Macroscopic Flow Research Articles

Characteristics of traumatic brain injury models: from macroscopic blood flow changes to microscopic mitochondrial changes.

Controlled cortical impingement is a widely accepted method to induce traumatic brain injury to establish a traumatic brain injury animal model. A strike depth of 1 mm at a certain speed is recommended for a moderate brain injury and a depth of > 2 mm is used to induce severe brain injury. However, the different effects and underlying mechanisms of these two model types have not been proven. This study investigated the changes in cerebral blood flow, differences in the degree of cortical damage, and differences in motor function under different injury parameters of 1 and 2 mm at injury speeds of 3, 4, and 5 m/s. We also explored the functional changes and mitochondrial damage between the 1 and 2 mm groups in the acute (7 days) and chronic phases (30 days). The results showed that the cerebral blood flow in the injured area of the 1 mm group was significantly increased, and swelling and bulging of brain tissue, increased vascular permeability, and large-scale exudation occurred. In the 2 mm group, the main pathological changes were decreased cerebral blood flow, brain tissue loss, and cerebral vasospasm occlusion in the injured area. Substantial motor and cognitive impairments were found on day 7 after injury in the 2 mm group; at 30 days after injury, the motor function of the 2 mm group mice recovered significantly while cognitive impairment persisted. Transcriptome sequencing showed that compared with the 1 mm group, the 2 mm group expressed more ferroptosis-related genes. Morphological changes of mitochondria in the two groups on days 7 and 30 using transmission electron microscopy revealed that on day 7, the mitochondria in both groups shrank and the vacuoles became larger; on day 30, the mitochondria in the 1 mm group became larger, and the vacuoles in the 2 mm group remained enlarged. By analyzing the proportion of mitochondrial subgroups in different groups, we found that the model mice had different patterns of mitochondrial composition at different time periods, suggesting that the difference in the degree of damage among traumatic brain injury groups may reflect the mitochondrial changes. Taken together, differences in mitochondrial morphology and function between the 1 and 2 mm groups provide a new direction for the accurate classification of traumatic brain injury. Our results provide reliable data support and evaluation methods for promoting the establishment of standard mouse controlled cortical impingement model guidelines.

Particle-scale heat and mass transfer processes during the pyrolysis of millimeter-sized lignite particles with solid heat carriers

• Devolatilization kinetics is coupled with the heat and mass transfer processes. • Evolution of pore structure in coal pyrolysis with solid heat carrier is included. • Multi-physical fields inside a lignite particle during pyrolysis is elaborated. • The intraparticle mass transfer mechanism influences the secondary reaction. The study of mesoscale solid fuel pyrolysis behavior is a key bridge between the microscopic reaction kinetics and macroscopic multiphase reaction flow in a reactor. In this work, a one-dimensional nonstationary model was developed in spherical coordinate system for the pyrolysis of millimeter-scale lignite particles with solid heat carriers, in which a modified multistep kinetic model (MSM) was coupled with a series of correlative transient heat and mass conservation equations in conjunction with the dusty gas model (DGM). The MSM for coal pyrolysis was modified by adding a set of kinetic equations of pseudo-elementary secondary reactions, making the kinetic model comparable to the Chemical Percolation Devolatilization model in terms of accuracy. In addition, a semiempirical submodel for the evolution of pore structure parameters (porosity, pore size, and permeability) was incorporated to obtain the precise mechanism of volatile species transport in the porous coal matrix. The modified MSM and pore evolution submodel were validated against literature data. The coupling effects between intraparticle transport processes and transient devolatilization kinetics were investigated. It was concluded that notable multiphysical fields (pressure, concentration, and velocity) emerged within lignite particles during pyrolysis, governed by the heating history. In addition, it should be noted that the direction and mechanism of volatiles transfer within porous lignite particles varied during pyrolysis, which can significantly affect secondary reactions. Flow reversal and shifts in the dominant mass transfer mode influenced the secondary reaction pathway, in addition to the impact of volatiles flow velocity on the reaction degree.

