Wellposedness of the discontinuous ODE associated with two-phase flows

We consider the initial value problem \dot x (t) = v(t,x(t)) \;\mbox{ for } t\in (a,b), \;\; x(t_0)=x_0 which determines the pathlines of a two-phase flow, i.e.\ $v=v(t,x)$ is a given velocity field of the type $$v(t,x)= \begin{cases} v^+(t,x) &\text{ if } x \in \Omega^+(t)\\ v^-(t,x) &\text{ if } x \in \Omega^-(t) \end{cases}$$ with $\Omega^\pm (t)$ denoting the bulk phases of the two-phase fluid system under consideration. The bulk phases are separated by a moving and deforming interface $\Sigma (t)$. Since we allow for flows with phase change, these pathlines are allowed to cross or touch the interface. Imposing a kind of transversality condition at $\Sigma (t)$, which is intimately related to the mass balance in such systems, we show existence and uniqueness of absolutely continuous solutions of the above ODE in case the one-sided velocity fields $v^\pm:\overline{{\rm gr}(\Omega^\pm)}\to \mathbb{R}^n$ are continuous in $(t,x)$ and locally Lipschitz continuous in $x$. Note that this is a necessary prerequisite for the existence of well-defined co-moving control volumes for two-phase flows, a basic concept for mathematical modeling of two-phase continua

The determinants of public sector size: Theoretical approaches and empirical estimates for local government in the Federal Republic of Germany

The growth of government has become a global phenomenon which, over the years, has attracted a great deal of attention and continues to do so. This growth has not been uniform, neither in time nor in space, and it was the factors underlying these differences interest has focused on. No comprehensive theory, however, has yet emerged from the prolific and varied literature. Instead, a number of approaches was developed, each of them an incomplete explanation of a complex phenomenon. A problem common to all of them when it comes to testing hypotheses empirically, is the measurement of total public sector economic activity. For lack of data, it is usually approximated by public expenditures. Most probably, this understates the role of government in economic life, since many of its activities, while unrecorded in the budget, redirect resources just as taxation and public spending do. Typical examples are consumer and worker safety regulation, public utility price and output regulation in certain industries, and tax expenditures.

Thermodynamically consistent modeling for dissolution/growth of bubbles in an incompressible solvent

We derive mathematical models of the elementary process of dissolution/growth of bubbles in a liquid under pressure control. The modeling starts with a fully compressible version, both for the liquid and the gas phase so that the entropy principle can be easily evaluated. This yields a full PDE system for a compressible two-phase fluid with mass transfer of the gaseous species. Then the passage to an incompressible solvent in the liquid phase is discussed, where a carefully chosen equation of state for the liquid mixture pressure allows for a limit in which the solvent density is constant. We finally provide a simplification of the PDE system in case of a dilute solution

Global Strong Solutions for a Class of Heterogeneous Catalysis Models

We consider a mathematical model for heterogeneous catalysis in a finite three-dimensional pore of cylinder-like geometry, with the lateral walls acting as a catalytic surface. The system under consideration consists of a diffusion-advection system inside the bulk phase and a reaction-diffusion-sorption system modeling the processes on the catalytic wall and the exchange between bulk and surface. We assume Fickian diffusion with constant coefficients, sorption kinetics with linear growth bound and a network of chemical reactions which possesses a certain triangular structure. Our main result gives sufficient conditions for the existence of a unique global strong $L^2$-solution to this model, thereby extending by now classical results on reaction-diffusion systems to the more complicated case of heterogeneous catalysis.Comment: 30 page

Continuum thermodynamics of chemically reacting fluid mixtures

We consider viscous, heat conducting mixtures of molecularly miscible chemical species forming a fluid in which the constituents can undergo chemical reactions. Assuming a common temperature for all components, we derive a closed system of partial mass and partial momentum balances plus a mixture balance of internal energy. This is achieved by careful exploitation of the entropy principle and requires appropriate definitions of absolute temperature and chemical potentials, based on an adequate definition of thermal energy excluding diffusive contributions. The resulting interaction forces split into a thermo-mechanical and a chemical part, where the former turns out to be symmetric in case of binary interactions. For chemically reacting systems and as a new result, the chemical interaction force is a contribution being non-symmetric outside of chemical equilibrium. The theory also provides a rigorous derivation of the so-called generalized thermodynamic driving forces, avoiding the use of approximate solutions to the Boltzmann equations. Moreover, using an appropriately extended version of the entropy principle and introducing cross-effects already before closure as entropy invariant couplings between principal dissipative mechanisms, the Onsager symmetry relations become a strict consequence. With a classification of the factors in the binary products of the entropy production according to their parity--instead of the classical partition into so-called fluxes and driving forces--the apparent anti-symmetry of certain couplings is thereby also revealed. If the diffusion velocities are small compared to the speed of sound, the Maxwell-Stefan equations follow in the case without chemistry, thereby neglecting wave phenomena in the diffusive motion. This results in a reduced model with only mass being balanced individually. In the reactive case ..

Global existence for a class of reaction-diffusion systems with mass action kinetics and concentration-dependent diffusivities

In this work we study the existence of classical solutions for a class of reaction-diffusion systems with quadratic growth naturally arising in mass action chemistry when studying networks of reactions of the type $A_i+A_j \rightleftharpoons A_k$ with Fickian diffusion, where the diffusion coefficients might depend on time, space and on all the concentrations $c_i$ of the chemical species. In the case of one single reaction, we prove global existence for space dimensions $N\leq 5$. In the more restrictive case of diffusion coefficients of the type $d_i(c_i)$, we use an $L^2$-approach to prove global existence for $N\leq 9$. In the general case of networks of such reactions we extend the previous method to get global solutions for general diffusivities if $N\leq 3$ and for diffusion of type $d_i(c_i)$ if $N\leq 5$. In the latter quasi-linear case of $d_i(c_i)$ and for space dimensions $N=2$ and $N=3$, global existence holds for more than quadratic reactions. We can actually allow for more general rate functions including fractional power terms, important in applications. We obtain global existence under appropriate growth restrictions with an explicit dependence on the space dimension $N$