Simultaneous Flows Research Articles

Two-fluid plasma model for radial Langmuir probes as a converging nozzle with sonic choked flow, and sonic passage to supersonic flow

Using the Lambert function, Guittienne et al. [Phys. Plasmas 25, 093519 (2018)] derived two-fluid solutions for radial Langmuir probes in collisionless and isothermal plasma. In this Brief Communication, we point out the close analogy with classical compressible fluid dynamics, where the simultaneous flows of the ion and electron fluids experience opposite electrostatic body forces in the inward radial flow of the plasma, which behaves as a converging nozzle. Hence, the assumed boundary condition of sonic flow of the repelled species at the probe is explained as choked flow. The sonic passage from subsonic to supersonic flow of the attracted species at the sonic radius is also interpreted using classical fluid dynamics. Moreover, the Lambert function can provide a general solution for one-dimensional, isothermal compressible fluids, with several applications.

Layscapes. Cartografía crítica de la ciudad conectada

El imperativo digital ha supuesto la definitiva inmersión de la sociedad en un universo complejo. El establecimiento de las tecnologías electrónicas de la información y la comunicación inaugura así un nuevo paradigma en las maneras de vivir de los seres humanos incidiendo en sus estilos de vida y en sus relaciones, cuyo reflejo urbano es un territorio emergente resultado de la hibridación del mundo físico y el digital. En este contexto parece necesario encontrar una cartografía adecuada a ese nuevo territorio. El artículo explora las cartografías digitales, cuya lógica operativa se traslada a los esquemas mentales del ser humano, generándose un proceso comunicativo entre ambos agentes. Estos flujos interactivos y simultáneos proveen nuevas capas de significado a lo físico - ciudad, personas, entorno – a partir de la integración de la capa digital al espacio físico tradicional. Desde esta lógica de capas se plantea Layscapes como una herramienta de diálogo entre el paisaje urbano contemporáneo y el habitante. La hermenéutica planteada por este modelo de conocimiento crítico obtiene como resultado una metamodelización en capas donde se identifican los elementos, las relaciones y las dinámicas espacio-temporales del territorio emergente. El habitante/usuario parte de estos datos para realizar su intervención territorial en un proceso de retroalimentación informativa. Las consideraciones finales muestran cómo la tecnología establecida tras la Revolución Digital abre nuevos espacios de diálogo con el fenómeno urbano, proponiendo nuevos modelos territoriales de carácter abierto y redefiniendo el papel del arquitecto en la producción urbana.

(Invited) Epitaxial Growth of 2D Transition Metal Dichalcogenide Monolayers, Alloys and Heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) comprise a compelling class of new materials with intriguing properties that arise from their ultra-thin form, thickness-dependent bandstructure, large exciton binding energies and unusual spin and valley polarization physics. The majority of the research that has been carried out thus far employs ultra-thin TMD flakes exfoliated from bulk crystals but future device development requires the ability to synthesize large area single crystal films and heterostructures. Our research is aimed at the development of an epitaxial growth technology for TMDs and related layered chalcogenides based on gas source/metalorganic chemical vapor deposition (CVD/MOCVD) to enable monolayer growth over large substrate areas and layer-by-layer growth of heterostructures. Our recent studies have focused on the epitaxial growth of WSe2 and (Mo,W)S2 monolayer films, alloys and heterostructures using metal hexacarbonyl and hydride chalcogen precursors on substrates including sapphire and hexagonal boron nitride (hBN). A multi-step precursor modulation growth method was developed to independently control nucleation density and the lateral growth rate of TMD domains on the substrate. This approach also enables an estimation of metal-species surface diffusivity and domain growth rate as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Using the multi-step growth process, coalesced monolayer and few-layer TMD films were obtained on sapphire substrates up to 2” in diameter at growth rates on the order of ~ 1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire resulting from a merging of 0o and 60o oriented domains. Room temperature photoluminescence (PL) measurements demonstrate strong emission peaks in the range of ~1.6 eV for WSe2 and ~2.0 eV for WS2 monolayers with lateral variations that may arise from residual strain in the films. (Mo,W)S2 alloy growth using simultaneous flows of Mo and W hexacarbonyl precursors reveals evidence of atomic scale ordering of Mo and W atoms within the monolayer. Growth and characterization of MoS2/WS2 and WSe2/WS2 vertical heterostructures will also be discussed.