Study on thermal protection characteristics of non-enclosed thermal cloak

The aerodynamic heat of hypersonic vehicle nose cone can reach MWm<sup>-2</sup> magnitude during flight, which could be transferred to the interior of hypersonic vehicle in the form of conduction and radiation. High efficient thermal insulation material is significant to keep internal electronic components working safely. Thermal metamaterials can regulate the macroscopic heat flow path, which have been developing rapidly and have a wide application prospect in the field of thermal protection. In this paper, a non-enclosed theoretical thermal cloak is designed to guide heat flow around hypersonic vehicle nose cone by using the transformation multithermotics, which can control thermal conduction and radiation simultaneously. A multi-layer structure is designed as cloak's simplified approximation due to the anisotropic parameters. Based on the software COMSOL, the thermal protection characteristics and heat transfer mechanism of the cloak and multi-layer structure are studied numerically. The results show that heat can flow around the object in the form of conduction and radiation in both theoretical thermal cloak and multi-layer structure, so the heat transferred to the inner area is decreased. Compared with the thermal insulation material, the heating rate of the protected area slows down, and the temperature at the front of the hypersonic vehicle nose cone is significantly reduced. However, the improvement of the thermal protection performance of cloak and multi-layer structures requires that the solid and radiative thermal conductivities of the material be lower than those of the original thermal insulation materials. To solve this problem, a non-enclosed theoretical extrapolation thermal cloak is further proposed. The solid and radiative thermal conductivities of extrapolation thermal cloak are non-singular, which could be higher than those of the original thermal insulation materials. Numerical simulation results show that the extrapolation thermal cloak can guide heat flow around object, so the thermal protection capability is improved significantly. Compared with the thermal insulation materials, the temperature of the front of the hypersonic vehicle nose cone is reduced by 100 K, and the temperature of the inner central zone of the hypersonic vehicle nose cone is reduced by 10 K. The non-enclosed extrapolation thermal cloak provides a new approach for thermal protection and is suitable for complex target areas, showing great application potential in thermal protection.

VISCOELASTIC PROPERTIES OF FERROFLUIDS WITH CLUSTERED PARTICLES

Clustered (multi-core) magnetic fluids provoke a considerable interest of researchers and practitioners, because they are very promising for various technical and biomedical applications. These fluids contain clusters (clustered particles), which, in turn, consist of ferromagnetic nanoparticles retained together by a polymer shell. The typical size of a cluster varies from a few tens to several hundreds of nanometers, while the sizes of individual single-domain ferroparticles of which it is composed vary from 5 to 12 nm. The rheological phenomena in such fluids (strong magnetorheological effect and slow viscoelastic relaxation) are predetermined by the association of the clustered particles under the action of an external magnetic field into heterogeneous structures and aggregates and the dynamics and disruption of these aggregates in macroscopic deformational flows. In this work, we propose a theoretical model for viscoelastic effects in clustered magnetic fluids. The model is based on the idea of aggregating composite particles into linear chain-like aggregates. In the order of magnitude, the theoretical results agree with experimental data.

Macroscopic traffic flow modelling: from kinematic waves to autonomous vehicles

Abstract This survey offers a unified up-to-date presentation of macroscopic models of traffic flow, pointing out their main characteristics and possible drawbacks. The presentation is completed by several pictures illustrating the models’ features. Some open problems and future research directions are also given to inspire the reader.

3-step calibration process of a microscopic traffic flow simulation platform for mixed traffic scenarios

Microscopic traffic flow simulation is used in multiple fields nowadays, resulting in varying objectives and requirements regarding model calibration. Traffic engineers analyse the impact on traffic flow using simulation tools and in automobile industry simulation is used for developing and analysing automated driving functions and their functionalities. We developed a simulation platform with pre-defined applications, allowing operators to test the impact of automated driving on traffic flow by varying mixed traffic scenarios. As calibration of the base case for different domains is a time-consuming process, we present in this paper a 3-step calibration process, which allows efficient model calibration by focusing on macroscopic traffic flow dynamics as well as microscopic driving behaviour.

