Merging Flows Research Articles

Cloud–Cloud Collision: Formation of Hub-filament Systems and Associated Gas Kinematics. Mass-collecting Cone—A New Signature of Cloud–Cloud Collision

Massive star-forming regions (MSFRs) are commonly associated with hub-filament systems (HFSs) and sites of cloud–cloud collision (CCC). Recent observational studies of some MSFRs suggest a possible connection between CCC and the formation of HFSs. To understand this connection, we analyzed the magnetohydrodynamic simulation data from Inoue et al. This simulation involves the collision of a spherical turbulent molecular cloud with a plane-parallel sea of dense molecular gas at a relative velocity of about 10 km s−1. Following the collision, the turbulent and nonuniform cloud undergoes shock compression, rapidly developing filamentary structures within the compressed layer. We found that CCC can lead to the formation of HFSs, which is the combined effect of turbulence, shock compression, magnetic field, and gravity. The collision between the cloud components shapes the filaments into a cone and drives inward flows among them. These inward flows merge at the vertex of the cone, rapidly accumulating high-density gas, which can lead to the formation of massive star(s). The cone acts as a mass-collecting machine, involving a nongravitational early process of filament formation, followed by gravitational gas attraction to finalize the HFS. The gas distribution in the position–velocity (PV) and position–position spaces highlights the challenges in detecting two cloud components and confirming their complementary distribution if the colliding clouds have a large size difference. However, such CCC events can be confirmed by the PV diagrams presenting gas flow toward the vertex of the cone, which hosts gravitationally collapsing high-density objects, and by the magnetic field morphology curved toward the direction of the collision.

ДОЦІЛЬНІСТЬ СТВОРЕННЯ ЕЛЕКТРОННОГО ОБЛІКУ ДОРОЖНІХ ЗНАКІВ

Introduction. Road signs form the framework of the driver's decision-making system in the process of driving a vehicle, in places of merging, separation and intersection of traffic and pedestrian flows. One of the most important characteristics of road signs is their brightness and ability to retroreflectivity at night. Signs should have clear feedback. The more fully and clearly the driver is informed about the conditions and the required mode of movement, the more accurate and error-free his actions are. Problem statement. The absence of accounting of all installed signs on the roads, signs that need to be replaced, signs that are at the end of their service life, as well as signs that meet or do not meet the requirements of regulatory documents in terms of retroreflective qualities. Objective. To analyze the software for electronic accounting of road signs, to determine the need and feasibility of developing and installing such software in road facilities. Materials and methods. Theoretical and empirical methods of scientific research are used to analyze the software for electronic accounting of road signs. Results. The analysis of foreign sources on the available software for electronic accounting of road signs was carried out.

MixMatch: Flow Matching for Mixnet Traffic

Mixnets provide communication anonymity against network adversaries by routing packets independently via multiple hops, delaying them artificially at each hop, and introducing cover traffic. We show that these features (particularly the use of cover traffic) significantly diminish the effectiveness of state-of-the-art flow correlation techniques developed to link the two ends of a Tor connection. In this work, we propose novel methods to determine whether a set of endpoints exchanges packets via a mixnet and demonstrate their effectiveness by applying them to the Nym mixnet. We consider Nym in both an idealized lab setup and the official live network, and propose and compare three classifiers to conduct flow matching on it. Our statistical classifier tests whether egress packet timestamps are consistent with ingress timestamps and the (known) routing delay characteristic of the mixnet. In contrast, our two deep learning (DL) classifiers learn to distinguish matched from unmatched flow pairs from collected datasets directly, rather than relying on priors that describe the delay distribution. All three classifiers use our flow merging technique, which enables testing a match for sets of communicating endpoints of any cardinality. Considering a use case where two observed endpoints communicate exclusively to exchange a file through Nym, we find that flow matching is fast and accurate in the idealized lab setup. If flow pairs are aligned using all network observations in a download, we achieve a TPR of circa 0.6 (DL) and 0.47 (statistical) at an FPR of 10^-2 after only processing 100 observations. We evaluate classifier performance under key variations of this setup: the absence of loop cover traffic, an increased or decreased average per-mix delay, larger communicating sets (three endpoints) with faster responders, and the presence of realistic network effects (live network). The classifiers' matching performance diminishes on the live network where packet losses and variable propagation delays exist, reducing DL TPR to circa 0.26 and statistical TPR to circa 0.28 at an FPR of 10^-2. Informed by the insights of our analyses, we outline countermeasures that can be deployed in mixnets such as Nym to mitigate flow matching threats.

