, and the autocorrelations. Taking advantage of the simplicity of the NAR models, we investigate maximal space-time reduction. Reduction in space dimension is unlimited, and NAR models with two Fourier modes can perform well. The NAR model’s stability limits time reduction, with a maximal time step smaller than that of the K-mode Galerkin system. We report a potential criterion for optimal space-time reduction: the NAR models achieve minimal relative error in the energy spectrum at the time step, where the K-mode Galerkin system’s mean Courant–Friedrichs–Lewy (CFL) number agrees with that of the full model.

Developing accurate, efficient, and robust closure models is essential in the construction of reduced order models (ROMs) for realistic nonlinear systems, which generally require drastic ROM mode truncations. We propose a deep residual neural network (ResNet) closure learning framework for ROMs of nonlinear systems. The novel ResNet-ROM framework consists of two steps: (i) In the first step, we use ROM projection to filter the given nonlinear system and construct a spatially filtered ROM. This filtered ROM is low-dimensional, but is not closed. (ii) In the second step, we use ResNet to close the filtered ROM, i.e., to model the interaction between the resolved and unresolved ROM modes. We emphasize that in the new ResNet-ROM framework, data is used only to complement classical physical modeling (i.e., only in the closure modeling component), not to completely replace it. We also note that the new ResNet-ROM is built on general ideas of spatial filtering and deep learning and is independent of (restrictive) phenomenological arguments, e.g., of eddy viscosity type. The numerical experiments for the 1D Burgers equation show that the ResNet-ROM is significantly more accurate than the standard projection ROM. The new ResNet-ROM is also more accurate and significantly more efficient than other modern ROM closure models.

Closure models started from Chou's work have been developed for more than 70 years, aiming at providing analytical tools to describe turbulent flows in the spectral space. In this study, a preliminary attempt is presented to introduce a closure model in the physical space, using the velocity structure functions as key parameters. The present closure model appears to qualitatively reproduce the asymptotic scaling behav-iors at small and large scales, despite some inappropriate behaviors such as oscillations. Therefore, further improvements of the present model are expected to provide appropriate descriptions of turbulent flows in the physical space. [ABSTRACT FROM AUTHOR]

A proper orthogonal decomposition (POD) technique has been successfully employed to develop reduced-order models for flow control purposes. For complex flows, higher POD modes also play a significant role in the stability and accuracy of the reduced-order model, thus require a closure, as in turbulent flows. In the presence of nonhomogeneous boundary conditions, developing a closure model becomes a challenging task. This paper discusses nonlinear closure modeling approaches for homogeneous and nonhomogeneous boundary conditions. Burgers’ equations, both one-dimensional and two-dimensional, are considered as the governing equations to develop reduced-order models with different boundary conditions. Homogeneous and nonhomogeneous boundary conditions are considered to demonstrate the effectiveness of the proposed closure modeling technique in boundary control applications. Numerical results show that the proposed closure model improves the accuracy of the reduced-order model. [ABSTRACT FROM AUTHOR]

In porous burners, the flame thickness can be smaller than the pore size, resulting in sharp and locally-anchored flame fronts. The presence of such steep gradients at the pore level is a major hurdle for the derivation of volume-averaged models, particularly for the highly non-linear reaction rates. With the intent to address the difficulties associated with volume-averaging for porous media combustion, this work makes use of 3D Pore-Level Direct Numerical Simulations including conjugate heat transfer and complex chemistry in burners of finite length. These detailed 3D simulations are compared to their 1D volume-averaged counterpart, with effective properties estimated directly on the computational domains and of identical thermo-chemical scheme. Discrepancies in terms of burning rate, profiles, and a priori analysis on the reaction rates are discussed. Various pore sizes and geometries are considered. At the pore level, it is shown that preheating, wrinkling and wall quenching are the three main factors driving the global burning rate. Importantly, hydrodynamic dispersion is shown to have an indirect role on combustion processes. From the observations of combustion at pore scale, a new closure for reaction rates based on a flamelet assumption is proposed. It accounts for flame wrinkling and eliminates the unwanted effect of hydrodynamic dispersion on burning rate. [ABSTRACT FROM AUTHOR]