Modeling the Impact of Route Recommendations in Road Traffic

In this work, effects of real-time route recommendations between two alternative routes are analyzed. The aim is to assess the consequences of route planner applications (apps) on road congestion. For this purpose, the considered model takes the form of a state-dependent switching system that describes a supply-demand mechanism, based on established macroscopic traffic flow models. Through a comprehensive stability analysis of the system, with an emphasis on the dependence on the system parameters (such as road capacities, critical densities, traffic demand), it is shown that real-time recommendations can cause congestion in the network and failure to satisfy user demand.

Rock-on-a-chip: "Seeing" the association/disassociation of an adaptive polymer in solutions flowing through porous media.

The flow and transport of polymer solutions through porous media are ubiquitous in myriad scientific and engineering applications. With escalating interest in adaptive polymers, understanding the flow dynamics of their solutions is indispensable (yet lacking). Here, the hydrophobic-effect-driven reversible associations in a self-adaptive polymer (SAP) solution and its flow characteristics in a microfluidic-based "rock-on-a-chip" device have been analyzed. The hydrophobic aggregates were fluorescent labeled; this enabled a direct visualization of the in situ association/disassociation of the polymer supramolecular assemblies in pore spaces and throats. Furthermore, the influence of this adaptation on the macroscopic flow behavior of the SAP solution was analyzed by comparing its flow with that of two partially-hydrolyzed polyacrylamide (the molecular weight (MW)-equivalent HPAM-1 and ultrahigh-MW HPAM-2) solutions in the semi-dilute regime with similar initial viscosities. At low flow rates (with shear predominance), the SAP solution showed a low shear viscosity compared to HPAM-1, indicating a higher shear susceptibility for association than chain entanglement. Although the SAP exhibited the same elastic instability as the non-adaptive polymers above a threshold flow rate, the adaptable structure of the former advanced the onset of its viscoelastic-governed flow, providing a stronger flow resistance, possibly through an extension resistance. Furthermore, 3D-media analysis indicated that the reversible association/disassociation of SAP increased the accessible pore space during nonaqueous-liquid displacement, facilitating oil production.

Unified gas-kinetic scheme with simplified multi-scale numerical flux for thermodynamic non-equilibrium flow in all flow regimes

In this paper, a BGK-type kinetic model for diatomic gases is proposed to describe the high-temperature thermodynamic non-equilibrium effect, which is a phenomenological relaxation model with continuous distribution modes of rotational and vibrational energies. In order to obtain the correct Prandtl number and reasonable relaxation rate of heat fluxes, the equilibrium distribution function is constructed by using a multi-dimensional Hermitian expansion around Maxwellian distribution. Based on this kinetic model equation, a unified gas-kinetic scheme (UGKS) with simplified multi-scale numerical flux is proposed for thermodynamic non-equilibrium flows involving the excitation of molecular vibrational degrees of freedom in all flow regimes. The present UGKS keeps the basic conservation laws of the macroscopic flow variables and microscopic gas distribution function in a discretized space. In order to improve the efficiency of UGKS, a simplified multi-scale numerical flux is directly constructed from the characteristic difference solution of complex kinetic model equation. Furthermore, the applications of unstructured discrete velocity space (DVS) and a simple integration error correction reduce the number of velocity mesh significantly and make the present method be rather efficient for flow simulation in all flow regimes. The new scheme is examined in a series of cases, such as Sod’s shock tube, high non-equilibrium shock structure, hypersonic flow around a circular cylinder with Knudsen (Kn) number Kn=0.01, supersonic rarefied flow over a flat plate with a sharp leading edge, and hypersonic rarefied flow past a blunt wedge. The present UGKS results agree well with the benchmark data of DSMC and other validated methods.

A combined first and second order model for a junction with ramp buffer

Second order macroscopic traffic flow models are able to reproduce the so-called capacity drop effect, i.e., the phenomenon that the outflow of a congested region is substantially lower than the maximum achievable flow. Within this work, we propose a first order model for a junction with ramp buffer that is solely modified at the intersection so that the capacity drop is captured. Theoretical investigations motivate the new choice of coupling conditions and illustrate the difference to purely first and second order models. The numerical example considering the optimal control of the onramp merging into a main road highlights that the combined model generates similar results as the second order model.