Elasto-inertial instabilities in the merging flow of viscoelastic fluids.

Many engineering and natural phenomena involve the merging of two fluid streams through a T-junction. Previous studies of such merging flows have been focused primarily upon Newtonian fluids. We observed in our recent experiment with five different polymer solutions a direct change from an undisturbed to either a steady vortical or unsteady three-dimensional flow at the T-junction with increasing inertia. The transition state(s) in between these two types of merging flow patterns is, however, yet to be known. We present here a systematic experimental study of the merging flow of polyethylene oxide (PEO) solutions with varying polymer concentrations and molecular weights. Two new paths of flow development are identified with the increase of Reynolds number: one is the transition in very weakly viscoelastic fluids first to steady vortical flow and then to a juxtaposition state with an unsteady elastic eddy zone in the middle and a steady inertial vortex on each side, and the other is the transition in weakly viscoelastic fluids first to a steady vortical and/or a juxtaposition state and then to a fully unsteady flow. Interestingly, the threshold Reynolds number for the onset of elastic instabilities in the merging flow is not a monotonic function of the elasticity number, but instead follows a power-law dependence on the polymer concentration relative to its overlap value. Such a dependence turns out qualitatively consistent with the prediction of the McKinley-Pakdel criterion.

Simulation modeling in infrastructure modernization projects of the Russian part of the Central Eurasian Corridor

The article presents a brief characteristic of international transport corridors running through Russia. Their key nodes and the need for their sustainable development are identified. The use of structurization, analysis and synthesis methods allowed the study to obtain quantitative and qualitative characteristics of transport infrastructure facilities of Russian cities, through which the railway branches of the Central Eurasian Corridor pass. The rules of interaction between the key elements that make up the structure of each of the research objects have been established. A set of such rules and object characteristics became the input parameters for the simulation model of the problem domain. The problem area includes all bottlenecks in the transportation network formed by the intersection and merging of traffic flows. State diagrams illustrating the peculiarities of the behavior of object groups of the simulation model on the example of the city of Orenburg are developed.

EXPERIMENTAL AND THEORETICAL CHARACTERIZATION OF TWO-PHASE FLOW DISTRIBUTION IN UNBALANCED FLOW NETWORKS

This study investigates the two-phase flow distribution of R-1234yf into two-branched channels. The effect of unbalanced pressure drop and tube diameter on the flow distribution was especially characterized and modeled. The experiments were conducted for a mass flux ranging from 393 kg/m<sup>2</sup>s to 1179 kg/ m<sup>2</sup>s, and the inlet quality was fixed at 0.2. The flow distribution was mainly governed by the pressure drop ratio between two branched channels. The mal-distribution under the unbalanced pressure drop condition was mitigated by increasing mass flux, whereas sustained mal-distribution was observed under unbalanced tube diameter conditions. Correspondingly, a theoretical representation of the phenomenon based on the principle of minimum entropy production was developed and adopted to predict two-phase flow distribution in an unbalanced flow network. The characteristics of the experimental data were clearly and quantitatively reflected in the prediction results. Under the unbalanced pressure drop condition, the predictions agreed well with the experimental data, maintaining the maximum deviation within ± 30%, whereas it exceeded ± 30% under the unbalanced tube diameter condition. The analysis of such theoretical formulation suggests the necessity of appropriate pressure drop models of flow impact, contraction, and merging at the outlet channel, which are compatible with the extremization of entropy production for further improving the prediction accuracy without compromising its generality.

Image-Based Feedback of Multi-Component Microdroplets for Ultra-Monodispersed Library Preparation.

