scholarly journals Numerical Models of 3-D Galactic Dynamos

1993 ◽  
Vol 157 ◽  
pp. 345-348
Author(s):  
J.S. Panesar ◽  
N. Moore ◽  
A.H. Nelson
Keyword(s):  
Numerical Models ◽  
Density Wave ◽  
Vertical Component ◽  
Small Scale ◽  
Galactic Centre ◽  
Model Parameters ◽  
Magnetic Intensity ◽  
Field Mode ◽  
The Galaxy ◽  
Scale Turbulence

We describe here the results of 3-D numerical simulations of an αω-dynamo in galaxies with differential rotation, small scale turbulence, and a shock wave induced by a stellar density wave. A non-linear quenching mechanism for the dynamo instability is used, and with the model parameters employed the field achieves a steady state which closely resembles observed fields in galaxies. The magnetic field vectors are parallel to the plane in the disc, with the magnetic intensity decreasing away from the plane. The vectors are also nearly parallel to the spiral arms in the disc, and the field direction is axisymmetric about the galactic centre, but with significant increase of intensity in the arms. The magnetic intensity rises steeply towards the centre of the galaxy, where the field becomes dominated by the vertical component. Nowhere in the parameter range covered is the bi-symmetric field mode dominant.

scholarly journals 3–D Models of Galaxy Magnetic Fields with Spiral Shocks

1990 ◽  
Vol 140 ◽  
pp. 133-134
Author(s):  
J. Panesar ◽  
A.H. Nelson
Keyword(s):  
Gas Flow ◽  
Density Wave ◽  
Shock Velocity ◽  
Small Scale ◽  
Hydrodynamic Simulation ◽  
Dynamo Equation ◽  
Scale Turbulence ◽  
Wave Induced ◽  
The Magnetic Field ◽  
Stellar Density

We report here some preliminary results of 3–D numerical simulations of an α–ω dynamo in galaxies with differential rotation, small–scale turbulence, and a shock wave induced by a stellar density wave. We obtain the magnetic field from the standard dynamo equation, but include the spiral shock velocity field from a hydrodynamic simulation of the gas flow in a gravitational field with a spiral perturbation (Johns and Nelson, 1986).

Coupled Thermo-Hydro-Geochemical Models of Engineered Barrier Systems: The Febex Project

2000 ◽  
Vol 663 ◽  
Author(s):  
J. Samper ◽  
R. Juncosa ◽  
V. Navarro ◽  
J. Delgado ◽  
L. Montenegro ◽  
...  
Keyword(s):  
Large Scale ◽  
Reference Model ◽  
Numerical Models ◽  
Full Scale ◽  
Small Scale ◽  
Model Parameters ◽  
In Situ Test ◽  
Wide Range ◽  
Engineered Barrier

ABSTRACTFEBEX (Full-scale Engineered Barrier EXperiment) is a demonstration and research project dealing with the bentonite engineered barrier designed for sealing and containment of waste in a high level radioactive waste repository (HLWR). It includes two main experiments: an situ full-scale test performed at Grimsel (GTS) and a mock-up test operating since February 1997 at CIEMAT facilities in Madrid (Spain) [1,2,3]. One of the objectives of FEBEX is the development and testing of conceptual and numerical models for the thermal, hydrodynamic, and geochemical (THG) processes expected to take place in engineered clay barriers. A significant improvement in coupled THG modeling of the clay barrier has been achieved both in terms of a better understanding of THG processes and more sophisticated THG computer codes. The ability of these models to reproduce the observed THG patterns in a wide range of THG conditions enhances the confidence in their prediction capabilities. Numerical THG models of heating and hydration experiments performed on small-scale lab cells provide excellent results for temperatures, water inflow and final water content in the cells [3]. Calculated concentrations at the end of the experiments reproduce most of the patterns of measured data. In general, the fit of concentrations of dissolved species is better than that of exchanged cations. These models were later used to simulate the evolution of the large-scale experiments (in situ and mock-up). Some thermo-hydrodynamic hypotheses and bentonite parameters were slightly revised during TH calibration of the mock-up test. The results of the reference model reproduce simultaneously the observed water inflows and bentonite temperatures and relative humidities. Although the model is highly sensitive to one-at-a-time variations in model parameters, the possibility of parameter combinations leading to similar fits cannot be precluded. The TH model of the “in situ” test is based on the same bentonite TH parameters and assumptions as for the “mock-up” test. Granite parameters were slightly modified during the calibration process in order to reproduce the observed thermal and hydrodynamic evolution. The reference model captures properly relative humidities and temperatures in the bentonite [3]. It also reproduces the observed spatial distribution of water pressures and temperatures in the granite. Once calibrated the TH aspects of the model, predictions of the THG evolution of both tests were performed. Data from the dismantling of the in situ test, which is planned for the summer of 2001, will provide a unique opportunity to test and validate current THG models of the EBS.