Visualization and investigation of healing mechanism in carbon fiber reinforced Elium® composites

In this study, carbon fiber/Elium® composites are manufactured using the resin infusion manufacturing technique. Artificial cracks representing impact-induced delamination are incorporated during layup. The laminates are subsequently healed using several healing parameters. The bonding mechanisms are investigated through microscopic analyses and X-ray Computed Tomography (XCT). The mechanical performance of the healed composites is investigated through four-point bending tests. It is found that as the healing temperature is increased, the cracks are gradually healed by interdiffusion and macroscopic resin flow across each crack interface. The 3D models generated from XCT images show an increase in solid volume fraction from 48% to 70% at the optimum healing parameters. The flexural strength and flexural modulus of the undamaged composites subjected to optimized healing condition are significantly increased relative to the room temperature cured pristine samples. The residual flexural strength of the damaged samples healed at the optimum parameters is 96% of the reference condition. The SEM images confirm that delamination from the previously-healed crack interfaces is the most dominant failure mode in the healed samples, in contrast to fiber fracture and brittle failure in the undamaged reference composite samples. The results show that the damaged composite laminates can be effectively healed and repaired to as good as undamaged laminates using the interdiffusion mechanism.

In‐Situ Capillary Pressure and Its Interrelationships With Flow Characteristics During Steady‐State Three‐Phase Flow in Water‐Wet Berea Sandstone

AbstractWe present a detailed multi‐level investigation of steady‐state three‐phase flow processes in a water‐wet Berea sandstone sample. We use high‐resolution micro‐computed tomography images acquired during micro‐scale core‐flooding experiments which include successive gas injection, waterflooding, and nonspreading oil flooding steps. We develop and employ robust techniques to comprehensively characterize and probe the complex interrelationships between the pore‐scale three‐phase capillary pressures, fluid occupancy, displacement events, and the macroscopic flow behavior. The results demonstrate that the local capillary pressures involving the invading phase rise or drop in compliance with the changes in the macroscopic flow ratios. For instance, the gas‐oil capillary pressures were at their highest levels during the gas injection process (highest gas/oil fractional flow) and dropped during the subsequent processes. More importantly, the oil‐water capillary pressure is found to sharply rise during the gas injection process, implying that the gas‐displacing‐oil‐displacing‐water double displacements are the dominant double displacement mechanism during this experiment. For the waterflooding test, the local gas‐oil capillary pressures consistently decrease due to water‐displacing‐oil‐displacing‐gas double displacement events. Moreover, the results show that the variations in the gas‐liquid capillary pressures are more significant than those in the oil‐water capillary pressures. Also, the variations in all capillary pressures during gas injection are higher than in the subsequent processes. The higher variations in capillary pressure values are attributed to more localized and less uniform pore‐scale events across the medium. Finally, the capillary pressure gradients along the flow direction are incorporated in the three‐phase relative permeability measurements.

An efficient implementation of the graphics processing unit-accelerated single-step and simplified lattice Boltzmann method for irregular fluid domains

In this article, an efficient implementation of the graphics processing unit (GPU)-accelerated single-step and simplified lattice Boltzmann method for curved (irregular) fluid domains (ISSLBM) is presented, allowing the algorithm to predict the macroscopic flow variables in realistic scenarios, such as the wind flow influenced by complex terrains. The fluid domain is approximated and reconstructed with two- and three-dimensional non-uniform rational B-splines functions, allowing customized refinements for desired regions. The model accuracy is investigated by conducting a two-dimensional flow around a circular profile for cases with low Reynolds numbers (Re = 20 and 40). Furthermore, the model is also used to simulate a highly turbulent wind flow (Re = 10 × 106) around the Bolund hill, located in Denmark. Numerical and experimental results reported in the literature are directly compared with the results from the ISSLBM algorithm, producing results with excellent agreement in all metrics. The computational performance is also analyzed, showing that the GPU-accelerated ISSLBM is significantly faster than other simulations reported in the literature.

