1,576 research outputs found
Long-term variations of turbulent transport coefficients in a solar-like convective dynamo simulation
The Sun, aside from its eleven year sunspot cycle is additionally subject to
long term variation in its activity. In this work we analyse a solar-like
convective dynamo simulation, containing approximately 60 magnetic cycles,
exhibiting equatorward propagation of the magnetic field, multiple frequencies,
and irregular variability, including a missed cycle and complex parity
transitions between dipolar and quadrupolar modes. We compute the turbulent
transport coefficients, describing the effects of the turbulent velocity field
on the mean magnetic field, using the test-field method. The test-field
analysis provides a plausible explanation of the missing cycle in terms of the
reduction of in advance of the reduced surface activity,
and enhanced downward turbulent pumping during the event to confine some of the
magnetic field at the bottom of the convection zone, where local maximum of
magnetic energy is observed during the event. At the same time, however, a
quenching of the turbulent magnetic diffusivities is observed, albeit
differently distributed in depth compared to the other transport coefficients.
Therefore, dedicated mean-field modelling is required for verification.Comment: 11 pages, 12 figures, accepted by AN for 14th Potsdam Thinksho
Deep conservation of histone variants in Thermococcales archaea
Histones are ubiquitous in eukaryotes where they assemble into nucleosomes, binding and wrapping DNA to form chromatin. One process to modify chromatin and regulate DNA accessibility is the replacement of histones in the nucleosome with paralogous variants. Histones are also present in archaea but whether and how histone variants contribute to the generation of different physiologically relevant chromatin states in these organisms remains largely unknown. Conservation of paralogs with distinct properties can provide prima facie evidence for defined functional roles. We recently revealed deep conservation of histone paralogs with different properties in the Methanobacteriales, but little is known experimentally about these histones. In contrast, the two histones of the model archaeon Thermococcus kodakarensis, HTkA and HTkB, have been examined in some depth, both in vitro and in vivo. HTkA and HTkB exhibit distinct DNA-binding behaviours and elicit unique transcriptional responses when deleted. Here, we consider the evolution of HTkA/B and their orthologs across the order Thermococcales. We find histones with signature HTkA- and HTkB-like properties to be present in almost all Thermococcales genomes. Phylogenetic analysis indicates the presence of one HTkA- and one HTkB-like histone in the ancestor of Thermococcales and long-term maintenance of these two paralogs throughout Thermococcales diversification. Our results support the notion that archaea and eukaryotes have convergently evolved histone variants that carry out distinct adaptive functions. Intriguingly, we also detect more highly diverged histone-fold proteins, related to those found in some bacteria, in several Thermococcales genomes. The functions of these bacteria-type histones remain unknown, but structural modelling suggests that they can form heterodimers with HTkA/B-like histones
Data-driven simulations of magnetic field evolution in Active Region 11429: Magneto-friction method using PENCIL code
Coronal magnetic fields evolve quasi statically over long time scales and
dynamically over short time scales. As of now there exists no regular
measurements of coronal magnetic fields, and therefore generating the coronal
magnetic field evolution using the observations of the magnetic field at the
photosphere is of fundamental requirement to understand the origin of the
transient phenomena from the solar active regions. Using the magnetofriction
(MF) approach, we aim to simulate the coronal field evolution in the solar
active region 11429. The MF method is implemented in open source \PC along with
a driver module to drive the initial field with different boundary conditions
prescribed from observed vector magnetic fields at the photosphere. In order to
work with vector potential and the observations, we prescribe three types of
bottom boundary drivers with varying free-magnetic energy. The MF simulation
reproduces the magnetic structure, which better matches to the sigmoidal
morphology exhibited by AIA images at the pre-eruptive time. We found that the
already sheared field further driven by the sheared magnetic field, will
maintain and further build the highly sheared coronal magnetic configuration,
as seen in AR 11429. Data-driven MF simulation is a viable tool to generate the
coronal magnetic field evolution, capturing the formation of the twisted flux