Micro Gas Turbine Cycle Humidification for Increased Flexibility: Numerical and Experimental Validation of Different Steam Injection Models

2018 ◽  
Vol 141 (2) ◽  
Author(s):  
Ward De Paepe ◽  
Massimiliano Renzi ◽  
Marina Montero Carrerro ◽  
Carlo Caligiuri ◽  
Francesco Contino
Keyword(s):  
Combustion Chamber ◽  
Gas Turbines ◽  
Numerical Models ◽  
Steam Injection ◽  
Power Production ◽  
Cycle Performance ◽  
Small Scale ◽  
Model Parameters ◽  
Complete Combustion ◽  
The Impact

With the current shift from centralized to more decentralized power production, new opportunities arise for small-scale combined heat and power (CHP) production units like micro gas turbines (mGTs). However, to fully embrace these opportunities, the current mGT technology has to become more flexible in terms of operation—decoupling the heat and power production in CHP mode—and in terms of fuel utilization—showing flexibility in the operation with different lower heating value (LHV) fuels. Cycle humidification, e.g., by performing steam injection, is a possible route to handle these problems. Current simulation models are able to correctly assess the impact of humidification on the cycle performance, but they fail to provide detailed information on the combustion process. To fully quantify the potential of cycle humidification, more advanced numerical models—preferably validated—are necessary. These models are not only capable of correctly predicting the cycle performance, but they can also handle the complex chemical kinetics in the combustion chamber. In this paper, we compared and validated such a model with a typical steady-state model of the steam injected mGT cycle based on the Turbec T100. The advanced one is an in-house MATLAB model, based on the NIST database for the characterization of the properties of the gaseous compounds with the combustion mechanisms embedded according to the Gri-MEch 3.0 library. The validation one was constructed using commercial software (Aspen Plus), using the more advance Redlich-Kwong-Soave (RKS)- Boston-Mathias(BM) property method and assuming complete combustion by using a Gibbs reactor. Both models were compared considering steam injection in the compressor outlet or in the combustion chamber, focusing only on the global cycle performance. Simulation results of the steam injection cycle fueled with natural gas and syngas showed some differences between the two presented models (e.g., 5.9% on average for the efficiency increase over the simulated steam injection rates at nominal power output for injection in the compressor outlet); however, the general trends that could be observed are consistent. Additionally, the numerical results of the injection in the compressor outlet were also validated with steam-injection experiments in a Turbec T100, indicating that the advanced MATLAB model overestimates the efficiency improvement by 25–45%. The results show the potential of simulating the humidified cycle using more advanced models; however, in future work, special attention should be paid to the experimental tuning of the model parameters in general and the recuperator performance in particular to allow correct assessment of the cycle performance.

scholarly journals Effect of Small-Scale Wildfires on the Air Parameters near the Burning Centers

Atmosphere ◽  
2021 ◽  
Vol 12 (1) ◽  
pp. 75
Author(s):  
Egor Loboda ◽  
Denis Kasymov ◽  
Mikhail Agafontsev ◽  
Vladimir Reyno ◽  
Yevgeniy Gordeev ◽  
...  
Keyword(s):  
Refractive Index ◽  
Wind Velocity ◽  
Large Scale ◽  
Vertical Component ◽  
Thermal Characteristics ◽  
Small Scale ◽  
Forest Steppe ◽  
Turbulence Scale ◽  
Fire Front ◽  
Scale Turbulence