Modeling mixed traffic flows of human-driving vehicles and connected and autonomous vehicles considering human drivers’ cognitive characteristics and driving behavior interaction

Connected and autonomous vehicles (CAVs) are developing rapidly nowadays. In the near future, we may see human-driving vehicles (HVs) and CAVs running on the same road. The characteristics of human drivers and the interaction between HVs and CAVs, therefore, need to be further studied. All these factors will have a great impact on traffic operation. This paper aims to explore a modeling method to analyze the operation mechanism of mixed traffic flows of HVs and CAVs. Considering HV drivers’ cognitive behavioral characteristics and HV–CAV interaction effects, the forward and lateral movement rules of HVs and CAVs are proposed based on the Cellular Automata model. Then the fundamental diagram, congestion degree, lane-changing frequency, and time-spatial diagram are obtained by numerical simulation. The results show that the presence of CAVs is positively related to macroscopic traffic parameters, including velocity, flow and critical density. CAVs help to relieve traffic congestion and instabilities. The congestion degree of pure CAV traffic flow is about 1/3 that of pure HV traffic flow. When the CAV penetration rate is relatively low, the interaction between HV and CAV has a significant impact on traffic operation. Specifically, when the CAV penetration rate is below 0.5∼0.6, increasing the CAV penetration rate will increase the lane-changing frequency of the whole traffic flow.

On non-Darcian flow behavior in a rough-walled fracture filled with a porous medium

Fluid flow through filled rock fractures is relevant to numerous engineering applications and geophysical processes. However, a proper understanding of the flow behaviors in filled fractures is still far from being completed. To investigate the fluid flow in a rough-walled fracture filled with a porous medium, we systematically compare the micro- and macroscopic flow behaviors within filled fractures and corresponding open fractures and porous media based on a series of pore-scale simulation results. We find that the filling medium has a dual role in altering the flow regime, as the filling particles can not only cut the flow domain into several pore channels and thus limit the generation of large-span eddies, but also force the fluid to flow around them and promote the generation of local eddies. Compared with porous media, the fluid flow within filled fractures is confined by fracture walls, which greatly alters the media permeability and the flow nonlinearity therein. By introducing the hydraulic radius and Darcy-Weisbach equation, we develop a new semi-empirical model to estimate the fluid flow through filled fractures using phenomenological coefficients of corresponding open fractures and porous media. The proposed model, satisfying dimensional consistency, shows high fidelity in predicting water flow through filled fractures. More importantly, this model can be successfully reduced to the typical form of the Forchheimer equation applied to open fractures and porous media. The results and findings are expected to be useful in predicting and evaluating the fluid flow in complex geological setting.

Slag Flow in the Packed Bed With Varied Properties and Bed Conditions: Numerical Investigation

Molten slag, which is primarily generated in the blast furnace (BF) cohesive zone, trickles down through the coke packed bed in the form of films, rivulets, or droplets in the lower zone of the BF. During its downward flow, there are significant interactions occurring between slag and other phases such as gas, coke particles, hot metal, and fine powders. In terms of these interactions, slag flow behavior can greatly affect BF productivity and be associated with furnace irregularities such as channeling, hanging, and slipping. Hence, understanding the interactions between phases is useful to maximizing BF efficiency in terms of operating cost, reliability, and production capacity. In the current study, a Volume of Fluid (VOF) modeling technique was applied to track the movement of individual slag droplets in the packed bed at a mesoscopic level, considering various bed permeabilities, more wide-ranging slag properties, and different wettability between slag and packing particles. Results demonstrate the significant role of modeling at a mesoscopic level in understanding macroscopic slag flow behavior. Modeling work helps to visualize the trickling behavior of slag droplets in more realistic and complex conditions representing a BF, and clarify the mechanisms of the different flow patterns generated for variations in operating conditions. Transient flow characteristics such as localized slag accumulation and droplet morphology were identified and analyzed in relation to complex condition changes. The current modeling proved to be a valuable tool to provide a foundation for better understanding the slag flow behavior and its interactions with other phases in the BF lower zone.

