A search for energy-dependence of the Kes 73/1E 1841-045 morphology in GeV

While the Kes 73/1E 1841-045 system had been confirmed as an extended GeV source, whether its morphology depends on the photon energy or not deserves our further investigation. Adopting data collected by Fermi Large Area Telescope (LAT) again, we look into the extensions of this source in three energy bands individually: 0.3-1 GeV, 1-3 GeV and 3-200 GeV. We find that the 0.3-1 GeV morphology is point-like and is quite different from those in the other two bands, although we cannot robustly reject a unified morphology for the whole LAT band.Comment: Approved for publication in PoS as a proceeding of the 7th International Fermi Symposium (IFS2017

Investigation and prediction of slug flow characteristics in highly viscous liquid and gas flows in horizontal pipes

Slug flow characteristics in highly viscous liquid and gas flow are studied experimentally in a horizontal pipe with 0.074 m ID and 17 m length. Results of flow regime map, liquid holdup and pressure gradient are discussed and liquid viscosity effects are investigated. Applicable correlations which are developed to predict liquid holdup in slug body for low viscosity flow are assessed with high viscosity liquids. Furthermore, a mechanistic model is developed for predicting the characteristics of slug flows of highly viscous liquid in horizontal pipes. A control volume is drawn around the slug body and slug film in a slug unit. Momentum equations with a momentum source term representing the significant momentum exchange between film zone and slug body are applied. Liquid viscosity effects are considered in closure relations. The mechanistic model is validated by comparing available pressure gradient and mean slug liquid holdup data produced in the present study and those obtained from literature, showing satisfactory capabilities over a large range of liquid viscosity

Fermi Large Area Telescope Observations of the Fast-dimming Crab Nebula in 60-600 MeV

Context: The Crab pulsar and its nebula are the origin of relativistic electrons which can be observed through their synchrotron and inverse Compton emission. The transition between synchrotron-dominated and inverse-Compton-dominated emissions takes place at $\approx 10^9$ eV. Aims: The short-term (weeks to months) flux variability of the synchrotron emission from the most energetic electrons is investigated with data from ten years of observations with the Fermi Large Area Telescope (LAT) in the energy range from 60 MeV to 600 MeV. Methods: The off-pulse light-curve has been reconstructed from phase-resolved data. The corresponding histogram of flux measurements is used to identify distributions of flux-states and the statistical significance of a lower-flux component is estimated with dedicated simulations of mock light-curves. The energy spectra for different flux states are reconstructed. Results: We confirm the presence of flaring-states which follow a log-normal flux distribution. Additionally, we discover a low-flux state where the flux drops to as low as 18.4% of the intermediate-state average flux and stays there for several weeks. The transition time is observed to be as short as 2 days. The energy spectrum during the low-flux state resembles the extrapolation of the inverse-Compton spectrum measured at energies beyond several GeV energy, implying that the high-energy part of the synchrotron emission is dramatically depressed. Conclusions: The low-flux state found here and the transition time of at most 10 days indicate that the bulk ($>75$%) of the synchrotron emission above $10^8$ eV originates in a compact volume with apparent angular size of $\theta\approx0.4" t_\mathrm{var}/(5 \mathrm{d})$. We tentatively infer that the so-called inner knot feature is the origin of the bulk of the $\gamma$-ray emission.Comment: Accepted by A&A on 05.05.2020; Original version submitted on 19.09.201

Experimental Study of Two-Phase Air–Water Flow in Large-Diameter Vertical Pipes

Recently, due to an increase in production demand in nuclear and oil and gas industries, the requirement to migrate toward larger pipe sizes for future developments has become essential. However, it is interesting to note that almost all the research on two-phase gas–liquid flow in vertical pipe upflow is based on small-diameter pipes (D_100 mm), and the experimental work on the two-phase gas–liquid flow in large-diameter (D>100 mm) vertical pipes is scarce. Under the above circumstances, the application of modelling tools=correlations based on small-diameter pipes in predicting flow behaviour (flow pattern, void fraction, and pressure gradient) poses severe challenges in terms of accuracy. The results presented in this article are motivated by the need to introduce the research work done to the industries where the data pertaining to large-diameter vertical pipes are scarce and there is a lack of understanding of two-phase gas-liquid flow behaviour in large-diameter (D>100 mm) vertical pipes. The unique aspect of the results presented here is that the experimental data have been generated for a 254-mm inner diameter vertical pipe that forms an excellent basis for the assessment of modelling tools=correlations. This article (i) presents the results of a systematic investigation of the flow patterns in large-diameter vertical pipes and identifies the transition between subsequent flow patterns, (ii) compares it directly with the existing large- (150 mm) and small-diameter data (28mm and 32 mm) in the same air–water superficial velocity range, (iii) exemplifies that the existing available empirical correlations=models=codes are significantly in error when applied to large-diameter vertical pipes for predictions, and last (iv) assesses the predictive capability of a well-known commercial multiphase flow simulator

Self-Organization of Balanced Nodes in Random Networks with Transportation Bandwidths

We apply statistical physics to study the task of resource allocation in random networks with limited bandwidths along the transportation links. The mean-field approach is applicable when the connectivity is sufficiently high. It allows us to derive the resource shortage of a node as a well-defined function of its capacity. For networks with uniformly high connectivity, an efficient profile of the allocated resources is obtained, which exhibits features similar to the Maxwell construction. These results have good agreements with simulations, where nodes self-organize to balance their shortages, forming extensive clusters of nodes interconnected by unsaturated links. The deviations from the mean-field analyses show that nodes are likely to be rich in the locality of gifted neighbors. In scale-free networks, hubs make sacrifice for enhanced balancing of nodes with low connectivity.Comment: 7 pages, 8 figure

Hydrodynamic Flow Behaviour in Large Diameter Vertical Riser: Experimental and Simulation Studies

An experimental campaign has been performed to investigate the hydrodynamic behaviour in 12m high and 254mm nominal diameter horizontal flowline-vertical riser setup using air-water as working fluid. The data generated from near riser base and flowline pressure variations were used to characterize the stable and unstable flows encountered during the experiments. A model to study the dynamic behaviour of the large diameter horizontal flowline-vertical riser system is developed using OLGA software (v5.1). Experimental results were compared with simulated model. The major objective of undertaking this work is to investigate the performance of the well-known code for flows encountered in large diameter horizontal flowline-vertical riser. Additionally based on the results, also identify the areas of the improvement in the simulator

Effect of upstream conditions on the two phase flow in the large diameter vertical pipe

Two phase flow in large diameter vertical pipes is of considerable importance to various fields of multiphase flows. Recently, due to increase in demand in various energy sectors considerable attention has been paid in understanding the flow behavior in large diameter vertical pipes. This paper reports the experimental results of flow pattern identification in relatively large diameter (254mm) vertical pipe. The present work also investigates the effect of upstream conditions on the flow patterns occurring in the vertical test section. The observed flow patterns are classified as bubbly, agitated bubbly, unstable slug and churn/froth flow. Absence of hydrodynamic slugging i.e. non occurrence of Taylor bubble in the riser is also reported. It is found that agitated bubbly and churn/froth type of flows dominates in the vertical large diameter pipe under the condition where conventional small diameter pipe indicates slug flow. Experimental results demonstrate the sensitivity of the flow patterns occurring in the vertical section with the upstream conditions in the flowline. The statistical analysis performed on the data along with visual observations, suggests that liquid slugs formed in flowline dissipate as they travelled up the riser, due to the high velocity distorted bubble clusters