Flow Evolution Process Research Articles

Study on the flow pattern and evolution process of an effervescent atomizer

Study on the flow pattern and evolution process of an effervescent atomizer

Coupled fluid flow, solute transport and dissolution processes in discrete fracture networks: An advanced Discontinuous Galerkin model

Modeling dissolution processes in discrete fracture networks (DFNs) is a challenging task. Challenges are related to the highly nonlinear coupling between flow, mass transport, and reactive processes associated with fracture aperture evolution by dissolution. Further, advection-dominated transport due to fast fluid flow in fractures renders the problem more complex from a computational point of view, as traditional numerical methods may introduce unphysical oscillations or excessive numerical diffusion. The Discontinuous Galerkin (DG) method is known to be suitable for the simulation of advection-dominated transport. In this work, an advanced DG model is developed to model transport with dissolution in DFNs. We propose an upwind formulation to deal with the upstream concentration at the intersection of several fractures. The upstream concentration at an intersection node is calculated based on the average nodal concentrations of all the fractures having an inflow at that node, weighted by the volumetric fluxes of these fractures. The dispersion term is discretized with the Mixed Finite Element (MFE) method, which ensures the continuity of the dispersive flux at the intersection of fractures with different apertures. The obtained nonlinear coupled flow-transport-dissolution equations are discretized in time with a high-order scheme via the method of lines (MOL). Numerical examples and comparisons with standard finite element (FE) and finite volume (FV) solutions are performed to investigate the correctness and efficiency of the developed model. Results show that the new DG-DFN model avoids unphysical oscillations encountered with the standard FE method and strongly reduces the numerical diffusion observed with the upwind FV scheme. The DG-DFN model is then used to investigate the effect of the dissolution rate on the flow, transport, and aperture evolution processes for a single fracture and for a DFN. A quasi-linear evolution of the fracture aperture is observed for low dissolution rates. For high dissolution rates, a funnel-shaped enlargement is observed with a significant widening for the fractures near the inlet and minor effects for those away from the injection location.

Effect of crossflow oscillation Strouhal number on circular cylinder wake

The wake flow features and aerodynamic characteristics of a circular cylinder in an oscillating crossflow were experimentally investigated. The study focused on crossflow oscillation Strouhal numbers that were smaller than the natural wake vortex-shedding Strouhal numbers. Crossflow oscillations were generated using a downstream rotating plate method. The flow evolution processes were observed using laser-light sheet assisted smoke flow visualization technique. Wake instability and time-averaged velocities were assessed using a one-component hot-wire anemometer and particle image velocimetry. Pressure distributions on the cylinder's surface were quantified using a linear pressure scanner, obtaining pressure coefficient distributions and drag forces. The wake instability Strouhal number, velocity vectors, streamline patterns, and recirculation bubble geometries in the wake region of the circular cylinder were determined. The wake turbulence properties were analyzed using the triple-decomposition method, including turbulence intensities and Lagrangian integral length and time scales. The results of natural and oscillating crossflows were compared. The wake vortex-shedding Strouhal number was lower than the natural Strouhal number but higher than the crossflow oscillation Strouhal number. It was primarily influenced by the oscillation Strouhal number and the Reynolds number of the crossflow. Critical crossflow Reynolds and oscillation Strouhal numbers were identified, beyond which the wake vortex-shedding Strouhal number reached a constant value. The crossflow oscillation intensity did not significantly affect the wake vortex-shedding behavior. The study provided quantitative descriptions and discussions of recirculation bubble geometries and statistical turbulence properties. Furthermore, the crossflow oscillations led to a substantial reduction in the drag coefficient experienced by the circular cylinder.