rope and its eruption.Comment: 15 pages, 5 figures, to appear in Research in Astronomy and
Astrophysic
Metagenomic and functional analysis of hindgut microbiota of a wood-feeding higher termite
From the standpoints of both basic research and biotechnology, there is considerable interest in reaching a clearer understanding of the diversity of biological mechanisms employed during lignocellulose degradation. Globally, termites are an extremely successful group of wood-degrading organisms and are therefore important both for their roles in carbon turnover in the environment and as potential sources of biochemical catalysts for efforts aimed at converting wood into biofuels. Only recently have data supported any direct role for the symbiotic bacteria in the gut of the termite in cellulose and xylan hydrolysis. Here we use a metagenomic analysis of the bacterial community resident in the hindgut paunch of a wood-feeding ‘higher’ Nasutitermes species (which do not contain cellulose-fermenting protozoa) to show the presence of a large, diverse set of bacterial genes for cellulose and xylan hydrolysis. Many of these genes were expressed in vivo or had cellulase activity in vitro, and further analyses implicate spirochete and fibrobacter species in gut lignocellulose degradation. New insights into other important symbiotic functions including H_2 metabolism, CO_2-reductive acetogenesis and N_2 fixation are also provided by this first system-wide gene analysis of a microbial community specialized towards plant lignocellulose degradation. Our results underscore how complex even a 1-μl environment can be
Рекомендации по ограничению динамических перенапряжений в обмотке ротора асинхронизированного турбогенератора
In this paper, a Volume-of-Fluid (VOF)-based approach for the Direct Numerical Simulation (DNS) of reactive mass transfer in gas–liquid flows is described. At the interface, local thermodynamic equilibrium is assumed and modelled by Henry's law. First numerical simulation results are presented for non-reactive and reactive mass transfer from rising gas bubbles to a surrounding liquid. For the evaluation of reactive mass transfer simulations with a consecutive, competitive reaction system in the liquid, a local selectivity is employed
Dynamo generated field emergence through recurrent plasmoid ejections
Magnetic buoyancy is believed to drive the transport of magnetic flux tubes
from the convection zone to the surface of the Sun. The magnetic fields form
twisted loop-like structures in the solar atmosphere. In this paper we use
helical forcing to produce a large-scale dynamo-generated magnetic field, which
rises even without magnetic buoyancy. A two layer system is used as
computational domain where the upper part represents the solar atmosphere.
Here, the evolution of the magnetic field is solved with the stress--and--relax
method. Below this region a magnetic field is produced by a helical forcing
function in the momentum equation, which leads to dynamo action. We find
twisted magnetic fields emerging frequently to the outer layer, forming
arch-like structures. In addition, recurrent plasmoid ejections can be found by
looking at space--time diagrams of the magnetic field. Recent simulations in
spherical coordinates show similar results.Comment: 4 pages, 8 figures, To appear in the proceedings of the IAU273
"Physics of Sun and Star Spots
Characterisation of a Coriolis flow meter for fuel consumption measurements in realistic drive cycle tests
When testing light-duty and heavy-duty vehicles on chassis dynamometers, as in the WLTP, or engines on engine test benches, as in the WHDC, it is required to measure the fuel consumption. In the preferable case, the measurement of the fuel consumption is carried out with suitable flow meters. These require high measurement accuracy in a wide flow range, independent of the fuel type, as the flow rate range is often very large and depends on the power range of the vehicle engines. Moreover, the fuel flow rate in the test cycles is very dynamically related to the loads. In the scope of the ongoing EMPIR Joint Research Project 20IND13 SAFEST the dynamic flow behaviour as well as the measurement accuracy of flow meters for different types of fuels are investigated. This paper presents first results from the realisation of dynamic flow profiles, and flow measurements with a Coriolis Flow Meter with different representative fuels in a wide density and viscosity range and a wide flow rate range at different fuel temperatures
Исследование спектральных свойств когерентного дифракционного излучения от периодических структур