The results of seminatural experiments on the study of steppe and field wildfires characteristic of the steppe and forest-steppe zones of Western Siberia are presented. Using infrared (IR) thermography methods, the main thermal characteristics of the fire front are derived, the flame turbulence scale is estimated, and changes in the structure function of the air refractive index are analyzed in the vicinity of a fire. The effect of a model fire on the change of meteorological parameters (wind velocity components, relative air humidity, and temperature) is ascertained. Large-scale turbulence is observed in the front of a seminatural fire, which is absent in laboratory conditions. The predominance of large-scale turbulence in a flame results in turbulization of the atmosphere in the vicinity of a combustion center. Strong heat release in the combustion zone and flame turbulence increase the vertical component of the wind velocity and produce fluctuations in the air refractive index, which is an indicator of atmospheric turbulization. This creates prerequisites for the formation of a proper wind during large fires. Variations in the gas and aerosol compositions of the atmosphere are measured in the vicinity of the experimental site.

scholarly journals Heightened Faraday complexity in the inner 1 kpc of the galactic centre

2021 ◽  
Vol 502 (3) ◽  
pp. 3814-3828
Author(s):  
J D Livingston ◽  
N M McClure-Griffiths ◽  
B M Gaensler ◽  
A Seta ◽  
M J Alger
Keyword(s):  
Galactic Plane ◽  
Small Scale ◽  
Order Structure ◽  
Galactic Centre ◽  
Outer Scale ◽  
Large Rotation ◽  
Stellar Feedback ◽  
Upper Limit ◽  
Centre Region ◽  
Scale Turbulence

ABSTRACT We have measured the Faraday rotation of 62 extra-galactic background sources in 58 fields using the CSIRO Australia Telescope Compact Array (ATCA) with a frequency range of 1.1–3.1 GHz with 2048 channels. Our sources cover a region $\sim 12\, \times 12\, \mathrm{deg^{ 2}}$ (∼1 kpc) around the Galactic Centre region. We show that the Galactic Plane for |l| < 10° exhibits large Rotation Measures (RMs) with a maximum |RM| of $1691.2 \pm 4.9\, \mathrm{rad}\, \mathrm{m}^{-2}$ and a mean $|\mathrm{RM}| = 219 \pm 42\, \mathrm{rad}\, \mathrm{m}^{-2}$. The RMs decrease in magnitude with increasing projected distance from the Galactic Plane, broadly consistent with previous findings. We find an unusually high fraction (95 per cent) of the sources show Faraday complexity consistent with multiple Faraday components. We attribute the presences of multiple Faraday rotating screens with widely separated Faraday depths to small-scale turbulent RM structure in the Galactic Centre region. The second-order structure function of the RM in the Galactic Centre displays a line with a gradient of zero for angular separations spanning 0.83°–11° (∼120–1500 pc), which is expected for scales larger than the outer scale (or driving scale) of magneto-ionic turbulence. We place an upper limit on any break in the SF gradient of 66 arcsec, corresponding to an inferred upper limit to the outer scale of turbulence in the inner 1 kpc of the Galactic Centre of 3 pc. We propose stellar feedback as the probable driver of this small-scale turbulence.

scholarly journals Influence of the Nonaxisymmetric Dark Halo Shape on Galaxies Outer Spiral Pattern Morphology

2021 ◽  
pp. 73-85
Author(s):  
Maria Butenko ◽  
Iraida Belikova ◽  
Svetlana Khokhlova ◽  
Nikolay Kuzmin ◽  
Gennadiy Ivanchenko ◽  
...  
Keyword(s):  
Galactic Disk ◽  
Stellar Disk ◽  
Small Scale ◽  
Model Parameters ◽  
Dark Halo ◽  
Spiral Pattern ◽  
The Galaxy ◽  
Scale Structures ◽  
Optical Radius ◽  
Small Scale Structures