Refined composite multivariate multiscale complexity-entropy causality plane analysis for gas-liquid two-phase flow

Abstract This paper presents a refined composite multivariate multiscale complexity-entropy causality plane (RCMMCECP) to explore the dynamics features of gas–liquid two-phase flow. Firstly, we employ a series of typical nonlinear time series to confirm the effectiveness of the RCMMCECP, including seven chaotic systems, two random processes, and one periodic process. The comparison results of the proposed method and conventional multivariate multiscale complexity-entropy causality plane (MMCECP) confirm the stability performance of the proposed RCMMCECP. Above all, the RCMMCECP enhances the reliability of the statistical complexity measure over large time scales and exhibits good continuity and noise-resistant ability in multiscale analysis. Then, we employ the RCMMCECP to analyze the upstream and downstream conductance signals. The experimental results demonstrate that the RCMMCECP can characterize the change of complexity and structural stability in the gas-liquid two-phase flow evolution process, effectively revealing its dynamics features.

Modeling and simulation of traffic flow based on memory effect and driver characteristics

In this paper, an improvement of the full velocity difference model is presented, which comprehensively considers driver characteristics which include aggressive and timid behaviors and memory effect. The stability condition of the improved model is obtained by making use of the small perturbation method. The Burgers, Korteweg–de Vries and modified Korteweg–de Vries equations are derived by means of the reductive perturbation method. Finally, through numerical simulation, the traffic flow evolution process of the improved model under two typical conditions is observed. The first is the starting process, and the second is the propagation process of disturbance in traffic flow. The results of numerical simulation and theoretical analysis demonstrate that the multiple driver characteristics considered in the improved model have important influences on traffic flow stability and the ease of traffic jams, and the model can quickly recover to its original state in case of the disturbance. • A new car following model is presented, which considers memory effect and driver characteristics including aggressive and timid behaviors. • The stability conditions of the extended model are obtained with the use of the small perturbation method. • The Burgers, Korteweg–de Vries and modified Korteweg–de Vries equations are derived by means of the reduced perturbation method. • The starting process and the propagation process of disturbance are carried out, the results are much closer to actual traffic.

Transient characteristics during start-up of dual pumps in the prefabricated pumping station

Abstract This work presents a numerical analysis of transient flow fields in the prefabricated pumping station with two identical pumps to clarify the flow evolution process during the start-up using the computational fluid dynamics (CFD) technique. The disturbance between neighboring components and external characteristics of the pump were analyzed and compared. The results demonstrate that the increase of pump head can be divided into two stages: lagging behind the increase of impeller speed and rapid increase of pump head. There is apparent interference between the two pumps, which not only affects the internal flow field distribution of the prefabricated pump station but also severely impacts the pump's performance. The transient impact during the pump start-up threatens the safety of the components of the prefabricated pump station, and the interaction between the pumps causes the pump to deviate from the design working condition. The investigation of results of the complex flow in the prefabricated pumping station during start-up is of great reference to improving the design of the prefabricated pumping station.

An improved car-following model considering the preceding vehicle’s velocity feedback and desired inter-vehicle distance

In this paper, an improved car-following model is raised by taking the preceding vehicle’s velocity feedback and desired inter-vehicle distance into account. The asymptotic and local stability criteria are obtained via control theory method to analyze the effect of the preceding vehicle’s velocity feedback and desired inter-vehicle distance on dynamic characteristics of traffic flow. In addition, numerical simulations are constructed on vehicle starting and traffic flow evolution processes, which show that the proposed car-following model can better present the characteristics of traffic flow. The preceding vehicle’s velocity feedback and desired inter-vehicle distance play a vital role on vehicle starting process and the stability of traffic flow.

Experimental Study on Interaction between Nanosecond Pulsed Laser and Normal Shock Wave

Nanosecond pulsed lasers possess two remarkable advantages: a high peak power density and the ability to break down air to form plasma readily. Therefore, they have significant practical value in the drag reduction of a supersonic body. An experimental investigation is conducted on the fundamental physical phenomenon of the interaction of the pulsed laser plasma with a normal shock wave to reveal the mechanism of drag reduction. Moreover, a high-precision schlieren system is developed to measure complex wave structures with a time resolution of up to 30 ns and a spatial resolution up to 1 mm. A high-speed particle image velocimetry system is set up to measure the velocity and vorticity of the flow field quantitatively; the system has a time resolution of up to 500 ns. The characteristics of the spherical shock wave and the high-temperature and low-density region induced by the laser plasma are presented. The flow characteristics and evolution process of the laser plasma under a normal shock wave are substantially revealed. The cause of the supersonic drag reduction by the pulsed laser plasma is illustrated with numerical simulation results. The following results are obtained in this study: the initial Mach number of the shock wave induced by the laser plasma increases with the laser energy, and the shape of the wave gradually evolves from a droplet shape to a spherical shape. The propagation velocity decreases with time and is close to the sound velocity after 50 μs. The shape of the initial high-temperature and low-density region is approximately spherical; it subsequently destabilizes to form a sharp spike structure in the laser’s incident direction. Ultimately, the region evolves into a double-vortex ring structure with upper and lower symmetry; the size of this region increases with the laser energy.