Numerical modelling of simultaneous overtopping and seepage flows with application to dike breaching

Dike failures due to overtopping frequently occur during floods, causing serious damage to settlements and the environment. For risk management, it is of great importance to understand the mechanisms of dike failure and predict this process accurately. In this study, a three-dimensional numerical model is developed to simulate both overtopping and seepage flows during dike breaching. The governing equations describing the free water flow are extended and applied to simulate also the filtration through porous media. A non-equilibrium sediment transport model is utilized for simulation of the bed deformation. The numerical model is firstly applied to flows over a solid embankment in order to validate the flow part of the model, and then the dike breaching is simulated such that the bed deformation model is also incorporated. By comparison with the experimental results, it is shown that the developed model reproduces overflow and the dike failure processes reasonably well.

An experimental investigation and modeling of micro array replication with Zr-based bulk metallic glass using a hot embossing process

Zirconium-based bulk metallic glass (Zr-based BMG) is a potential material for micro/nano molds. In this study, we investigated the flow characteristics of Zr-based BMG in the supercooled liquid region (SLR) with a series of uniaxial compression tests. The Newtonian viscosity model of Zr-based BMG is constructed by fitting with the experimental results based on the Arrhenius equation. The hot embossing process which can be seen as a combination of two kinds of simultaneous flows is theoretically analyzed based on Navier-Stokes (N-S) equations. The velocity and pressure fields in a BMG sample have been evaluated. Two kinds of filling modes are proposed considering the interfacial tension and oxide layer that forms on the BMG surface. The exact solutions are obtained when the cavities of the micro arrays are circular, triangular and rectangular in shape based on N-S equations. The theoretical solutions, considered surface resistance, are in good agreement with our experimental results. These models can be used to describe the fillings of metallic glass in micro hot embossing process. Finally, we studied the effects of micro cavity size and shape on BMG filling, and found that the cavities of micro rectangular array are easier filling in same feature size, however, the cavities of micro circular array are easier filling in same area. The exact models developed in this paper can be used to calculate the BMG filling heights in the cavities of micro arrays and verify the reliability of simulation results in future mold designs.

Aspherical Supernovae and Oblique Shock Breakout

AbstractIn an aspherical supernova explosion, shock emergence is not simultaneous and non-radial flows develop near the stellar surface. Oblique shock breakouts tend to be easily developed in compact progenitors like stripped-envelop core collapse supernovae. According to Matzner et al. (2013), non-spherical explosions develop non-radial flows that alters the observable emission and radiation of a supernova explosion. These flows can limit ejecta speed, change the distribution of matter and heat of the ejecta, suppress the breakout flash, and most importantly engender collisions outside the star. We construct a global numerical FLASH hydrodynamic simulation in a two dimensional spherical coordinate, focusing on the non-relativistic, adiabatic limit in a polytropic envelope to see how these fundamental differences affect the early light curve of core-collapse SNe.

Uniform multicommodity flows in the hypercube with random edge‐capacities

ABSTRACTWe give two results for multicommodity flows in the d‐dimensional hypercube with independent random edge‐capacities distributed like a random variable C where . Firstly, with high probability as , the network can support simultaneous multicommodity flows of volume close to between all antipodal vertex pairs. Secondly, with high probability, the network can support simultaneous multicommodity flows of volume close to between all vertex pairs. Both results are best possible. © 2016 Wiley Periodicals, Inc. Random Struct. Alg., 50, 437–463, 2017

Towards a single step process to create high purity gold structures by electron beam induced deposition at room temperature

