926 research outputs found
Viscous accretion discs around rotating black holes
The stationary hydrodynamic equations for transonic viscous accretion discs
in Kerr geometry are derived. The consistent formulation is given for the
viscous angular momentum transport and the boundary conditions on the horizon
of a central black hole. An expression for the thickness of the disc is
obtained from the vertical Euler equation for general accretion flows with
vanishing vertical velocity. Different solution topologies are identified,
characterized by a sonic transition close to or far from the marginally stable
orbit. A numerical method is presented that allows to integrate the structure
equations of transonic accretion flows. Global polytropic solutions for the
disc structure are calculated, covering each topology and a wide range of
physical conditions. These solutions generally possess a sub-Keplerian angular
momentum distribution and have maximum temperatures in the range
K. Accretion discs around rotating black holes are hotter and
deposit less angular momentum on the central object than accretion discs around
Schwarzschild black holes.Comment: 15 pages LateX, requires mn.sty, accepted for publication in the
MNRAS, figures available at http://www.lsw.uni-heidelberg.de/~jpeitz/PV
Pressure-driven Instabilities in Cylindrical Geometry: A New General Criterion
A new criterion for pressure-driven interchange instabilities in cylindrical
geometry is derived, based on an alternate use of the Energy Principle. This
criterion is inequivalent to Suydam's criterion and does not contain the
magnetic shear. In fact, it is shown that Suydam's criterion relates to the
instability of the slow magnetosonic branch, while the present criterion
relates to the Alfv\'enic one, which is the most dangerous of the two. These
findings explain why pressure-driven modes nearly always exist even if Suydam's
criterion is satisfied by a large margin.Comment: 4 pages. Submitted to Phys. Rev. Let
3+1 formulation of non-ideal hydrodynamics
The equations governing dissipative relativistic hydrodynamics are formulated
within the 3+1 approach for arbitrary spacetimes. Dissipation is accounted for
by applying the theory of extended causal thermodynamics (Israel-Stewart
theory). This description eliminates the causality violating infinite signal
speeds present in the conventional Navier-Stokes equation. As an example we
treat the astrophysically relevant case of stationary and axisymmetric
spacetimes, including the Kerr metric. The equations take a simpler form
whenever the inertia due to the dissipative contributions can be neglected.Comment: 24 pages, LateX, uses mn.sty and AMS fonts, no figures, accepted for
publication in the MNRAS, also available via
http://www.lsw.uni-heidelberg.de/~jpeitz/Personal.htm
Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds
Two three-dimensional magnetohydrodynamical simulations of strongly
magnetized conical jets, one with a poloidal and one with a helical magnetic
field, have been performed. In the poloidal simulation a significant sheath
(wind) of magnetized moving material developed and partially stabilized the jet
to helical twisting. The fundamental pinch mode was not similarly affected and
emission knots developed in the poloidal simulation. Thus, astrophysical jets
surrounded by outflowing winds could develop knotty structures along a straight
jet triggered by pinching. Where helical twisting dominated the dynamics,
magnetic field orientation along the line-of-sight could be organized by the
toroidal flow field accompanying helical twisting. On astrophysical jets such
structure could lead to a reversal of the direction of Faraday rotation in
adjacent zones along a jet. Theoretical analysis showed that the different
dynamical behavior of the two simulations could be entirely understood as a
result of dependence on the velocity shear between jet and wind which must
exceed a surface Alfven speed before the jet becomes unstable to helical and
higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September
Plasma physics in clusters of galaxies
Clusters of galaxies are the largest self-gravitating structures in the
universe. Each cluster is filled with a large-scale plasma atmosphere, in which
primordial matter is mixed with matter that has been processed inside stars.
