76 research outputs found
Critical exponents in zero dimensions
In the vicinity of the onset of an instability, we investigate the effect of
colored multiplicative noise on the scaling of the moments of the unstable mode
amplitude. We introduce a family of zero dimensional models for which we can
calculate the exact value of the critical exponents for all the
moments. The results are obtained through asymptotic expansions that use the
distance to onset as a small parameter. The examined family displays a variety
of behaviors of the critical exponents that includes anomalous exponents:
exponents that differ from the deterministic (mean-field) prediction, and
multiscaling: non-linear dependence of the exponents on the order of the
moment
DNS of vertical plane channel flow with finite-size particles: Voronoi analysis, acceleration statistics and particle-conditioned averaging
We have performed a direct numerical simulation of dilute turbulent
particulate flow in a vertical plane channel, fully resolving the phase
interfaces. The flow conditions are the same as those in the main case of
"Uhlmann, M., Phys. Fluids, vol. 20, 2008, 053305", with the exception of the
computational domain length which has been doubled in the present study. The
statistics of flow and particle motion are not significantly altered by the
elongation of the domain. The large-scale columnar-like structures which had
previously been identified do persist and they are still only marginally
decorrelated in the prolonged domain. Voronoi analysis of the spatial particle
distribution shows that the state of the dispersed phase can be characterized
as slightly more ordered than random tending towards a homogeneous spatial
distribution. It is also found that the p.d.f.'s of Lagrangian particle
accelerations for wall-normal and spanwise directions follow a lognormal
distribution as observed in previous experiments of homogeneous flows. The
streamwise component deviates from this law presenting significant skewness.
Finally, a statistical analysis of the flow in the near field around the
particles reveals that particle wakes present two regions, a near wake where
the velocity deficit decays as 1/x and a far wake with a decay of approximately
1/(x*x).Comment: accepted for publication in Int. J. Multiphase Flo
The integral equation approach to kinematic dynamo theory and its application to dynamo experiments in cylindrical geometry
The conventional magnetic induction equation that governs hydromagnetic
dynamo action is transformed into an equivalent integral equation system. An
advantage of this approach is that the computational domain is restricted to
the region occupied by the electrically conducting fluid and to its boundary.
This integral equation approach is first employed to simulate kinematic dynamos
excited by Beltrami-like flows in a finite cylinder. The impact of externally
added layers around the cylinder on the onset of dynamo actions is
investigated. Then it is applied to simulate dynamo experiments within
cylindrical geometry including the von Karman sodium (VKS) experiment and the
Riga dynamo experiment. A modified version of this approach is utilized to
investigate magnetic induction effects under the influence of externally
applied magnetic fields which is also important to measure the proximity of a
given dynamo facility to the self-excitation threshold.Comment: 22 pages, 14 figure
Generation of magnetic field by dynamo action in a turbulent flow of liquid sodium
We report the observation of dynamo action in the VKS experiment, i.e., the
generation of magnetic field by a strongly turbulent swirling flow of liquid
sodium. Both mean and fluctuating parts of the field are studied. The dynamo
threshold corresponds to a magnetic Reynolds number Rm \sim 30. A mean magnetic
field of order 40 G is observed 30% above threshold at the flow lateral
boundary. The rms fluctuations are larger than the corresponding mean value for
two of the components. The scaling of the mean square magnetic field is
compared to a prediction previously made for high Reynolds number flows.Comment: 4 pages, 5 figure
Dose repartition in alveoli, alveolar ducts and bronchi of rats exposed to radon and its progeny, preliminary results
Recently, micronuclei scoring has been performed to estimate doses delivered to the lungs after heterogenous irradiation induced after inhalation of radon and its progeny. These studies were limited to the deep lung after either extraction of alveolar macrophages (AM) by lavage (Taya et al., 1994, Johnson and Newton, 1994) or enzymatic dissociation of lung cells to initiate fibroblast cultures (Khan et al., 1994). Dose estimates were performed after comparison with micronuclei formation induced in vitro by irradiation with alpha particles
Learning categories with spiking nets and spike timing dependent plasticity
An exploratory study of learning a neural network for categorisation shows that commonly used leaky integrate and fire neurons and Hebbian learning can be effective. The system learns with a standard spike timing dependent plasticity Hebbian learning rule. A two layer feed forward topology is used with a presentation mechanism of inputs followed by outputs a simulated ms. later to learn Iris flower and Breast Cancer Tumour Malignancy categorisers. An exploration of parameters indicates how this may be applied to other tasks
Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma
Magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. However, experimental demonstration of the turbulent dynamo mechanism has remained elusive, since it requires plasma conditions that are extremely hard to re-create in terrestrial laboratories. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization
Pleuro-pulmonary tumours detected by clinical and chest X-ray analyses in rats transplanted with mesothelioma cells
New strategies for cancer therapy must be developed, especially in severe neoplasms such as malignant pleural mesothelioma. Animal models of cancer, as close as possible to the human situation, are needed to investigate novel therapeutical approaches. Orthotopic transplantation of cancer cells is then relevant and efforts should be made to follow up tumour evolution in animals. In the present study, we developed a method for the orthotopic growth of mesothelioma cells in the pleural cavity of Fischer 344 and nude rats, along with a procedure for clinical survey. Two mesothelioma cell lines, of rat and human origin, were inoculated by transthoracic puncture. Body weight determination and chest X-ray analyses permitted the follow-up of tumour evolution by identifying different stages. Autopsies showed that tumours localized on the whole pleural cavity (diaphragm, parietal pleura), mediastinum and pericardium. Tumour morphology and antigenic characteristics were consistent with those of the inoculated cells and were similar in both types of rats inoculated with the same cell type. These results demonstrate that mesothelioma formation in rats can be followed up by clinical and radiographic survey after gentle intrathoracic inoculation of mesothelioma cells, thus allowing the definition of stages of interest for further experimental trials. © 1999 Cancer Research Campaig
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