Correlation Decay and Recurrence Asymptotics for Some Robust Nonuniformly Hyperbolic Maps

We study a robust class of multidimensional non-uniformly hyperbolic transformations considered by Oliveira and Viana (Ergod. Theory Dyn. Syst. 28:501–533, 2008). For an open class of Hölder continuous potentials with small variation we show that the unique equilibrium state has exponential decay of correlations and that the distribution of hitting times is asymptotically exponential. Furthermore, using that the equilibrium states satisfy a weak Gibbs property we also prove log-normal fluctuations of the return times around their average.     

The interface migration in shear-banded micellar solutions

In this work, we study the diffusion of the interface between bands in wormlike micellar solutions that exhibit shear banding flow regimes, namely, systems undergoing coexistence of states of different shear rates along a constant stress plateau. The migration of the interface between bands possessing different birefringence levels is predicted by the BMP (Bautista-Manero-Puig) model in which a structural parameter (the fluidity) presents two states with differing order separated by an interface. The mechanical potential derived from the constitutive equations and a diffusion term for the structure evolution equation predict various time scales of interface migration at the inception of shear flow and under shear-rate changes along the plateau stress. It is shown that the extremes of the plateau (binodals) correspond to the minima in the mechanical potential as a function of fluidity or shear rate. We also predict the dependence of the diffusive length scale on the applied shear rate.     

Uniform flow of viscoelastic fluids past a confined falling cylinder

Uniform steady flow of viscoelastic fluids past a cylinder placed between two moving parallel plates is investigated numerically with a finite-volume method. This configuration is equivalent to the steady settling of a cylinder in a viscoelastic fluid, and here, a 50% blockage ratio is considered. Five constitutive models are employed (UCM, Oldroyd-B, FENE-CR, PTT and Giesekus) to assess the effect of rheological properties on the flow kinematics and wake patterns. Simulations were carried out under creeping flow conditions, using very fine meshes, especially in the wake of the cylinder where large normal stresses are observed at high Deborah numbers. Some of the results are compared with numerical data from the literature, mainly in terms of a drag coefficient, and significant discrepancies are found, especially for the constant-viscosity constitutive models. Accurate solutions could be obtained up to maximum Deborah numbers clearly in excess of those reported in the literature, especially with the PTT and FENE-CR models. The existence or not of a negative wake is identified for each set of model parameters.     

Flow of elasto-viscoplastic liquids through an axisymmetric expansion–contraction

In this work, we used a recently proposed constitutive model that addresses the elastic behavior of viscoplastic liquids. We used the equation in an expansion–contraction axisymmetric flow, and compared the results with experimental data found in the literature. Steady, inertialess numerical solutions were obtained by solving the balance equations of mass and momentum via the finite element method. We obtained the velocity and stress fields for different combinations of the governing parameters, and observed that elasticity leads to significant changes in the shape and position of the yield surface, affecting both the extra-stress and the rate of deformation fields in the cavity. The trends observed are in qualitative agreement with visualization results available in the literature.     

Group-invariant solutions for the Ricci curvature equation and the Einstein equation

We consider the pseudo-Euclidean space $({\mathbb{R}}^n,g)$, $n\ge 3$, with coordinates $x=(x_1,\dots ,x_n)$ and metric $g_{ij}={\delta }_{ij}{?}_i$, ${?}_i=\pm 1$, where at least one ${?}_i$ is positive, and also tensors of the form $A={\sum }_{i,j}{A}_{ij}d{x}_{i}d{x}_j$, such that $A_{ij}$ are differentiable functions of x. For such tensors, we use Lie point symmetries to find metrics $\stackrel{￣}{g}=\frac{1}{u^2}g$ that solve the Ricci curvature and the Einstein equations. We provide a large class of group-invariant solutions and examples of complete metrics $\stackrel{￣}{g}$ defined globally in ${\mathbb{R}}^n$. As consequences, for certain functions $\stackrel{￣}{K}$, we show complete metrics $\stackrel{￣}{g}$, conformal to the pseudo-Euclidean metric g, whose scalar curvature is $\stackrel{￣}{K}$.     

Gevrey global solvability of non-singular real first-order differential operators

In this article we deal with Gevrey global solvability of non-singular first-order operators defined on an n-dimensional s-Gevrey manifold, s > 1. As done by Duistermaat and Hörmander in the C ^∞ framework, we show that Gevrey global solvability is equivalent the existence of a global cross section.     

Hybrid stochastic approach for the modelling and analysis of fire safety systems

This paper approaches the problem of analysing control strategies for Fire Safety Systems. The components of Fire Safety Systems present behaviours of different nature and therefore the use of a hybrid modelling formalism is necessary. Petri net is used to model the discrete dynamics. Algebraic and differential equations are used for the continuous one. In order to realistically evaluate the performance of Fire Safety Systems, failures and other uncertainties, such as people’s behaviour, should be included in the model. Due to the model complexity, results are obtained by Monte Carlo simulation.     

Double-Regularization Proximal Methods, with Complementarity Applications

We consider the variational inequality problem formed by a general set-valued maximal monotone operator and a possibly unbounded “box” in , and study its solution by proximal methods whose distance regularizations are coercive over the box. We prove convergence for a class of double regularizations generalizing a previously-proposed class of Auslender et al. Using these results, we derive a broadened class of augmented Lagrangian methods. We point out some connections between these methods and earlier work on “pure penalty” smoothing methods for complementarity; this connection leads to a new form of augmented Lagrangian based on the “neural” smoothing function. Finally, we computationally compare this new kind of augmented Lagrangian to three previously-known varieties on the MCPLIB problem library, and show that the neural approach offers some advantages. In these tests, we also consider primal-dual approaches that include a primal proximal term. Such a stabilizing term tends to slow down the algorithms, but makes them more robust. This author was partially supported by CNPq, Grant PQ 304133/2004-3 and PRONEX-Optimization.     

On the Borel property for solutions to systems of complex vector fields

In this work we study the Borel property for smooth solutions to systems of complex vector fields associated to locally integrable structures. Inspired by the recent article [6], in which the Borel property was studied for generic submanifolds of the complex space, we prove similar results in this more general set up. In particular we obtain, for the case of corank one structures, a necessary and sufficient condition for the validity of the Borel property.