717 research outputs found
Role of soft-iron impellers on the mode selection in the VKS dynamo experiment
A crucial point for the understanding of the von-K\'arm\'an-Sodium (VKS)
dynamo experiment is the influence of soft-iron impellers. We present numerical
simulations of a VKS-like dynamo with a localized permeability distribution
that resembles the shape of the flow driving impellers. It is shown that the
presence of soft-iron material essentially determines the dynamo process in the
VKS experiment. % An axisymmetric magnetic field mode can be explained by the
combined action of the soft-iron disk and a rather small -effect
parametrizing the induction effects of unresolved small scale flow
fluctuations
Triadic resonances in non-linear simulations of a fluid flow in a precessing cylinder
We present results from three-dimensional non-linear hydrodynamic simulations
of a precession driven flow in cylindrical geometry. The simulations are
motivated by a dynamo experiment currently under development at
Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in which the possibility of
generating a magnetohydrodynamic dynamo will be investigated in a cylinder
filled with liquid sodium and simultaneously rotating around two axes. In this
study, we focus on the emergence of non-axisymmetric time-dependent flow
structures in terms of inertial waves which - in cylindrical geometry - form
so-called Kelvin modes. For a precession ratio
the amplitude of the forced Kelvin mode
reaches up to one fourth of the rotation velocity of the cylindrical container
confirming that precession provides a rather efficient flow driving mechanism
even at moderate values of . More relevant for dynamo action might
be free Kelvin modes with higher azimuthal wave number. These free Kelvin modes
are triggered by non-linear interactions and may constitute a triadic resonance
with the fundamental forced mode when the height of the container matches their
axial wave lengths. Our simulations reveal triadic resonances at aspect ratios
close to those predicted by the linear theory except around the primary
resonance of the forced mode. In that regime we still identify various free
Kelvin modes, however, all of them exhibit a retrograde drift around the
symmetry axis of the cylinder and none of them can be assigned to a triadic
resonance. The amplitudes of the free Kelvin modes always remain below the
forced mode but may reach up to 6% of the of the container's angular velocity.
The properties of the free Kelvin modes will be used in future simulations of
the magnetic induction equation to investigate their ability to provide for
dynamo action.Comment: 26 pages, 14 figures, submitted to New J. Phy
Towards a precession driven dynamo experiment
The most ambitious project within the DREsden Sodium facility for DYNamo and
thermohydraulic studies (DRESDYN) at Helmholtz-Zentrum Dresden-Rossendorf
(HZDR) is the set-up of a precession-driven dynamo experiment. After discussing
the scientific background and some results of water pre-experiments and
numerical predictions, we focus on the numerous structural and design problems
of the machine. We also outline the progress of the building's construction,
and the status of some other experiments that are planned in the framework of
DRESDYN.Comment: 9 pages, 6 figures, submitted to Magnetohydrodynamic
a retrospective study
Introduction Emergency treatment of major sub-/total traumatic amputations
continue to represent a clinical challenge due to high infection rates and
serious handicaps. Effective treatment is based on two columns: surgery and
antimicrobial therapy. Detailed identification of pathogen spectrum and
epidemiology associated with these injuries is of tremendous importance as it
guides the initial empiric antibiotic regimen and prevents adverse septic
effents. Methods In this retrospective study 51 patients with major traumatic
amputations (n = 16) and subtotal amputations (n = 35) treated from 2001 to
2010 in our trauma center were investigated. All patients received emergency
surgery, debridement with microbiological testing within 6 h after admission
and empircic antimicrobial therapy. Additionally to baseline patient
characteristics, the incidence of positive standardized microbiologic testing
combined with clinical signs of infection, pathogen spectrum, administered
antimicrobial agents and clinical complications were analyzed. Results 70.6%
of the patients (n = 36) acquired wound infection. In 39% wounds were
contaminated on day 1, whereas the mean length of duration until first
pathogen detection was 9.1 ± 13.4 days after injury. In 37% polymicrobial
colonization and 28% Pseudomonas were responsible for wound infections during
hospitalization. In 45% the empirc antimicrobial therapy focussed on Gram
positive strains did not cover the detected bacteria, according antimicrobial
resistogram. It was significantly more often found in infections associated
with Pseudomonas (p 0.02) or polymicrobial wound infections. Conclusions This
epidemiologic study reveals a pathogen shift from Gram-positive to Gram-
negative strains with high incidence of Pseudomonas and polymicrobial
infections in sub-/total major traumatic amputations. Therefore, empiric
antimicrobial treatment historically focussing on Gram-positive strains must
be adjusted. We recommend the use of Piperacillin/Tazobactam for these
injuries. As soon as possible antimicrobial treatment should be changed from
empiric to goal directed therapy according to the microbiological tests and
resistogram results
Impact of high prevalence of pseudomonas and polymicrobial gram-negative infections in major sub-/total traumatic amputations on empiric antimicrobial therapy: a retrospective study
INTRODUCTION: Emergency treatment of major sub-/total traumatic amputations continue to represent a clinical challenge due to high infection rates and serious handicaps. Effective treatment is based on two columns: surgery and antimicrobial therapy. Detailed identification of pathogen spectrum and epidemiology associated with these injuries is of tremendous importance as it guides the initial empiric antibiotic regimen and prevents adverse septic effents. METHODS: In this retrospective study 51 patients with major traumatic amputations (n = 16) and subtotal amputations (n = 35) treated from 2001 to 2010 in our trauma center were investigated. All patients received emergency surgery, debridement with microbiological testing within 6 h after admission and empircic antimicrobial therapy. Additionally to baseline patient characteristics, the incidence of positive standardized microbiologic testing combined with clinical signs of infection, pathogen spectrum, administered antimicrobial agents and clinical complications were analyzed. RESULTS: 70.6% of the patients (n = 36) acquired wound infection. In 39% wounds were contaminated on day 1, whereas the mean length of duration until first pathogen detection was 9.1 ± 13.4 days after injury. In 37% polymicrobial colonization and 28% Pseudomonas were responsible for wound infections during hospitalization. In 45% the empirc antimicrobial therapy focussed on Gram positive strains did not cover the detected bacteria, according antimicrobial resistogram. It was significantly more often found in infections associated with Pseudomonas (p 0.02) or polymicrobial wound infections. CONCLUSIONS: This epidemiologic study reveals a pathogen shift from Gram-positive to Gram-negative strains with high incidence of Pseudomonas and polymicrobial infections in sub-/total major traumatic amputations. Therefore, empiric antimicrobial treatment historically focussing on Gram-positive strains must be adjusted. We recommend the use of Piperacillin/Tazobactam for these injuries. As soon as possible antimicrobial treatment should be changed from empiric to goal directed therapy according to the microbiological tests and resistogram results
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