Highly pure metallic structures can be deposited by electron beam induced deposition and they have many important applications in different fields. The organo-metallic precursor is decomposed and deposited under the electron beam, and typically it is purified with post-irradiation in presence of O2. However, this approach limits the purification to the surface of the deposit. Therefore, ‘in situ’ purification during deposition using simultaneous flows of both O2 and precursor in parallel with two gas injector needles has been tested and verified. To simplify the practical arrangements, a special concentric nozzle has been designed allowing deposition and purification performed together in a single step. With this new device metallic structures with high purity can be obtained more easily, while there is no limit on the height of the structures within a practical time frame. In this work, we summarize the first results obtained for ‘in situ’ Au purification using this concentric nozzle, which is described in more detail, including flow simulations. The operational parameter space is explored in order to optimize the shape as well as the purity of the deposits, which are evaluated through scanning electron microscope and energy dispersive x-ray spectroscopy measurements, respectively. The observed variations are interpreted in relation to other variables, such as the deposition yield. The resistivity of purified lines is also measured, and the influence of additional post treatments as a last purification step is studied.

Pktgen: Measuring performance on high speed networks

Pktgen is a tool for high-speed packet generation and testing. It runs in the Linux kernel, and is designed to accommodate a wide range of network performance tests. Pktgen consists of a packet generator, a receiver, and a protocol that defines the format for test packets. This paper describes the design of pktgen, and discusses its usage as a capable performance testing tool. The design is focused on performance, and in order to generate packets at high packet rate, pktgen takes advantage of multicore systems and multi-queue features on modern network cards. To support realistic heterogeneous traffic patterns, pktgen can generate multiple simultaneous UDP flows. Pktgen supports measurements of throughput, latency, packet inter-arrival time, and packet delay variation with a wide-range of fine-grained user specified rates.We evaluate and compare pktgen to other tools, and show that it is more than an order of magnitude faster than current Linux applications such as Iperf and Netperf, and has the same upper limit on performance as special-purpose high-speed tools, such as DPDK and Netmap, when it comes to throughput measurements with user-specified rate.

Debris flows of 28 June 2014 near the Arshan village (Siberia, Republic of Buryatia, Russia)

On 28 June 2014, high intensity rainfall resulted in seven simultaneous debris flows going down the slopes of the Tunka Ridge in the vicinity of Arshan village, which is a balneological and alpine resort (51° 54′ 31″ N, 102° 25′ 44″ E). The debris flows caused one life loss and several injuries, 112 buildings were damaged, and 15 were completely destroyed. The total volume of the transported deposits amounted to 3 × 106 m3. Debris flows’ formation began with the failure of weak sediments in the hanging cirques. Similar phenomena had not been recorded in the study area for over 40 years. The article presents a complete picture of the event and analysis of geological, geomorphological, and meteorological conditions for debris flows formation, for which extreme local rainfall was the major cause.

Sink location to find optimal shelters in evacuation planning

The sink location problem is a combination of network flow and location problems: from a given set of nodes in a flow network a minimum cost subset W has to be selected such that given supplies can be transported to the nodes in W. In contrast to its counterpart, the source location problem which has already been studied in the literature, sinks have, in general, a limited capacity. Sink location has a decisive application in evacuation planning, where the supplies correspond to the number of evacuees and the sinks to emergency shelters. We classify sink location problems according to capacities on shelter nodes, simultaneous or non-simultaneous flows, and single or multiple assignments of evacuee groups to shelters. Resulting combinations are interpreted in the evacuation context and analyzed with respect to their worst-case complexity status. A mixed integer programming formulation is given which is shown to have the integrality property. Generic solution methods for uncapacitated problems are based on source location and modifications of the network. In the capacitated case, for which source location cannot be applied, we suggest alternative approaches which work in the original network. It turns out that latter class algorithms are superior to the former ones. This is established in numerical tests including random data as well as real-world data from the city of Kaiserslautern, Germany.