Dynamic Assessment of Road Network Vulnerability Based on Cell Transmission Model

The road network maintaining stability is critical for guaranteeing urban traffic function. Therefore, the vulnerable links need to be identified accurately. Previous vulnerability research under static condition compared the operating states of the old equilibrium before the event and the new equilibrium after the event to assess vulnerability ignoring the dynamic variation process. Does road network vulnerability change over time? This paper combines the vulnerability assessment with the traffic flow evolution process, exploring the road network vulnerability evaluation from the perspective of time dimension. More accurate identification and evaluation of vulnerable nodes and links can help to strengthen the ability of road network resisting disturbances. A modified dynamic traffic assignment (DTA) model is established for dynamic path selection (reselect the shortest path at the end of each link) based on the dynamic user optimal (DUO) principle. A modified cell transmission model is established to simulate the traffic flow evolution processes. The cumulative and time-varying index of vulnerability assessment is established from the viewpoint of traveler’s time loss. Then the road network vulnerability assessment combined the traffic flow model with the vulnerability index. The road network vulnerability assessment of Bao’an Central District of Shenzhen, China, reveals that road network vulnerability does contain a dynamic process, and vulnerable links in each phase can be exactly identified by the model. Results showed that the road network would have a large vulnerability during the disordered phase when the main road fails. Therefore, prioritizing the smooth flow of main roads can weaken the impact of road network vulnerability exposure.

Numerical and Experimental Studies of an Oil Slick Recovery Method That Uses a Free Surface Vortex.

To study the flow characteristics of water and oil in a free surface vortex with an oil slick on the water surface, the flow phenomenon was simulated using FLUENT software and compared with the experimental phenomenon. The volume of the fluid model was used to obtain the oil–gas–water three-phase eddy current field, yielding the flow structure and evolution process of the free surface vortex. The results reveal that the oil and water distribution follows a specific rule, from the beginning of the vortex at the free surface, through continuous downward extension and finally reaching stability. A few other parameters were also calculated, including the vertical distribution of the vortex core radius, the maximum tangential velocity and the radial velocity at the vortex core radius, and the variation of the velocity components of each phase in the flow field with position and time. The research reveals the oil transportation characteristics of free surface vortices and provides a method for recovering an oil slick using its surface vortex characteristics.

Effects of pulsation intensity on the flow and dispersion of pulsed dual plane jets

The effects of jet pulsation intensity on the flow and dispersion characteristics of dual parallel plane jets were experimentally studied. The jet pulsations were generated by acoustic excitations. The laser-light-sheet-assisted smoke flow visualization method was used to study the flow evolution processes. The lateral spreading characteristics of the jets were measured using the binary edge detection method. A hotwire anemometer was used to measure the velocity pulsations, mean velocities, turbulence intensities, and Lagrangian turbulent length and time scales. The dispersion characteristics of the jet fluids were measured by the tracer-gas concentration detection method. The jet Reynolds number and excitation Strouhal number were fixed at 600 and 0.106, respectively. The jet pulsation intensities were varied from 0 to 1.6. As the jets were pulsed, the turbulence intensity, lateral spread width, and jet fluid dispersion capability markedly increased compared with those of the non-pulsed jets. The larger the pulsation intensity was, the higher the turbulence intensity, lateral spread width, and jet fluid dispersion. The Lagrangian time and length scales of the turbulent eddies decreased as the jet pulsation intensity increased because the mushroom-shaped structures generated by the jet pulsations broke up into small turbulent eddies caused by the strong vortex-stretching effect.

