88 research outputs found
Auf Wolfsspuren - unterwegs mit dem Kamerateam des WDR
Am 29.09.2008 hieĂ es fĂŒr drei Mitglieder des NABU-Kreisverbands Höxter auf Wolfssuche gehen. Zusammen mit einem Kamerateam des WDR informierten sich Theo Elberich, Heinrich Hachmeier und JĂŒrgen KrĂ€utle ĂŒber neueste Nachweise eines Wolfs im Reinhardswald. Treffpunkt mit dem Kamerateam war Beverungen. Dieses Kamerateam, bestehend aus Michael BLASCHKE und Ulrich HAUFE, dreht unter anderem seit 12 Jahren fĂŒr die Redaktion der WDR-Lokalzeit OWL (dienstags, 19:30 Uhr) Naturfilme von vier bis sechs Minuten LĂ€nge. Von Beverungen ging es hinauf nach Langental, knapp jenseits der Landesgrenze Nordrhein-Westfalens in Hessen gelegen. Von hier hatten wir einen guten Ausblick auf den Reinhardswald. Die Kamera schwenkte ĂŒber die Höhen des Waldes, um dem Fernsehzuschauer einen ersten Eindruck von dem Gebiet zu vermitteln, in das vor wenigen Monaten der Wolf zurĂŒckgekehrt ist. Leider schien an diesem Tag die Sonne nicht, so dass die HöhenzĂŒge teilweise im Dunst verschwanden
Kinetics of silicide formation by thin films of V on Si and SiO_2 substrates
The reaction rate of vacuumâevaporated films of V of the order of 1000 Ă
thick is investigated by MeV He backscattering spectrometry. On substrates of singleâcrystal Si and for anneal times up to several hours in the temperature range 570â650°C, VSi_2 is formed at a linear rate in time. The activation energy of the process is 1.7±0.2 eV. The presence of oxygen in amounts of 10% can significantly decelerate the reaction. On substrates of SiO_2 in the temperature range 730â820°C and anneal times of several hours or less, V_3Si is formed at a squareâroot rate in time. The activation energy of this process is 2.0±0.2 eV
A global method for coupling transport with chemistry in heterogeneous porous media
Modeling reactive transport in porous media, using a local chemical
equilibrium assumption, leads to a system of advection-diffusion PDE's coupled
with algebraic equations. When solving this coupled system, the algebraic
equations have to be solved at each grid point for each chemical species and at
each time step. This leads to a coupled non-linear system. In this paper a
global solution approach that enables to keep the software codes for transport
and chemistry distinct is proposed. The method applies the Newton-Krylov
framework to the formulation for reactive transport used in operator splitting.
The method is formulated in terms of total mobile and total fixed
concentrations and uses the chemical solver as a black box, as it only requires
that on be able to solve chemical equilibrium problems (and compute
derivatives), without having to know the solution method. An additional
advantage of the Newton-Krylov method is that the Jacobian is only needed as an
operator in a Jacobian matrix times vector product. The proposed method is
tested on the MoMaS reactive transport benchmark.Comment: Computational Geosciences (2009)
http://www.springerlink.com/content/933p55085742m203/?p=db14bb8c399b49979ba8389a3cae1b0f&pi=1
Weak-strong uniqueness of solutions to entropy-dissipating reaction-diffusion equations
We establish a weak-strong uniqueness principle for solutions to
entropy-dissipating reaction-diffusion equations: As long as a strong solution
to the reaction-diffusion equation exists, any weak solution and even any
renormalized solution must coincide with this strong solution. Our assumptions
on the reaction rates are just the entropy condition and local Lipschitz
continuity; in particular, we do not impose any growth restrictions on the
reaction rates. Therefore, our result applies to any single reversible reaction
with mass-action kinetics as well as to systems of reversible reactions with
mass-action kinetics satisfying the detailed balance condition. Renormalized
solutions are known to exist globally in time for reaction-diffusion equations
with entropy-dissipating reaction rates; in contrast, the global-in-time
existence of weak solutions is in general still an open problem - even for
smooth data - , thereby motivating the study of renormalized solutions. The key
ingredient of our result is a careful adjustment of the usual relative entropy
functional, whose evolution cannot be controlled properly for weak solutions or
renormalized solutions.Comment: 32 page
Comparison of numerical methods for simulating strongly non-linear and heterogeneous reactive transport problems â the MoMaS benchmark case
International audienceAlthough multicomponent reactive transport modeling is gaining wider application in various geoscience fields, it continues to present significant mathematical and computational challenges. There is a need to solve and compare the solutions to complex benchmark problems, using a variety of codes, because such intercomparisons can reveal promising numerical solution approaches and increase confidence in the application of reactive transport codes. In this contribution, the results and performance of five current reactive transport codes are compared for the 1D and 2D sub-problems of the so-called "Easy Test Case" of the MoMaS benchmark (Carrayrou et al., this issue). As a group, the codes include iterative and non-iterative operator splitting, and global implicit solution approaches. The 1D Easy Advective and 1D Easy Diffusive scenarios were solved using all codes and, in general, there was good agreement, with solution discrepancies limited to regions with rapid concentration changes. Computational demands were typically consistent with what was expected for the various solution approaches. The most important outcome of the benchmark exercise is that all codes are able to generate comparable results for problems of significant complexity and computational difficulty
- âŠ