Wind‐driven modifications to the residual circulation in an ebb‐tidal delta: Arcachon Lagoon, Southwestern France

AbstractA combination of observations and analytical solutions was used to determine the modifications caused by wind forcing on the residual or nontidal circulation in an ebb‐tidal delta. Observations were obtained in the lower Arcachon Lagoon, southwestern France. The basic nontidal circulation was established with acoustic Doppler current profilers (ADCPs) that were (i) moored in the delta's two deepest channels, and (ii) towed along a cross‐lagoon transect. The bathymetry of the lower lagoon, or ebb‐tidal delta, featured two channels: North Pass (>9 m) and South Pass (>20 m). The basic nontidal circulation consisted of mostly inflow with weak surface outflow in the South Pass, and laterally sheared bidirectional flow, dominated by outflow, in the North Pass. Analytical solutions and comparison of observed dynamical terms suggested that, in addition to the conventionally accepted influence of tidal nonlinearities, density gradients contributed to the basic nontidal circulation in the lagoon. Observations also indicated that wind forcing altered the basic circulation by driving simultaneous upwind flows in both passes. This response was supported by an analytical solution to wind‐driven flows over the bathymetry of the transect sampled. The response to seaward winds was to enhance inflow in South Pass and reduce outflow in North Pass. Landward winds caused diminished inflow in South Pass and increased outflow in North Pass.

All capacities modular cost survivable network design problem using genetic algorithm with completely connection encoding

AbstractWe study the survivable network design problem (SNDP) for simultaneous unicast and anycast flows in networks where the link cost follows All Capacities Modular Cost (ACMC) model. Given a network modeled by a connected, undirected graph and a set of flow demands, this problem aims at finding a set of connections with a minimized network cost in order to protect the network against any single failure. This paper proposes a new Genetic Algorithm with an efficient encoding to solve the SNDP in networks with ACMC model (A-SNDP). Our encoding scheme is simple and allows large search space. Extensive simulation results on real large topology instances show that the proposed algorithm is much more efficient than the Tabu Search and other conventional Genetic Algorithm in terms of minimizing the network cost.

A multi-objective approach for solving the survivable network design problem with simultaneous unicast and anycast flows

In this paper, we consider the survivable network design problem for simultaneous unicast and anycast flow requests. We assume that the network is modeled by a connected and undirected graph. This problem aims at finding a set of connections with a minimized network cost in order to protect the network against any single failure. The cost is computed using the all capacities modular cost (ACMC) model and a set of flow demands. We name it as ACMC-based survivable network design problem (A-SNDP). It is proved that the problem is NP-hard. We introduce a multi-objective approach to solve A-SNDP. The objectives are to minimize the network cost (NCost) and the network failure (NFail). Extensive simulation results on instances of Polska, Germany and Atlanta networks showed the efficiency of the multi-objective approach for solving A-SNDP.

Fuzzy Logic Based Rate Control Scheme for ODMRP in Mobile Ad hoc Networks

On Demand Multicast Routing Protocol (ODMRP) is a popular solution designed for ad hoc networks with mobile hosts. Its efficiency, simplicity, and robustness to mobility render it one of the most widely used multicast routing protocols in Mobile Ad hoc NETworks (MANET). In ODMRP, there is no input rate control for upper layer traffic. So, it's possible that high dense traffic flow causes congestion in networks. In this work, an enhancement to ODMRP is proposed referred to as fuzzy logic based Rate Control ODMRP (FRC-ODMRP). FRC-ODMRP attempts to adapt the arrival rate from upper layers to the state in the network by using feedback information from receivers of the multicast group. Accordingly, source comes up with a decision whether to increase or decrease its transmission rate based on information collected from the receivers. In this research, delay and packet delivery ratio reconsidered as indicators of congestion in addition to number of received packets. Simulation results demonstrate that FRC-ODMRP achieves significant performance improvements in comparison to conventional ODMRP and QoS-ODMRP. Indeed, it efficiently handles simultaneous traffic flows such that no one could dominate available bandwidth of networks.