In this paper, we develop data-driven closure/correction terms to increase the pressure and velocity accuracy of reduced order models (ROMs) for fluid flows. Specifically, we propose the first pressure-based data-driven variational multiscale ROM, in which we use the available data to construct closure/correction terms for both the momentum equation and the continuity equation. Our numerical investigation of the two-dimensional flow past a circular cylinder at R e = 50 , 000 in the marginally-resolved regime shows that the novel pressure data-driven variational multiscale ROM yields significantly more accurate velocity and pressure approximations than the standard ROM and, more importantly, than the original data-driven variational multiscale ROM (i.e., without pressure components). In particular, our numerical results show that adding the closure/correction term in the momentum equation significantly improves both the velocity and the pressure approximations, whereas adding the closure/correction term in the continuity equation improves only the pressure approximation. • Proposed new data-driven reduced order models for turbulent flows. • Put forth novel data-driven correction/closure terms to increase the ROM accuracy. • Developed novel pressure correction terms for the pressure Poisson reduced order formulation. [ABSTRACT FROM AUTHOR]

In multiphase flows, the mathematical and physical nature of the phenomena leads to the postulation of closure models to describe the exchange of momentum across the interface as well as other properties such as heat and mass, and various models are available for this purpose . However, unlike dispersed flows, in the case of free surface flows there is a lack of correlations for the drag coefficient, which is an important issue in the management of this type of flow. In this paper, the interfacial density force was calculated dynamically without the use of any correlation through the modeling of the viscous tensor and the normal vector at the interface. The model was applied to test cases as vertical shear flow, dam break and Rayleigh–Taylor instability. The comparison of the results was satisfactory and good agreement with numerical and experimental data from the literature, as well as with the standard approach also employed was observed. These results indicate that the model is a good alternative for stratified flows, and it can also be used directly to calculate the drag coefficient when the standard formulation is preferred. [ABSTRACT FROM AUTHOR]

We develop new interface-unaware sub-scale dynamics (IU-SSD) multimaterial cell closure models for the Lagrangian stage of cell-centered arbitrary Lagrangian-Eulerian method. The IU-SSD closure models do not require information about the interface in the multimaterial cell and consist of two stages. The first stage makes use of a constant volume fraction model, wherein the main ingredient is the construction of a nodal Riemann solver that is capable of handling multimaterial cells. This model provides the numerical flux at cell face and introduces the mechanism of velocity relaxation. The sub-scale interactions of the materials in a multimaterial cell are taken into account by incorporating the pressure relaxation model in the second stage. Two types of pressure relaxation models are designed utilizing the linear relation between viscous pressure and volume strain rate. The pressure relaxation models are implemented via a multi-step time marching procedure such that they satisfy the physically justified constraints: positivity of volume, positivity of internal energy and smooth relaxation of pressure. The performances of new IU-SSD models are demonstrated via a series of numerical tests involving both Lagrangian and arbitrary Lagrangian-Eulerian hydrodynamics calculations. • Second-order extension of the constant volume fraction model is made. • Two types of pressure relaxation models are constructed. • Plenty of numerical tests for validation are carried out. [ABSTRACT FROM AUTHOR]

In the numerical simulation of multi-material large deformation flow problems, the most important thing is tracking the material interfaces accurately while dealing with the large deformation of fluid simultaneously. The multi-material arbitrary Lagrangian Eulerian (MMALE) method coupled with the moment-of-fluid (MOF) interface reconstruction, was named a MOF-MMALE method and applied to multi-material large deformation flow problems. For the MOF-MMALE method, the mesh lines were allowed to cross the material interfaces and the mixed cells were introduced. In the mixed cells, the MOF interface reconstruction was used to determine the position and direction of the material interface. The numerical results of several typical examples, including the 2-material shock tube problem, the triple point problem, the Rayleigh-Taylor instability problem and the shock wave-Helium bubble interaction problem, show high accuracy and good resolution of the MOF-MMALE method, which is validated to be an effective way to simulate multi-material fluid flow problems with large deformation. [ABSTRACT FROM AUTHOR]