Flow behaviours and velocity fields of non-oscillating and transversely oscillating jets

The near-field flow characteristics and jet fluid dispersion properties of non-oscillating and transversely oscillating jets issued from a fluidic oscillator were studied experimentally. A V-shaped fluidic oscillator was designed to induce transverse jet oscillations by merging two bifurcated alternatively issuing side jets. The characteristic flow patterns, evolution processes, velocities, vorticities, turbulence intensities and jet fluid dispersion capabilities in the median plane of the jets were examined. The characteristic flow patterns were obtained using laser-light-sheet-assisted smoke flow visualization method. The flow velocities were measured using particle image velocimetry. The jet fluid dispersion properties were analysed using tracer gas detection method. The non-oscillating jet was axial momentum-dominated—the axial momentum of the jet was limited within a narrow region around the central axis and was significantly larger than the transverse momentum. Therefore, the transverse velocities and vorticities were small and the dispersion of the jet fluid was relatively low. The transversely oscillating jets meandered in the near-field so the flow patterns presented a violent breakup of vortical flow structures and large transverse widths. The streamlines expanded transversely outwards and caused a part of the axial momentum to transfer towards the transverse direction. The vorticities and transverse velocities were increased and the axial velocities were decreased, so the dispersion of jet fluid was augmented in the transversely oscillating jets.

Effects of Reynolds number on flow and mixing characteristics of a self-sustained swinging jet

The effects of Reynolds number on flow and mixing characteristics of a merged single swinging jet induced by a V-shaped fluidic oscillator were studied experimentally. The jet Reynolds number was varied from 80 to 3000. A high-speed digital camera was used to capture the instantaneous flow evolution processes using the laser light-sheet-assisted flow visualisation method. Long-exposure images together with the binary edge detection technique were used to identify the jet spread width. The velocities, turbulence intensities, as well as turbulence macro time and length scales in the central axis were measured using a one-component hot-wire anemometer. Measurement results using the tracer-gas concentration detection method provided information on the dispersion characteristics of the merged swinging and non-swinging jets. Four characteristic flow modes (non-swinging, subcritical swinging, critical swinging, and supercritical swinging jets) were identified within different ranges of jet Reynolds number. At Reynolds numbers <150 ± 10, no jet swinging happened. At Reynolds numbers between 150 ± 10 and 1200 ± 23, the instantaneous velocities of the merged jet presented periodic oscillations of small amplitudes and was denoted as the subcritical swinging jet. At Reynolds numbers between 1200 ± 23 and 2350 ± 50, the merged jet proceeded irregular swinging motion and was called the critical jet. At Reynolds numbers higher than 2350 ± 50, the time-evolving velocities of the merged jet presented periodic oscillations of large amplitudes imposed by large turbulent fluctuations and was termed the supercritical swinging jet. The measured results of the axial velocities and turbulence intensities in the central axis revealed that in the regime of the supercritical swinging jet, the momentum conversion from the axial to transverse direction was rapidly performed in the near field at the axial distance <2.4 times of the jet exit width due to the induced transverse jet oscillation. The transverse oscillations of the swinging jets induced large turbulence intensities and small turbulence macro time and length scales, and led to large jet mass dispersion. Therefore, the mixing characteristics of the flow was significantly improved due to large jet mass dispersion.

Analysis of regional river blocking by debris flows in response to climate change