Using devices with microfluidic channels can allow for precise control over liquids flowing through them. Merging flows of immiscible liquids can create emulsions with highly monodispersed microdroplets within a carrier liquid, which are ideal for miniaturised reaction vessels which can be generated with a high throughput of tens of thousands of droplets per second. Control of the size and composition of these droplets is generally performed by controlling the pumping system pushing the liquids into the device; however, this is an indirect manipulation and inadequate if absolute precision is required in the size or composition of the droplets. In this work, we extend the previous development of image-based closed-loop feedback control over microdroplet generation to allow for the control of not only the size of droplets but also the composition by merging two aqueous flows. The feedback allows direct control over the desired parameters of volume and ratio of the two components over a wide range of ratios and outperforms current techniques in terms of monodispersity in volume and composition. This technique is ideal for situations where precise control over droplets is critical, or where a library of droplets of different concentrations but the same volume is required.

Numerical simulation of thermal mixing of liquid sodium in a Y-junction

Numerical simulations are conducted to investigate the flow and heat transfer behaviour of liquid sodium in a Y-junction. Three thermal hydraulic parameters – the fluid momentum ratio, the temperature difference between the inlets of the two branches, and the inlet Reynolds number – are studied to understand their impact on flow development, mixing characteristics, and heat transfer in the main pipe. Results indicate that the momentum ratio is a key factor affecting flow development and temperature distribution. When the momentum ratio lies outside a certain range, flow impingement on the opposite wall occurs. If the ratio falls within this range, two jet flows merge into a turn/deflecting slab-shaped jet, accompanied by two pairs of counter-rotating vortices in the main pipe. The Reynolds number has a negligible impact on flow and thermal mixing patterns for the turbulent flows considered. However, lower Reynolds numbers result in stronger velocity variation and weaker temperature differences at the cross-section in the mixing area. Lastly, the inlet temperature difference has a very limited impact on flow and thermal mixing, and changes in physical property variables negligibly affect velocity and temperature distributions in flow and thermal mixing.

Density effects on streamwise-orientated vorticity at river confluences: a laboratory investigation

Small differences in the densities of a river confluence's tributaries (i.e. 0.5 kg m $^{-3}$ ) have been proposed to cause coherent streamwise-oriented vortices (SOVs) in its mixing interface. These secondary flow structures are thought to result from density-driven gravity currents being laterally confined between the converging flows. However, empirical evidence for density SOVs and the confined gravity current mechanism is lacking. To this end, experiments are carried out in a laboratory confluence permitting a spectrum of thermal density differences between its tributaries. Particle image velocimetry and laser-induced fluorescence are used simultaneously to study the mixing interface's dynamics. The sensitivity of the mixing interface's secondary flow structure to the confluence's momentum ratio and the magnitude of the density difference is evaluated. Density SOVs are confirmed in the mixing interface and are caused by the gravity currents being confined laterally as the opposing flows merge and accelerate downstream. The SOVs are largest and most coherent when the momentum of the dense channel is greater than that of the light channel. The dynamics of these secondary flow structures is strongly coupled to periodic vertically orientated Kelvin–Helmholtz instabilities. The striking similarities between the empirically reproduced SOVs herein and those recently observed at the Coaticook-Massawippi confluence (Quebec, Canada), despite a two-order magnitude difference in physical scale, suggest density SOVs are a scale-independent flow structure at confluences when specific, yet relatively common, hydraulic and density conditions align.

On the delineation of the flow separation zone in open-channel confluences

In the context of a T-shaped open-channel confluence, where the tributary flow separates at the downstream junction edge and creates a separation zone in the post-confluence channel, this paper provides a critical reflection upon the terminology and the delineation methods for the separation zone and its proxies: the recirculation zone and the reverse flow zone. Following the fluid-mechanical literature on flow separation, the separation zone is delineated by a dividing surface formed by the three-dimensional (3D) streamlines separating from the downstream junction edge. Based on a dense 3D time-averaged velocity field obtained from a Large-Eddy Simulation of a lab-style confluence flow, the separation zone turns out to be open. Near the bed, the dividing surface exhibits a gap, through which mainly tributary fluid enters the separation zone, leading to a limited discharge flowing through that zone. Downstream of the section where the merging flows are the most contracted, the dividing surface does not reattach to the inner bank, prohibiting the determination of a maximum length of the separation zone. Downstream of that section, the dividing surface gradually loses its role as a skeleton for the separation zone shear layer. Due to mixing, the separation zone and the adjacent maximum velocity zone gradually dissolve and transform into the flow recovery zone, as in a wake. By means of flow visualization with 3D streamlines, revealing different types of recirculating motions, the maximum recirculation zone extent is determined. Then three simplified methods commonly used in the literature to quantify the dimensions of the separation zone or its proxies are evaluated by comparison with the 3D streamline method. By adopting various simplifying assumptions about the flow, the pseudo-streamline method, the zero-unit-discharge method and the zero-velocity method yield definite reasonable maximum lengths for the recirculation zone and the reverse flow zone. With the exception of the zero-velocity method, all simplified methods yield reasonable estimates for the maximum widths and the corresponding contraction coefficients of the separation zone proxies. Still, contrary to the simplified methods, only the 3D streamline method can provide physical insight into the flow features occurring in the separation zone or its proxies.

