44 research outputs found
Quenched noise and over-active sites in sandpile dynamics
The dynamics of sandpile models are mapped to discrete interface equations.
We study in detail the Bak-Tang-Wiesenfeld model, a stochastic model with
random thresholds, and the Manna model. These are, respectively,
discretizations of the quenched Edwards-Wilkinson equation with columnar,
point-like and correlated noise, with the constraint that the interface
velocity is either zero or exactly one. The constraint, embedded in the
sandpile rules, gives rise to another noise component. This term has for the
Bak-Tang-Wiesenfeld model long-range on-site correlations and reveals that with
open boundary conditions there is no spatial translational invariance.Comment: 4 pages, 3 figure
Noisy Kuramoto-Sivashinsky equation for an erosion model
We derive the continuum equation for a discrete model for ion sputtering. We
follow an approach based on the master equation, and discuss how it can be
truncated to a Fokker-Planck equation and mapped to a discrete Langevin
equation. By taking the continuum limit, we arrive at the Kuramoto-Sivashinsky
equation with a stochastic noise term.Comment: latex (w/ multicol.sty), 4 pages; to appear in Physical Review E (Oct
1996
Self-organized criticality in a rice-pile model
We present a new model for relaxations in piles of granular material. The
relaxations are determined by a stochastic rule which models the effect of
friction between the grains. We find power-law distributions for avalanche
sizes and lifetimes characterized by the exponents and
, respectively. For the discharge events, we find a
characteristic size that scales with the system size as , with . We also find that the frequency of the discharge events
decrease with the system size as with .Comment: 4 pages, RevTex, multicol, epsf, rotate (sty files provided). To
appear Phys. Rev. E Rapid Communication (Nov or Dec 96
Universality classes for rice-pile models
We investigate sandpile models where the updating of unstable columns is done
according to a stochastic rule. We examine the effect of introducing nonlocal
relaxation mechanisms. We find that the models self-organize into critical
states that belong to three different universality classes. The models with
local relaxation rules belong to a known universality class that is
characterized by an avalanche exponent , whereas the models
with nonlocal relaxation rules belong to new universality classes characterized
by exponents and . We discuss the values
of the exponents in terms of scaling relations and a mapping of the sandpile
models to interface models.Comment: 4 pages, including 3 figure
Surface Critical Behavior in Systems with Non-Equilibrium Phase Transitions
We study the surface critical behavior of branching-annihilating random walks
with an even number of offspring (BARW) and directed percolation (DP) using a
variety of theoretical techniques. Above the upper critical dimensions d_c,
with d_c=4 (DP) and d_c=2 (BARW), we use mean field theory to analyze the
surface phase diagrams using the standard classification into ordinary,
special, surface, and extraordinary transitions. For the case of BARW, at or
below the upper critical dimension, we use field theoretic methods to study the
effects of fluctuations. As in the bulk, the field theory suffers from
technical difficulties associated with the presence of a second critical
dimension. However, we are still able to analyze the phase diagrams for BARW in
d=1,2, which turn out to be very different from their mean field analog.
Furthermore, for the case of BARW only (and not for DP), we find two
independent surface beta_1 exponents in d=1, arising from two distinct
definitions of the order parameter. Using an exact duality transformation on a
lattice BARW model in d=1, we uncover a relationship between these two surface
beta_1 exponents at the ordinary and special transitions. Many of our
predictions are supported using Monte-Carlo simulations of two different models
belonging to the BARW universality class.Comment: 19 pages, 12 figures, minor additions, 1 reference adde
Catheter Directed Thrombolysis for Treatment of Ilio-femoral Deep Venous Thrombosis is Durable, Preserves Venous Valve Function and May Prevent Chronic Venous Insufficiency
AbstractObjectivesTo investigate the results of catheter directed thrombolysis offered to patients with acute femoro-iliac deep venous thrombosis (DVT).DesignRetrospective analysis of all patients treated with this modality at Gentofte Hospital until December 2003.MaterialForty-five consecutive patients treated between June 1999 and December 2003 with a median age of 31 years. All patients had femoro-iliac DVT with an average anamnesis of 6 days.MethodsAll patients were treated by catheter directed infusion of alteplase into the popliteal vein. After thrombolysis residual venous stenoses were treated by percutaneous balloon angioplasty (PTA) and stenting. Patients were followed with color-duplex scanning for assessment of venous patency and reflux.ResultsForty-two of 45 (93%) of cases were treated successfully with reopening of the thrombosed vein segments. In 30 of 45 cases a residual stenosis was treated by PTA and stenting. Only one serious complication was observed: Compartment syndrome of the forearm where arterial punctures had been taken. After an average of 24 months follow-up were no cases of re-thrombosis among the 42 patients discharged with open veins. Only two of 41 with presumed normal venous valve function prior to DVT developed reflux during follow-up.ConclusionIn this selected patient group, catheter directed thrombolysis seems effective in treating acute DVT, it appears durable and preserves venous valve function in the majority. The method needs to be tested in a randomised controlled trial