The river-blocking effects of debris flows have become common in numerous catchments in response to climate and environmental changes, and these effects have caused multiple, overlapping, and interconnected chain reactions that have led to huge losses in alpine regions. Considering this issue, this article developed a quantitative method for the regional river-blocking hazard assessment of debris flows by analyzing the in-depth relations among river-blocking hazard formation processes, factors and evolution mechanisms. Taking the debris flows in the Parlung Zangbo Basin in China as a case study, a multidimensional analysis was performed to analyze the characteristics of the hazard sequence and its relationship with climate change, including changes in temperature and precipitation. Accordingly, a new step toward a more comprehensive hazard assessment is proposed by establishing both a model and a system for regional river-blocking hazard assessment to analyze the debris flow evolution processes and environmental dynamics. Specifically, the sources of loose material were quantitatively estimated by establishing mathematical models based on the geometrical characteristics of diverse moraines, and the debris flow runoff was scientifically determined by focusing on the analysis of moraine sources, the brittleness index of the sediment mass and the geomorphological connectivity. Next, through coupling with the hydrodynamic conditions of debris flows and river flows, methods were established to determine the blocking degree of debris flow hazards at the regional scale. Validated by a field study and a remote-sensing interpretation of actual debris flows, a debris-flow-induced river-blocking hazard map was obtained, and the assessment results were in accordance with the actual disaster situation. The analysis shows that the distribution of zones with high to very high levels of river blocking is closely correlated with the topographic characteristics and actual disaster sequences of debris flows. These findings suggest that the assessment results provide scientific support for engineering planning and hazard prevention in climate-sensitive areas; thus, the presented method may serve as pertinent guidance for regional river-blocking hazard assessments of debris flows in the Parlung Zangbo Basin and beyond.

A Traffic Flow Evolution Process toward Mixed Equilibrium with Multicriteria of Route Choice Behaviour

Based on the price-quantity adjustment behaviour principle of disequilibrium theory, the route choices of travellers are also affected by a quantity signal known as traffic flow, while the route cost is considered as a price signal in economics. Considering the quantity signal’s effect among travellers, a new route comfort choice behaviour criterion and its corresponding equilibrium condition are established. The network travellers are classified into three groups according to their route choice behaviour: travellers in the first group choose the shortest route following the route rapidity behaviour criterion with complete information forming the UE (user equilibrium) pattern, travellers in the second group choose the most comfortable route following the route comfort behaviour criterion with complete information forming the QUE (quantity adjustment user equilibrium) pattern, and travellers in the third group choose a route according to their perceived travel time with incomplete information forming the SUE (stochastic user equilibrium) pattern. The traffic flows of all three groups converge to a new UE-QUE-SUE mixed equilibrium flow pattern after interaction. To depict the traveller-diversified choice behaviour and the traffic flow interaction process, a mixed equilibrium traffic flow evolution model is formulated. After defining the route comfort indicator and the corresponding user equilibrium state, the equilibrium conditions of the three group flows are given under a mixed equilibrium pattern. In addition, an equivalent mathematical programming of the mixed equilibrium traffic flow evolution model is proposed to demonstrate that the developed model converges to the mixed equilibrium state. Finally, numerical examples are examined to evaluate the effect of route comfort proportions on the traffic network flow evolution and analyse the performance of the proposed model.

A discrete day-to-day link flow dynamic model considering travelers’ heterogeneous inertia patterns

ABSTRACT This study investigates the impact of heterogeneous psychological inertia of travelers on link flow evolution process on a day-to-day basis. The psychological inertia of a traveler is defined as his/her reluctance to reconsider his/her route choice, and is characterized by a sequence of binary parameters along time. Travelers are grouped into different classes by their inertia patterns. Based on these classes, a variational inequality formulation for the multi-class user equilibrium problem is presented. We develop a generic day-to-day link flow dynamic model by considering heterogeneous inertia patterns of the travelers. The convergence properties of the model are rigorously demonstrated. The developed model is formulated in a general form under mild assumptions. We further consider a special case of the generic model. We examine properties of this special case model, and investigate its relationship with existing models in the literature. Numerical experiments are conducted to demonstrate our theoretical results.

A Markov Traffic Model for Signalized Traffic Networks Based on Bayesian Estimation

In order to better understand the stochastic dynamic features of signalized traffic networks, we propose a Markov traffic model to simulate the dynamics of traffic link flow density for signalized urban traffic networks with demand uncertainty. In this model, we have four different state modes for the link according to different congestion levels of the link. Each link can only be in one of the four link state modes at any time, and the transition probability from one state to the other state is estimated by Bayesian estimation based on the distributions of the dynamic traffic flow densities, and the posterior probabilities. Therefore, we use a first-order Markov Chain Model to describe the dynamics of the traffic flow evolution process. We illustrate our approach for a small traffic network. Compared with the data from the microscopic traffic simulator SUMO, the proposed model can estimate the link traffic densities accurately and can give a reliable estimation of the uncertainties in the dynamic process of signalized traffic networks.