The results of numerical simulations of a gaseous galactic disk rotating in an external nonaxisymmetric potential of a dark halo are presented in the article. Analysis of two models of a nonaxisymmetric dark halo, in which a gaseous galactic disk rotates, has been carried out. In the first case, the halo is nonaxisymmetric within the optical radius of the disk, where the bulk of the visible matter of the galaxy is located, including the stellar disk. The model is ineffective for the external long-lived spiral structure formation in the disk periphery due to the nonaxisymmetry of dark halo. In the second series of calculations, we have employed the model with a symmetric halo inside the optical radius and a non-axisymmetric one outside of it. The results of the simulations confirm that nonaxisymmetry in the halo matter distribution is effectively generating the global spiral pattern at the periphery of the galaxy. One may observe such spiral structures in some galaxies, mainly in the ultraviolet range. Analysis of various model parameters has showed that the value of parameter " is the primary characteristic affecting the morphology of the forming spiral pattern. This value determines the degree of nonaxisymmetry of the halo. The Le parameter introduced in this work and responsible for the formation of small-scale structures in the transition region does not significantly affect the disk periphery. Moreover, the larger the value of Le, the smoother spirals are formed. As it has shown in this work the size of the computational grid does not significantly influence on the simulation results, revealing only small-scale structures which are not the subject of current work.

Micro Gas Turbine Cycle Humidification for Increased Flexibility: Numerical and Experimental Validation of Different Steam Injection Models

2018 ◽  
Author(s):  
Ward De Paepe ◽  
Massimiliano Renzi ◽  
Marina Montero Carrerro ◽  
Carlo Caligiuri ◽  
Francesco Contino
Keyword(s):  
Combustion Chamber ◽  
Gas Turbines ◽  
Numerical Models ◽  
Steam Injection ◽  
Power Production ◽  
Cycle Performance ◽  
Small Scale ◽  
Model Parameters ◽  
Complete Combustion ◽  
The Impact

With the current shift from centralized to more decentralized power production, new opportunities arise for small-scale Combined Heat and Power (CHP) production units like micro Gas Turbines (mGTs). However, to fully embrace these opportunities, the current mGT technology has to become more flexible in terms of operation — decoupling the heat and power production in CHP mode — and in terms of fuel utilization — showing flexibility in the operation with different Lower Heating Value (LHV) fuels. Cycle humidification e.g. by performing steam injection, is a possible route to handle these problems. Current existing simulation models are able to correctly assess the impact of humidification on the cycle performance, but they fail to provide detailed information on the combustion process. To fully quantify the potential of cycle humidification, more advanced numerical models — preferably validated — are necessary. These models are not only capable of correctly predicting the cycle performance, but they can also handle the complex chemical kinetics in the combustion chamber. In this paper, we compared and validated such a model with a typical steady-state model of the steam injected mGT cycle based on the Turbec T100. The advanced one is an in-house MATLAB® model, based on the NIST database for the characterization of the properties of the gaseous compounds with the combustion mechanisms embedded according to the Gri-MEch 3.0 library. The validation one was constructed using commercial software (Aspen® Plus), using the more advance RKS-BM property method and assuming complete combustion by using a Gibbs reactor. Both models were compared considering steam injection in the compressor outlet or in the combustion chamber, focussing only on the global cycle performance. Simulation results of the steam injection cycle fuelled with natural gas and syngas showed some differences between the two presented models (e.g. 5.9% on average for the efficiency increase over the simulated steam injection rates at nominal power output for injection in the compressor outlet); however, the general trends that could be observed are consistent. Additionally, the numerical results of the injection in the compressor outlet were also validated with steam-injection experiments in a Turbec T100, indicating that the advanced MATLAB® model overestimates the efficiency improvement by 25 % to 45 %. The results show the potential of simulating the humidified cycle using more advanced models; however, in future work, special attention should be paid to the experimental tuning of the model parameters in general and the recuperator performance in particular to allow correct assessment of the cycle performance.