Имитационно-стохастическое моделирование движения людских потоков при эвакуации людей из здания больницы

General data on fires in the Voronezh region are given. The general statistics on fires in medical institutions are considered. The fire safety assessment was carried out on the example of VGKBSMP No. 10. For this purpose, general information about the object of the study is provided. The analysis of the causes of fires in VGKBSMP No. 10 was carried out. The necessary initial data for calculating the time of evacuation of people from the hospital are given. The rules of evacuation from VGKBSMP No. 10 are presented. The existing life support services and their tasks in case of fire are considered. The peculiarities of evacuation from hospitals in the event of a fire of people with disabilities have been revealed. The regularities of the movement of human flows are considered. The diagrams of the movement of the flow of people at the borders of neighboring sections of evacuation routes are given, on which observers and people observed in the flow are indicated. The movement of the human flow across the border of adjacent sections of the path, as well as the features of the merging and separation of flows, is investigated. Possible options for merging human flows are considered. The simulation-stochastic modeling of the movement of human flows is considered. The results of calculating the time of evacuation of people from VGKBSMP No. 10 are given. The calculations were carried out using the Fogard program.

An optimal split ratio in design and control of a recompression supercritical CO2 Brayton system

The sCO2 Brayton cycle is considered an alternative to the conventional Brayton and Rankine cycles. The recompression Brayton cycle is advantageous because of its simple cycle layout and high cycle efficiency. The split ratio (SR) is defined as the recompression fraction of the working fluid that significantly affects the corresponding parameters of the system, particularly the cycle efficiency. However, studies focusing on the influence of SR on the design and off-design performance remain limited. The factors influencing the optimal SR and regulation mechanism under the design and off-design conditions of recompression cycle are considered in this study. The results demonstrate that the optimal SR under the design conditions is primarily affected by the high pressure, low pressure and efficiency of the recuperators. The optimal SR decreases with a reduction in the load demand, and the recompression loop can be cut off under a low load to improve the cycle efficiency. A simplified control can be achieved by controlling the temperature difference at the flow merge point to achieve a higher system efficiency. Moreover, the SR regulation mechanism is unaffected by the load control method. These findings are of significance to recompression configuration with different design parameters.

Cascade aircraft flows merging with multiple order changes

Nowadays, the problem of safe aircraft flows merging is considered usually as one-stage. However, real air-route schemes assume often a tree-like structure when primary flows are merged at some points and the resultant flows are merged further. In this case, the safety of merging should be controlled not only at the final point, but also at some intermediate ones. With that, aircraft can change their order between these merge points by means of usage of holding areas and/or path alignments. In this paper, the authors suggest a mixed integer linear programming formalization of such a problem. The suggested procedure is realized as a computational program with the optimization Gurobi library. Results of numerical simulations are presented.

Investigating mixing patterns of suspended sediment in a river confluence using high-resolution hyperspectral imagery