Thermal damage analysis of aircraft composite laminate suffered from lightning swept stroke and arc propagation

Lightning strike is a complicated process involving multi-field coupling. In order to investigate the thermal damage mechanism of carbon fiber reinforced composites subject to lightning swept stroke, a complete numerical method is presented. Numerical model of lightning discharge is established based on Magneto Hydro Dynamics (MHD) and calculated by FLUENT secondary development technology. Considering aerodynamic flow effect, channel formation and evolution process during lightning discharge is analyzed for lightning current waveform A. Thermal-electric Coupling model is presented according to Radial Basis Function (RBF) interpolation theory, which is implemented by compiling program to make lightning current and heat energy inject into composite laminate. Consequently, damage mechanism of composite laminate under lightning swept stroke is studied based on the coupled numerical model and element deletion method. Ablation damage morphology of composite laminate is analyzed to understand plasma expansion and reattachment in arc root. The results show that aerodynamic flow makes the lightning channel move fast and composite laminate is deteriorated due to thermal damage.

Distance-Based Congestion Pricing with Day-to-Day Dynamic Traffic Flow Evolution Process

This paper studies the distance-based congestion pricing in a network considering the day-to-day dynamic traffic flow evolution process. It is well known that, after an implementation or adjustment of a new congestion toll scheme, the network environment will change and traffic flows will be nonequilibrium in the following days; thus it is not suitable to take the equilibrium-based indexes as the objective of the congestion toll. In the context of nonequilibrium state, prior research proposed a mini–max regret model to solve the distance-based congestion pricing problem in a network considering day-to-day dynamics. However, it is computationally demanding due to the calculation of minimal total travel cost for each day among the whole planning horizon. Therefore, in order to overcome the expensive computational burden problem and make the robust toll scheme more practical, we propose a new robust optimization model in this paper. The essence of this model, which is an extension of our prior work, is to optimize the worst condition among the whole planning period and ameliorate severe traffic congestions in some bad days. Firstly, a piecewise linear function is adopted to formulate the nonlinear distance toll, which can be encapsulated to a day-to-day dynamics context. A very clear and concise model named logit-type Markov adaptive learning model is then proposed to depict commuters’ day-to-day route choice behaviors. Finally, a robust optimization model which minimizes the maximum total travel cost among the whole planning horizon is formulated and a modified artificial bee colony algorithm is developed for the robust optimization model.

Multiple patterns of heat and mass flow induced by the competition of forced longitudinal ventilation and stack effect in sloping tunnels

The understanding of heat and mass transfer phenomena and underlying flow and thermal mechanisms in a tunnel fire is critical for smoke control and human evacuation. Critical velocity has been acknowledged as the most critical parameter to guarantee the effectiveness of smoke control using longitudinal ventilation in tunnels. However, in slopping tunnels, even under the conditions where the expected smoke flow route and the corresponding critical velocity can be achieved, another flow pattern which has an opposite smoke flow direction and higher gas temperatures is also likely to be generated. This phenomenon has not attracted enough attention. In this paper, multiple patterns of heat and mass flow in a slopping tunnel induced by the competition of buoyancy and forced ventilation were examined using laboratory experiments, Computational Fluid Dynamics simulations and theoretical analysis. Both dynamic and steady state analyses were performed to explain the transient evolution behaviors and final steady solutions for these flow patterns. The constraints and the prerequisite conditions for the existence of multiple flow patterns or only a certain pattern were expressed in terms of three main influencing parameters including fire heat release rate, pressure rise induced by mechanical ventilation system and the fire source location. For the regime where both fan-dominated and buoyancy-dominated patterns can occur, the initial flow state and the flow evolution process determines the steady state solutions, indicating the activation time of the mechanical ventilation plays an important role in the final flow pattern. This study has particular relevance to understanding the complex heat and mass flow behaviors in a sloping tunnel fire and it provides new implications for smoke control.