Risk assessment of simultaneous debris flows in mountain townships

Many mountain towns in China are located on the joint alluvial fans of multiple and adjacent past debris flows, making them vulnerable to large, multiple, and simultaneous debris flows during heavy rainfall. Without emergency management planning, such flows, often appearing with interconnecting and chain-reaction processes, can lead to extensive loss of life and property. In the Wenchuan earthquake-affected area, such disasters are common. We analyzed the compound effects of simultaneous debris flow events, and proposed three quantitative methods of debris risk assessment based on kinetic energy, flow depth, and inundation depth. Validated using a field study of actual debris flow disasters, these analyses are useful in determining the type, quantity, distribution, economic worth, and susceptibility of hazard-affected objects in a region. Subsequently, we established a method to determine the vulnerability of different hazard-affected objects, particularly concerning the susceptibility indexes of buildings or structures. By analyzing the elements underlying hazard formation conditions, damage potential, and the socio-economic conditions of mountain townships, we proposed a systematic and quantitative method for risk analysis of mountain townships. Finally, the proposed method was applied to a case study of Qingping Township, which was affected by 21 simultaneous debris flows triggered by a 50-year return period precipitation event. The proposed method analyzed the superposition and chain-reaction effects of disasters and divided the affected area of the township into three risk zones. The analysis indicated that the calculated risk zones coincide with the actual distribution and severity of damage in the debris flow event, which suggests that the risk assessment is consistent with results from the actual disaster.

Providing multiple hard latency and throughput guarantees for packet switching networks on chip

In many-core architectures different distributed applications are executed in parallel. The applications may need hard guarantees for communication with respect to latency and throughput to cope with their constraints. Networks on Chip (NoC) are the most promising approach to handle these requirements in architectures with a large number of cores. Dynamic reservation of communication resources in virtual channel NoCs is used to enable quality of service for concurrent communication. This paper presents a router design supporting best effort and connection-oriented guaranteed service communication. The communication resources are shared dynamically between the two communication schemes. The key contribution is a concept for virtual channel reservation supporting different bandwidth and latency guarantees for simultaneous guaranteed service communication flows. Different to state-of-the-art, the used scheduling approach allows to give hard guarantees regarding throughput and latency. The concept enables to adjust the bandwidth and latency requirements of connections at run-time to cope with dynamically changing application requirements. Due to its distributed reservation process and resource allocation it offers good scalability for many-core architectures. The implementation of a router and the required extension of a network interface to support the proposed concept are presented. The software perspective is discussed. An algorithm is presented that is used to establish guaranteed service connections according to the applications bandwidth requirements. Simulation results are compared to state-of-the-art arbitration schemes and show significant improvements of latency and throughput, e.g. for an MPEG4 application. Synthesis results expose the low area overhead and impact on energy consumption which makes the concepts highly attractive for QoS-constraint many-core architectures.

Damped kink oscillations of flowing prominence threads

Transverse oscillations of thin threads in solar prominences are frequently reported in high-resolution observations. Two typical features of the observations are that the oscillations are damped in time and that simultaneous mass flows along the threads are detected. Flows cause the dense threads to move along the prominence magnetic structure while the threads are oscillating. The oscillations have been interpreted in terms of standing magnetohydrodynamic (MHD) kink waves of the magnetic flux tubes which support the threads. The damping is most likely due to resonant absorption caused by plasma inhomogeneity. The technique of seismology uses the observations combined with MHD wave theory to estimate prominence physical parameters. This paper presents a theoretical study of the joint effect of flow and resonant absorption on the amplitude of standing kink waves in prominence threads. We find that flow and resonant absorption can either be competing effects on the amplitude or both can contribute to damp the oscillations depending on the instantaneous position of the thread within the prominence magnetic structure. The amplitude profile deviates from the classic exponential profile of resonantly damped kink waves in static flux tubes. Flow also introduces a progressive shift of the oscillation period compared to the static case, although this effect is in general of minor importance. We test the robustness of seismological estimates by using synthetic data aiming to mimic real observations. The effect of the thread flow can significantly affect the estimation of the transverse inhomogeneity length scale. The presence of random background noise adds uncertainty to this estimation. Caution needs to be paid to the seismological estimates that do not take the influence of flow into account.

