Dalibor Pražák

On Finite Fractal Dimension of the Global Attractor for the Wave Equation with Nonlinear Damping

We prove that the global attractor to a semilinear damped wave equation has finite fractal dimension provided that the damping function and the lower order nonlinearity are smooth with certain polynomial growth.

A necessary and sufficient condition for the existence of an exponential attractor

We give a necessary and sufficient condition for the existence of an exponential attractor. The condition is formulated in the context of metric spaces. It also captures the quantitative properties of the attractor, i.e., the dimension and the rate of attraction. As an application, we show that the evolution operator for the wave equation with nonlinear damping has an exponential attractor.

Longtime behavior of a diffuse interface model for binary fluid mixtures with shear dependent viscosity

We consider a system which describes the behavior of a binary mixture of immiscible incompressible fluids with shear dependent viscosity by means of the diffuse interface approach. This system consists of Navier–Stokes type equations, characterized by a nonlinear stress-strain law, which are nonlinearly coupled with a convective Cahn–Hilliard equation for the order parameter. We analyze the corresponding dynamical system and, by means of the short trajectory method, we prove the existence of global and exponential attractors. We also discuss the dependence of an upper bound of the fractal dimension on the physical parameters of the system.

Moving contact line of a volatile fluid

Interfacial flows close to a moving contact line are inherently multiscale. The shape of the interface and the flow at meso- and macroscopic scales inherit an apparent interface slope and a regularization length, both named after Voinov, from the microscopic inner region. Here, we solve the inner problem associated with the contact line motion for a volatile fluid at equilibrium with its vapor. The evaporation or condensation flux is then controlled by the dependence of the saturation temperature on interface curvature-the so-called Kelvin effect. We derive the dependencies of the Voinov angle and of the Voinov length as functions of the parameters of the problem. We then identify the conditions under which the Kelvin effect is indeed the mechanism regularizing the contact line motion.

On the dynamics of equations with infinite delay

We consider a system of ordinary differential equations with infinite delay. We study large time dynamics in the phase space of functions with an exponentially decaying weight. The existence of an exponential attractor is proved under the abstract assumption that the right-hand side is Lipschitz continuous. The dimension of the attractor is explicitly estimated.

Regularity Results for a Cahn-Hilliard-Navier-Stokes System with Shear Dependent Viscosity

We analyze a diffuse interface model describing the behavior of a mixture of two incompressible fluids. More precisely, we consider Navier-Stokes type equations with power-law like shear dependent viscosity. Such equations are nonlinearly coupled with a convective Cahn-Hilliard equation for the order parameter. The resulting system is endowed with no-slip and no-flux boundary conditions. We prove some regularity properties of weak solutions under rather general conditions. This is a generalization of previous results already proven for single fluids. Some consequences of such results are also addressed.

EXPONENTIAL ATTRACTORS FOR ABSTRACT PARABOLIC SYSTEMS WITH BOUNDED DELAY

We show that a suitable adaptation of the so-called method of trajectories can be usedto construc atn exponential attracto for r a very general clas of nonlineas r reaction-diffusion systems wit ah bounde d delay.In particular, we assume that the dependence o thn e past histor iys controlled viaconvolution with a possibly singular measure. Assumin a priori thag tht e solutionsare bounded, a simple proof of the existence of an exponential attractor is given undervery little regularity requirements.

On Reducing the 2d Navier-Stokes Equations to a System of Delayed ODEs

We give a simple proof that projecting the 2d Navier-Stokes equations to sufficiently many eigenfunctions of the Stokes operator leads to a system of delayed ODEs. The proof is based on the repeated use of the so-called squeezing property. The reduced system is uniquely solvable and dissipative. Moreover, the solutions on the attractor to the full NSEs are in one-to-one correspondence to the solutions on a compact, invariant subset to a global attractor of the reduced system.

A note on the dimension of the global attractor for an abstract semilinear hyperbolic problem

We study a semilinear hyperbolic problem, written as a second-order evolution equation in an infinite-dimensional Hilbert space. Assuming existence of the global attractor, we estimate its fractal dimension explicitly in terms of the data. Despite its elementary character, our technique gives reasonable results. Notably, we require no additional regularity, although nonlinear damping is allowed.

Mechanical oscillators with dampers defined by implicit constitutive relations

We study the vibrations of lumped parameter systems, the spring being defined by the classical linear constitutive relationship between the spring force and the elongation while the dashpot is described by a general implicit relationship between the damping force and the velocity. We prove global existence of solutions for the governing equations, and discuss conditions that the implicit relation satisfies that are sufficient for the uniqueness of solutions. We also present some counterexamples to the uniqueness when these conditions are not met.