In a river confluence, the fluvial process is complex because of the merging of different flows from rivers with distinct morphologies. However, the mixing of suspended sediment in river confluences has been inadequately investigated owing to the low resolution of conventional techniques for measuring suspended sediment. In this study, we investigated the mixing of suspended sediment at the confluence of the Hwang and Nakdong Rivers in South Korea by analyzing the spectral characteristics of suspended sediment from high-resolution hyperspectral data. We retrieved the suspended sediment concentration (SSC) from high-resolution hyperspectral imagery using the conversion technique, which is a cluster-based machine learning regression with optical variability. Hyperspectral clustering was first applied to classify the water regions of the main river (Nakdong) and its tributary (Hwang). The clustering result at a near-field after the confluence point was strongly related to the mixing degree of flow and sediments from two different rivers. Through segmentation, regressors with hyperspectral clusters enabled accurate SSC measurement. In particular, they precisely retrieved the concentrations adjacent to the mixing layer, even when the relationship between the spectral data and suspended sediment concentrations of the main river and tributary was substantially different. Using a detailed SSC map, we compared two cases of mixing patterns by calculating the variation in sediment concentration along the downstream river confluence. We found that the mixing pattern was different because of the flow velocity and a large sandbar near the stagnant area even though river discharge was similar for both cases. In the slow mixing case, the sandbar caused a velocity-deficit with a wake that reduced the variance of SSC due to the irregular flow near the mixing layer. In this case, dual counter-rotating secondary flow cells also limited mixing across the post-confluence reach. However, when the wake and dual counter-rotating secondary flows weakened due to the erosion of the sandbar and lower flow velocity, the transverse mixing coefficient increased by approximately 40 % compared to the slow mixing case. This study demonstrated that the hyperspectral approach can be used to investigate the mixing of suspended sediment in greater detail in complex river systems, as it provides high-resolution information that conventional measurement methods cannot explore.

Integrated Real-Time Interchange Control System for Freeway Ramps and the Connecting Arterial

Contending with various recurrent congestion patterns on major freeway corridors has long been a prioritized task of the transportation community. Most of these patterns take place on a freeway interchange’s on-ramp and off-ramp segments where both exiting and merging flows often trigger a series of lane changes, resulting in significant freeway speed reduction and propagation of traffic queues to neighboring surface streets. To concurrently mitigate freeway traffic congestion and minimize any negative impacts on the nearby local arterials, this study has developed an Integrated Real-time Interchange Control (IRIC) system, which maximizes the benefits of both freeway and arterial users by fusing an integrated off-ramp signal control (IOSC) module, an off-ramp queue impact (OQI) model, a lane group-based (LGB) traffic model, and a ramp metering control strategy as well as arterial signal optimization modules. The proposed system with both ramp metering functions and local arterial signal design capabilities can determine whether to implement system-wide optimization or merely the ramp metering based on real-time detected traffic conditions. The evaluation results from experiments have confirmed the effectiveness of the developed real-time interchange control system, especially with respect to its ability of preventing ramp queue overflows to the neighboring arterials and minimizing the off-ramp spillback onto the freeway mainline.

Large eddy simulation of flow around two side-by-side circular cylinders at Reynolds number 3900

This paper investigates the flow dynamics around two circular cylinders in a side-by-side arrangement with different spacing ratios (T/D, T is the center-to-center cylinder spacing and D is the diameter) under a subcritical Reynolds number condition (Re = 3900). A three-dimensional (3D) numerical model was developed with Large Eddy Simulation (LES) technique. The model was well validated against published data of flow around a single cylinder at Re = 3900. Numerical simulations were conducted for flow around two side-by-side circular cylinders with T/D = 1.2, 1.5, 1.75, 2, 2.5, 3, 3.5, and 4. Based on the LES results, three wake regimes were identified: single bluff body regime (T/D = 1.2), biased flow regime (T/D = 1.5–2), and parallel vortex streets regime (T/D = 2.5–4). In the single bluff body regime with T/D = 1.2, the stable deflection of gap flow is also observed which indicates that there may exist a transition state from the single bluff body regime to the biased flow regime. In biased flow regime, the pairing and merging process of the outer vortices with the inner vortices are analyzed. The occurrence of the flip-flopping phenomenon is found to be related to the merging tendency between gap-side vortices in narrow wake region and free-flow-side vortices in wide wake region, and the relative phase of gap side vortices in transient state. In the parallel vortex streets regime, the phase relation of the vortex shedding process was analyzed. The time proportions of the in-phase mode and anti-phase mode are found to be varied with spacing ratio. As the spacing ratio increases, the wakes behind the cylinders lose their dependency on the anti-phase mode. The results of the present study were compared with the existing results at other Reynolds numbers. It is found that vortex shedding manner during the flip-over transitions is closely related to the spacing ratios and is independent of the Reynolds number.