scholarly journals A new downscaling method for sub-grid turbulence modeling

2017 ◽  
Vol 17 (11) ◽  
pp. 6531-6546
Author(s):  
Lucie Rottner ◽  
Christophe Baehr ◽  
Fleur Couvreux ◽  
Guylaine Canut ◽  
Thomas Rieutord
Keyword(s):  
High Resolution ◽  
Turbulence Modeling ◽  
Particle System ◽  
Numerical Models ◽  
Particle Systems ◽  
Small Scale ◽  
Grid Turbulence ◽  
Turbulent Structures ◽  
High Resolution Data ◽  
Scale Turbulence

Abstract. In this study we explore a new way to model sub-grid turbulence using particle systems. The ability of particle systems to model small-scale turbulence is evaluated using high-resolution numerical simulations. These high-resolution data are averaged to produce a coarse-grid velocity field, which is then used to drive a complete particle-system-based downscaling. Wind fluctuations and turbulent kinetic energy are compared between the particle simulations and the high-resolution simulation. Despite the simplicity of the physical model used to drive the particles, the results show that the particle system is able to represent the average field. It is shown that this system is able to reproduce much finer turbulent structures than the numerical high-resolution simulations. In addition, this study provides an estimate of the effective spatial and temporal resolution of the numerical models. This highlights the need for higher-resolution simulations in order to evaluate the very fine turbulent structures predicted by the particle systems. Finally, a study of the influence of the forcing scale on the particle system is presented.

scholarly journals A new downscaling method for sub-grid turbulence modeling

2016 ◽  
Author(s):  
L. Rottner ◽  
C. Baehr ◽  
F. Couvreux ◽  
G. Canut ◽  
T. Rieutord
Keyword(s):  
High Resolution ◽  
Turbulence Modeling ◽  
Particle System ◽  
Numerical Models ◽  
Atmospheric Model ◽  
Particle Systems ◽  
Small Scale ◽  
Grid Turbulence ◽  
Turbulent Structures ◽  
Scale Turbulence

Abstract. In this study we explore a new way to model sub-grid turbulence using particle systems. The ability of particle systems to model small scale turbulence is evaluated using high resolution numerical simulations. These high-resolution simulations have been performed with the research atmospheric model Meso-NH and averaged at larger scale from which a complete downscaling experience, via a particle system, have been performed. The particle simulations are compared to the high-resolution simulation for the representation of the wind fluctuations and the turbulent kinetic energy. Despite the simplicity of the physical model used to drive the particles, the results show that particle system is able to represent the average field. It is shown that this system is able to reproduce much finer turbulent structures than the numerical high-resolution simulations. In addition, this study provides an estimate of the effective spatial and temporal resolution of the numerical models. This highlights the need for higher resolution simulations to be able to evaluate the very fine turbulent structures predicted by the particle systems. Eventually a study of the influence of the forcing scale on the particle system is presented.

Numerical models of hydromagnetic dynamos

1975 ◽  
Vol 67 (4) ◽  
pp. 625-646 ◽  
Author(s):  
S. A. Jepps
Keyword(s):  
Large Scale ◽  
Numerical Models ◽  
Fluid Flows ◽  
Magnetic Reynolds Number ◽  
Small Scale ◽  
Initial Field ◽  
Finite Difference Equations ◽  
Induction Equation ◽  
Scale Turbulence ◽  
Magnetic Induction Equation

The magnetic induction equation is solved numerically in a sphere for a variety of prescribed fluid flows. The models considered are the so-called ‘αω dynamos’, in which both small-scale turbulence and large-scale shearing play a significant role. Solutions are obtained by marching the finite–difference equations forward in time from some initial field. For a critical value of the magnetic Reynolds numberRmsolutions which neither grow nor decay are found.Further calculations are performed with a ‘cut-off effect’ in which an attempt is made to simulate the effect of the Lorentz forces on the turbulence. For supercritical values of R, the magnetic field now stabilizes a t a finite value instead of increasing indefinitely. The form of the final field is compared with that produced at criticalRmin the absence of the cut-off effect.

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