• A fast, spectrally accurate algorithm for computing the Bingham closure. • Chebyshev interpolation provides a controllable error/efficiency tradeoff. • The algorithm enables large-scale continuum simulations of apolar particle suspensions. Continuum kinetic theories provide an important tool for the analysis and simulation of particle suspensions. When those particles are anisotropic, the addition of a particle orientation vector to the kinetic description yields a 2 d − 1 dimensional theory which becomes intractable to simulate, especially in three dimensions or near states where the particles are highly aligned. Coarse-grained theories that track only moments of the particle distribution functions provide a more efficient simulation framework, but require closure assumptions. For the particular case where the particles are apolar, the Bingham closure has been found to agree well with the underlying kinetic theory; yet the closure is non-trivial to compute, requiring the solution of an often nearly-singular nonlinear equation at every spatial discretization point at every timestep. In this paper, we present a robust, accurate, and efficient numerical scheme for evaluating the Bingham closure, with a controllable error/efficiency tradeoff. To demonstrate the utility of the method, we carry out high-resolution simulations of a coarse-grained continuum model for a suspension of active particles in parameter regimes inaccessible to kinetic theories. Analysis of these simulations reveals that inaccurately computing the closure can act to effectively limit spatial resolution in the coarse-grained fields. Pushing these simulations to the high spatial resolutions enabled by our method reveals a coupling between vorticity and topological defects in the suspension director field, as well as signatures of energy transfer between scales in this active fluid model. [ABSTRACT FROM AUTHOR]

Abstract: The use of residual K (K res ) approaches for prediction of fatigue crack growth rates in residual stress fields was studied. Finite element models of the samples were built and the measured residual stress data put into the model. The virtual crack closure technique (VCCT) was used to calculate Kres (stress intensity factor from residual stress) together with its changes with crack length using data from the part I paper. Local Kres values were used to calculate effective R values. Kop and ΔKeff values throughout the crack path in the weld. The master curve approach was used to relate these to corresponding values for crack growth rates. Predicted crack growth rates were compared with experimental results. Changes in crack growth rate found as the crack grows through the weld can successfully be predicted via application of this closure based model. Agreement between predictions and experimental data was best for tensile residual stress fields and was not as exact in compression. Possible reasons for this discrepancy are discussed. [Copyright &y& Elsevier]

A closure model for flow-induced fiber orientation is proposed that is based on a reconstruction of the orientation distribution function (ODF). An a priori analysis by using Monte-Carlo simulation was performed. The main results are as follows. For strong rotary Brownian motion (low Péclet numbers), the model performs well even by using only the second moment to reconstruct the ODF. By considering the fourth moment in the reconstruction procedure, the accuracy of the model is improved and the model is applicable to higher Péclet numbers. At very high Péclet numbers and for non-Brownian fibers, the solution predicted by the proposed model deviates from the reference solution. [ABSTRACT FROM AUTHOR]

Abstract: There are many natural and anthropogenic reasons why a gap can occur inside the forest. When a gap appears within a studied stand (e.g. near a flux tower which operated for some time, providing information about the ecosystem–atmosphere exchange), an assessment of new measurement conditions should be carried out. Using a three-dimensional approach for footprint estimation based on numerical solution of Reynolds-Averaged Navier–Stokes (RANS) equations, we investigated possible changes in air flow and CO2 flux footprints resulting from two suggested forest management activities – clear-cut and stripe-cut – around the flux tower located in 130-year-old spruce forest in the Solling highland, Germany. The model results show that degree of changes in flux footprints depends on the chosen management strategy. The clear-cut strategy produces the largest changes and the stripe-cut leads to weaker changes of investigated characteristics. The role of remote canopy sources increases, while the contribution of remote soil sources decreases with increased share of removed trees. In general, the investigated characteristics change differently for summer and winter due to the combined effects of phenology and upwind topography. [Copyright &y& Elsevier]