Global stability and bifurcation of macroscopic traffic flow models for upslope and downslope

A macro-continuum model of the traffic flow is derived from a micro-car-following model that considers both the upslope and downslope by using the transformation relationship between macro- and micro-variables. The perturbation propagation characteristics and stability conditions of the macroscopic continuum equation are discussed. For uniform flow in the initial equilibrium state, the stability conditions reveal that as the slope angle increased under the action of a small disturbance, the upslope stability increases and downslope stability decreases. Moreover, under a large disturbance, the global stability analysis is carried out by using the wavefront expansion technique for uniform flow in the initial equilibrium state. For the initial nonuniform flow, nonlinear bifurcation analysis such as Hopf bifurcation and saddle–node bifurcation is carried out at the equilibrium point. Subcritical Hopf bifurcation exists when the traffic flow state changes; thus, the limit cycle formed by the Hopf bifurcation is unstable. And the existence condition of saddle-node bifurcation is obtained. Simulation results verify the stability conditions of the model and determine the critical density range. The numerical simulation results show the existence of Hopf bifurcation in phase space, and the spiral saddle point of saddle-node bifurcation varies with the slope angle. Furthermore, the impact of the angle of both the upslope and downslope on the evolution of density waves is investigated.

Macroscopic traffic flow in multiple-loop networks

We present a network model for traffic flow on multiple-loop lines with a single intersection. We investigate the macroscopic traffic flow and fundamental diagram in the multiple-loop network. We propose a graph representation for traffic flow on multiple-loop lines with an intersection. The dynamic equations of vehicular densities on multiple-loop networks are derived by using the graph representation. Vehicular densities approach stable equilibrium values after long times irrespective of initial densities. The equilibrium densities and currents (flows) are obtained analytically for traffic flow on double-loop lines. Also, we obtain the equilibrium densities and currents (flows) on multiple-loop networks numerically. We show how the fundamental diagram depends on the fraction of loop-changing vehicles.

The impact of bimodal pore size distribution and wettability on relative permeability and capillary pressure in a microporous limestone with uncertainty quantification

Pore-scale X-ray imaging combined with a steady-state flow experiment was used to study the displacement processes during waterflooding in an altered-wettability carbonate, Ketton limestone, with more than two orders of magnitude difference in pore size between macropores and microporosity. We simultaneously characterized macroscopic and local multiphase flow parameters, including relative permeability, capillary pressure, wettability, and fluid occupancy in pores and throats. An accurate method was applied for porosity and fluid saturation measurements using greyscale based differential imaging without image segmentation. The relative permeability values were corrected by considering the measured saturation profile along the sample length to account for the so-called capillary end effect. The behaviour of relative permeability and capillary pressure was compared to other measurements in the literature to demonstrate the combined effects of wettability and pore structure. Typical oil-wet behaviour in resolvable macropores was measured from contact angle, fluid occupancy and curvature. The capillary pressure was negative while the oil relative permeability dropped quickly as oil was drained to low saturation and flowed through connected oil layers. Brine initially largely flowed through water-wet microporosity, and then filled the centre of large oil-wet pore bodies. Thus, the brine relative permeability remained exceptionally low until brine formed a connected flow path in the macropores leading to a substantial increase in relative permeability. Overall, this work demonstrates that not only wettability but also pore size distribution and microporosity have significant impact on displacement processes.

New Model of Oil Migration in Shale Nanopores Considering Microscopic Deformation Induced by Stress and Pore Pressure

During the exploitation of shale oil, the stress and pore pressure of the reservoir generally change, resulting in the deformation of the shale pore structure, which affects the oil migration in shale nanopores. Establishing an oil migration model and a permeability model that take into account the effects of reservoir stress and pore pressure is important for the numerical simulation shale oil development. In this paper, a new oil migration model in inorganic (IM) and organic (OM) nanopores of shale and a new shale apparent liquid permeability (ALP) model that consider the effects of reservoir stress and pore pressure are established. The molecular dynamics simulation data and experimental data are used to verify the validity of the proposed model. The results show that the model can reasonably describe the transport process of oil in IM and OM nanopores and calculate the ALP. The flow enhancement factor in inorganic and organic nanopores and the shale ALP are negatively correlated with the mean compressive stress and positively correlated with the pore pressure. When the shale bulk modulus is small, the flow enhancement factor and ALP are more sensitive to the stress and pore pressure. Moreover, the effects of stress or pore pressure on shale microscopic flow capacity which was evaluated by the flow enhancement factor and macroscopic flow capacity which was evaluated by ALP are consistent.