При пролете электрона вблизи периодической структуры генерируется поляризационное излучение, называемое дифракционным излучением от периодических структур (grating diffraction radiation). При пролете электрона параллельно решетке возникает хорошо изученное излучение Смита-Парселла. Однако при непараллельном пролёте также может возникать излучение. В данной работе проводится анализ спектральных характеристик когерентного дифракционного излучения от решеток, экспериментально полученных на ускорителе KEK-LUCX (г. Цукуба, Япония). Представлены спектры излучения, получаемые при разных углах ориентации решетки. Полученные спектральные характеристики сравниваются с теоретически рассчитанными.When an electron passes close to the periodic structure polarization radiation is generated, called diffraction radiation from periodic structures (grating diffraction radiation). When an electron passes parallel to the grating, well-studied Smith-Purcell radiation arises. However, in the case of a nonparallel flight, radiation can also occur. In this work, we analyze the spectral characteristics of coherent diffraction radiation from gratings, experimentally obtained at the accelerator KEK-LUCX (Tsukuba, Japan). Radiation spectrums obtained at different angles of grating orientation are presented. The obtained spectral characteristics are compared with theoretically calculated
Turbulent dynamo with advective magnetic helicity flux
Many astrophysical bodies harbor magnetic fields that are thought to be
sustained by a dynamo process. However, it has been argued that the production
of large-scale magnetic fields by mean-field dynamo action is strongly
suppressed at large magnetic Reynolds numbers owing to the conservation of
magnetic helicity. This phenomenon is known as {\it catastrophic quenching}.
Advection of magnetic fields by stellar and galactic winds toward the outer
boundaries and away from the dynamo is expected to alleviate such quenching.
Here we explore the relative roles played by advective and turbulent--diffusive
fluxes of magnetic helicity in the dynamo. In particular, we study how the
dynamo is affected by advection. We do this by performing direct numerical
simulations of a turbulent dynamo of type driven by forced
turbulence in a Cartesian domain in the presence of a flow away from the
equator where helicity changes sign. Our results indicate that in the presence
of advection, the dynamo, otherwise stationary, becomes oscillatory. We confirm
an earlier result for turbulent--diffusive magnetic helicity fluxes that for
small magnetic Reynolds numbers (\Rm\lesssim 100...200, based on the
wavenumber of the energy-carrying eddies) the magnetic helicity flux scales
less strongly with magnetic Reynolds number (\Rm^{-1/2}) than the term
describing magnetic helicity destruction by resistivity (\Rm^{-1}). Our new
results now suggest that for larger \Rm the former becomes approximately
independent of \Rm, while the latter falls off more slowly. We show for the
first time that both for weak and stronger winds, the magnetic helicity flux
term becomes comparable to the resistive term for \Rm\gtrsim 1000, which is
necessary for alleviating catastrophic quenching.Comment: 9 pages, 9 figures, accepted for publication in MNRA
Three-dimensional imaging of intracochlear tissue by scanning laser optical tomography (SLOT)
The presented study focuses on the application of scanning laser optical tomography (SLOT) for non-destructive visualization of anatomical structures inside the human cochlea ex vivo. SLOT is a laser-based highly efficient microscopy technique, which allows for tomographic imaging of the internal structure of transparent large-scale specimens (up to 1 cm3). Thus, in the field of otology this technique is best convenient for an ex vivo study of the inner ear anatomy. For this purpose, the preparation before imaging comprises mechanically assisted decalcification, dehydration as well as optical clearing of the cochlea samples. Here, we demonstrate results of SLOT visualizing hard and soft tissue structures of the human cochlea with an optical resolution in the micrometer range using absorption and autofluorescence as contrast mechanisms. Furthermore, we compare our results with the method of X-ray micro tomography (micro-CT, μCT) as clinical gold standard which is based only on absorption. In general, SLOT can provide the advantage of covering all contrast mechanisms known from other light microscopy techniques, such as fluorescence or scattering. For this reason, a protocol for antibody staining has been developed, which additionally enables selective mapping of cellular structures within the cochlea. Thus, we present results of SLOT imaging rodent cochleae showing specific anatomical structures such as hair cells and neurofilament via fluorescence. In conclusion, the presented study has shown that SLOT is an ideally suited tool in the field of otology for in toto visualization of the inner ear microstructure. © 2016 SPIE
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