Abstract: A normalization scheme for the numerical solution of the moment approximation equation in fiber suspension flows is presented. Here, normalization refers to rescaling the trace of the second moment tensor to unity at each time step. The equivalence between the normalization scheme and the quadratic closure model is analytically proved. The performance of the scheme is investigated in simple shear flow with respect to the quadratic and hybrid closures, and a stochastic Monte-Carlo simulator that provides exact solution. The proposed scheme is a computationally efficient alternative to the quadratic closure: it performs equally well and is more efficient regarding computational time. [Copyright &y& Elsevier]

Abstract: A direct 3D CFD modelling of airflow within model plant canopies was performed with separate models for the leaves and the branches of the canopy. The effect of the branches on airflow was modelled by introducing the 3D architecture of the canopies into the model and that of the leaves was modelled by adding drag force terms in the momentum and turbulent energy equations in porous sub-domains created around the branches. Wind tunnel experiments were used to validate the model. The experiments were carried out for inlet velocities of 10ms−1 and 15ms−1 with two artificial 1:10 scaled model trees, fully leafed, half leafed and leafless. Simulations were made with a full closure model (FCM) and with the presented integrated model (IM). FCM and IM gave a maximum percent error of 34 and 30, respectively for average air velocity, U. On horizontal average basis both methods gave fairly comparable accuracies with maximum errors of 16.6% for FCM and 13.7% for IM but on local basis both horizontally and vertically within the tree height the present approach gave a better accuracy. The present model also gave qualitatively good results for turbulent kinetic energy. [Copyright &y& Elsevier]

We investigate the scaling behaviour of a turbulent kinetic energy (TKE) closure model for stably stratified conditions. The mixing length scale for stable stratification is proportional to the ratio of the square root of the TKE and the local Brunt–Väisälä frequency, which is a commonly applied formulation. We analyze the scaling behaviour of our model in terms of traditional Monin–Obukov Similarity Theory and local scaling. From the model equations, we derive expressions for the stable limit behaviour of the flux–gradient relations and other scaling quantities. It turns out that the scaling behaviour depends on only a few model parameters and that the results obey local scaling theory. The analytical findings are illustrated with model simulations for the second GABLS intercomparison study. We also investigate solutions for the case in which an empirical correction function is used to express the eddy diffusivity for momentum as a function of the Richardson number (i.e. an increasing turbulent Prandtl number with stability). In this case, it seems that for certain parameter combinations the model cannot generate a steady-state solution. At the same time, its scaling behaviour becomes unrealistic. This shows that the inclusion of empirical correction functions may have large and undesired consequences for the model behaviour. [ABSTRACT FROM AUTHOR]

Abstract: The two-stream model (TSM; Chatfield and Brost, J. Geophys. Res. 92 (1987) 13263–13276) was developed to account for advection-like vertical mixing features of a passive tracer in the convective boundary layer (CBL). In the TSM, the tracer is advected by two-streams (that is, mean updraft and downdraft), of which the vertical profiles are prescribed as functions of the CBL similarity scales, that is, the CBL height and velocity scales. Compared to the results from large-eddy simulation (LES) and laboratory model, the TSM shows similar plume propagation behavior, but the plume propagation speed appears to be too slow. This drawback of the TSM is found to be mainly due to too weak updraft and downdraft. To improve the TSM, therefore, we modify the prescribed vertical profiles of the mean updraft and downdraft using the LES data. Compared to the original up and down draft velocity profiles, the modified profiles have significantly larger magnitudes in the lower CBL while having similar magnitudes in the upper CBL. The realism of the modified TSM simulation of vertical mixing in the CBL is tested for near surface and elevated tracer sources against the LES results of which validity has been well verified with laboratory experiments and observation. For comparison purpose, we introduce other non-local closure models such as the Blackadar model, the asymmetric convective model, and the transilient turbulence parameterization as well as a local K-theory-based scheme. The modified TSM not only predicts much more improved tracer propagation than the original TSM but also yields the most superior results among the models introduced in this study, showing a remarkably realistic description of the temporal behavior for the concentration distributions and the ground and source level concentrations. [Copyright &y& Elsevier]