Synergistic surface current mapping by spaceborne stereo imaging and coastal HF radar

Well validated optical and radar methods of surface current measurement at high spatial resolution (nominally <100 m) from space can greatly advance our ability to monitor earth's oceans, coastal zones, lakes and rivers. With interest growing in optical along‐track stereo techniques for surface current and wave motion determinations, questions of how to interpret such data and how to relate them to measurements made by better validated techniques arise. Here we make the first systematic appraisal of surface currents derived from along‐track stereo Sun glitter (ATSSG) imagery through comparisons with simultaneous synoptic flows observed by coastal HF radars working at frequencies of 13.9 and 24.5 MHz, which return averaged currents within surface layers of roughly 1 m and 2 m depth respectively. At our Tsushima Strait (Japan) test site, we found that these two techniques provided largely compatible surface current patterns, with the main difference apparent in current strength. Within the northwest (southern) comparison region, the magnitudes of the ATSSG current vectors derived for 13 August 2006 were on average 22% (40%) higher than the corresponding vectors for the 1‐m (2‐m) depth radar. These results reflect near‐surface vertical current structure, differences in the flow components sensed by the two techniques and disparities in instrumental performance. The vertical profile constructed here from ATSSG, HF radar and ADCP data is the first to resolve downwind drift in the upper 2 m of the open ocean. The profile e‐folding depth suggests Stokes drift from waves of 10‐m wavelength visible in the images.

THINC법을 이용한 비혼합 혼상류의 경계면 추적

기체와 액체의 유동 및 고체의 변형을 동시에 고려할 수 있는 혼상류모델을 이용하여 파동장을 해석하는 경우, 서로 다른 비혼합의 유체 경계면의 시간변형을 고정도로 추적하는 것이 대단히 중요하다. 본 연구에서는 경계면의 추적에 있어서 VOF(Volume of Fluid)법으로 대표되는 경계면 형상의 재구축이 필요한 Geometrical-type의 경계추적법의 대신에 Algebraic-type의 경계추적법인 THINC(Tangent of Hyperbola for INterface-Capturing)법을 적용하였다. THINC법은 경계면에 대한 형상의 구축이 필요하지 않으므로 VOF법에 비해 비교적 간단한 알고리즘을 가지며, 기존의 Navier-Stokes solver에로 적용성이 용이한 장점을 갖는다. 본 연구에서는 THINC법의 기본적인 이류특성을 고찰하고, 혼상류수치모델인 TWOPM(one-field Model for immiscible TWO-Phase flows)과 결합한 수치모델을 파동장에 적용하여 비혼합 혼상류에서 경계면의 추적능을 검토하였다. 그 결과, 혼상류의 경계면 추적에 있어서 상대적으로 간단한 알고리즘의 THINC법이 기존의 VOF법과 유사한 정도를 갖는 해석법이라는 것을 확인할 수 있었고, 따라서 향후 기포의 연행을 동반하는 쇄파 및 쇄파력의 해석 등에 그의 적용성이 기대된다. In the numerical simulation of wave fields using a multi-phase flow model that considers simultaneous flows of materials with different states such as gas, liquid and solid, there is need of an accurate representation of the interface separating the fluids. We adopted an algebraic interface capturing method called tangent of hyperbola for interface-capturing(THINC) method for the capture of the free-surface in computations of multi-phase flow simulations instead of geometrical-type methods such a volume of fluid(VOF) method. The THINC method uses a hyperbolic tangent functions to represent the surface, and compute the numerical flux for the fluid fraction functions. One of the remarkable advantages of THINC method is its easy applicability to incorporate various numerical codes based on Navier-Stokes solver because it does not require the extra geometric reconstruction needed in most of VOF-type methods. Several tests were carried out in order to investigate the advection of interfaces and to verify the applicability of the THINC method to wave fields based on the one-field model for immiscible two-phase flows (TWOPM). The numerical results revealed that the THINC method is able to track the interface between air and water separating the fluids although its algorithm is fairly simple.