This is a wonderful plasma physics laboratory. Our diagnostics are the data we
obtain from X-ray and radio telescopes. The thermal plasma is a strong X-ray
source; from this we determine its density and temperature. Radio data reveal a
relativistic component in the plasma, and first measurements of the
intracluster magnetic field have now been made. Energization of the particles
and the field must be related to the cosmological evolution of the cluster. The
situation is made even richer by the few galaxies in each cluster which host
radio jets. In these galaxies, electrodynamics near a massive black hole in the
core of the galaxy lead to a collimated plasma beam which propagates from the
nucleus out to supergalactic scales. These jets interact with the cluster
plasma to form the structures known as radio galaxies. The interaction disturbs
and energizes the cluster plasma. This complicates the story but also helps us
understand both the radio jets and the cluster plasma.Comment: 12 pages, 6 figures, 3 in color. Invited review, to appear in Physics
of Plasmas, May 2003. After publication it can be found at
http://ojps.aip.org/po
Two-dimensional vortex behavior in highly underdoped YBa_2Cu_3O_{6+x} observed by scanning Hall probe microscopy
We report scanning Hall probe microscopy of highly underdoped superconducting
YBa_2Cu_3O_{6+x} with T_c ranging from 5 to 15 K which showed distinct flux
bundles with less than one superconducting flux quantum (Phi_0) through the
sample surface. The sub-Phi_0 features occurred more frequently for lower T_c,
were more mobile than conventional vortices, and occurred more readily when the
sample was cooled with an in-plane field component. We show that these features
are consistent with kinked stacks of pancake vortices.Comment: 11 pages, 8 figures, accepted for publication in Physical Review
A new digital twin for enzymatic hydrolysis processes applied to model-based process design
Renewable raw materials containing starch and proteins are split into their main components using enzymatic hydrolysis processes. However, even small changes in temperature, pH or pressure may strongly affect the enzyme activity and stability. At the same time, natural fluctuations may lead to changes in the substrate composition. These mutually influencing factors place enormous demands on the design and control of enzymatic hydrolysis processes.
Individual enzymatic hydrolysis processes have already been modelled, but models for the hydrolysis of potato starch by α-amylase and glucoamylase and the proteolysis of organic sunflower seed meal by endopeptidase and exopeptidase in a stirred tank reactor, or even digital twins, are unavailable. Therefore, a new mechanistic model for the combined starch hydrolysis and proteolysis was developed.
Sigmoidal and double sigmoidal functions were implemented to map the temperature and pH-dependent enzyme activity. The model can simulate the enzymatic hydrolysis processes with an agreement of more than 90%. The new model was integrated into an existing digital twin of a 20 L stirred tank reactor to create a new stand-alone digital twin for enzymatic hydrolysis processes.
Applying the new digital twin core model, a model-based process design strategy based on the open-loop-feedback-optimal and model-based design of experiment strategies was established. By applying the new strategy, the amount of α-amylase and glucoamylase required for starch hydrolysis could be reduced by more than 30%. In addition, the required amount of endopeptidase and exopeptidase for proteolysis could be reduced by more than 50%. Compared to the classic design of experiments approach, the number of experiments required for process optimisation could be reduced by more than 50%.
The strategies resulting from this work can soon be used for the optimisation of the industrial organic nutrient media production from regenerative substrates for the cultivation of microorganisms such as Saccharomyces cerevisiae
On the Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets
Three-dimensional magnetohydrodynamical simulations of strongly magnetized
``light'' conical jets have been performed. An investigation of the transition
from sub-Alfv\'enic to super-Alfv\'enic flow has been made for nearly poloidal
and for helical magnetic fields. The jets are stable to asymmetric modes of jet
distortion provided they are sub-Alfv\'enic over most of their interior but
destabilize rapidly when they become on average super-Alfv\'enic. The jets are
precessed at the origin and the resulting small amplitude azimuthal motion is
communicated down the jet to the Alfv\'en point where it couples to a slowly
moving and rapidly growing helical twist. Significant jet rotation can
contribute to destabilization via increase in the velocity shear between the
jet and the external medium. Destabilization is accompanied by significant mass
entrainment and the jets slow down significantly as denser external material is
entrained. Synchrotron intensity images satisfactorily reveal large scale
helical structures but have trouble distinguishing a large amplitude elliptical
jet distortion that appears as an apparent pinching in an intensity image.
Smaller scale jet distortions are not clearly revealed in intensity images,
largely as a result of the relatively small total pressure variations that
accompany destabilization and growing distortions. Fractional polarization is
high as a result of the strong ordered magnetic fields except where the
intensity image suggests cancellation of polarization vectors by integration
through twisted structures.Comment: 27 pages, 11 figures, AASTeX, to appear in Oct 20 issue of ApJ,
postscript versions of Figures 5 and 6 are available at this URL
http://crux.astr.ua.edu/~rosen/tralf/hr.htm
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