Numerical Analysis of Merging Flows of Floc-Forming Fluids in a T-Junction Channel Using a Non-Newtonian Viscous Model Based on a Population

The flow of a floc-forming fluid in a T-junction channel was numerically analyzed using a non-Newtonian viscous model based on the population balance equation (PBE) of the floc size. The finite element method was applied to solve the velocity and pressure fields, and the PBE of the floc size distribution was computed to obtain the effective volume fraction of flocs. The Krieger-Dougherty model was used to evaluate the viscosity of suspension of flocs using the effective volume fraction. The numerical simulation captured the characteristic aggregation-breakage behavior of flocs in the flow. In the region where flows from two entrances merge, the effective volume fraction decreases because the breakage of flocs progresses. In the downstream channel, re-aggregation of flocs occurs near the channel centerline, and the effective volume fraction increases. This aggregation-breakage behavior of flocs was confirmed by the analysis of the floc size distribution.

Models of traffic and pedestrian flows for organization of smart traffic light traffic

The article defines the importance of organizing road traffic in urban transportation systems. The research targets are defined and all its key elements and characteristics are identified. The research target is a segment of the North-Western Chord of Moscow corresponding to Bolshaya Akademicheskay Street in the Koptevo District and limited by Mikhalkovskaya Street and 3rd Nizhnelikhoborskiy Proyezd. The research resulted in a model of a transport system graph for the area under study delineating traffic and pedestrian flows. The model takes into account the points of conflicts in crossing and merging of flows. The research findings became the basis for a mathematical model of traffic in problematic spots of the area under study. Conclusions are made as to the suitability of the findings and their use in the smart city system.

Расходные характеристики инжекционных водовыпусков-регуляторов

In cases where the supply of energy sources to hydraulic structures located on irrigation canals is economically unjustified, hydraulic automation means are widely used in Russia and abroad. The purpose of the research is to derive design dependencies to determine the flow rate of the injected and injected flows, taking into account hydraulic losses within the flow path of the structure, to construct the flow characteristic of the regulator with a nozzle and an output section of a constant area with its subsequent verification by the data of the hydraulic experiment. Injection flow regulators have several modifications: with a diffuser outlet section without restricting the outlet pipe with a nozzle, with a nozzle and diffuser, with a nozzle and an outlet section of a constant area. For regulators with a nozzle, dependences for hydraulic calculations have now been obtained, which can be considered conditionally “ideal”, since they do not take into account a number of local resistances and frictional resistances along the length that arise when flows merge with subsequent movement in the mixing chamber. As a result of the research, the authors for the first time obtained calculated dependencies for determining the flow rate of the injected and injected flows, taking into account hydraulic losses within the flow path of the structure, and constructed the flow characteristic of a regulator with a nozzle and an outlet section of a constant area. The flow characteristic is verified using available hydraulic experiment data. An analysis of the shape of the flow characteristics of the injection regulator was carried out by comparison with the generalized characteristics of hydraulic jet pumps. The practical significance of the research lies in the possibility of using the dimensionless characteristics of the injection water outlet to link it to specific operating conditions, as is customary when selecting pumps. To determine the flow rates in the regulator, a computer program has been registered.

Mathematical Correlation Study of Nanofluid Flow Merging Points in Entrance Regions

Here, hydrodynamic features of laminar forced nanofluid flow between two parallel plates are numerically investigated, and the results are mathematically discussed. The conventional understanding of developing flow in the entrance region of internal flows is based on the idea that boundary layers start forming at the inlet and merge at some point just before the fully developed section. However, because of the consideration of mass and flow conservation, the entire conception is required to be detailed with appropriate criteria according to the numerical simulations. Hence, nanofluid flow between two parallel plates is solved by ANSYS Fluent 19.3 for laminar forced in an isothermal condition. Two major criteria are studied to find the location of the boundary layer merging points: vorticity and velocity gradient in a direction perpendicular to the flow. The former presents the influential area of wall shear stress, and the latter is the direct infusion of the boundary layer induced by the solid walls. Vorticity for an irrotational flow is obtained by calculating the curl of the velocity. It is found that the merging points for the hydrodynamic boundary layers are considered before the fully developed region. For the first time, in this study, the results of various Reynolds numbers are collected, and correlations are proposed to predict the length of the boundary layer merging location by using a regression analysis of the data.