arXiv subjects

Peter Knabner

Publications and source records attributed to Peter Knabner.

22 records · Page 2Linked to original sources

arXiv2016

Building a Bridge from Moments to PDF's: A New Approach to Finding PDF Mixing Models

Probability density function (PDF) methods are a promising alternative to predicting the transport of solutes in groundwater under uncertainty. They make it possible to derive the evolution equations of the mean concentration and the concentration variance, used in moment methods. A mixing model, also known as a dissipation model, is essential for both methods. Finding a satisfactory mixing model is still an open question and due to the rather elaborate PDF methods, a difficult undertaking. Both the PDF equation and the concentration variance equation depend on the same mixing model. This connection is used to find and test an improved mixing model for the much easier to handle concentration variance. Subsequently, this mixing model is transferred to the PDF equation and tested. The newly proposed mixing model yields significantly improved results for both variance modelling and PDF modelling.

physics.flu-dyn↗

arXiv2014

Multiscale Modeling of Colloidal Dynamics in Porous Media: Capturing Aggregation and Deposition Effects

We investigate the influence of multiscale aggregation and deposition on the colloidal dynamics in a saturated porous medium. At the pore scale, the aggregation of colloids is modeled by the Smoluchowski equation. Essentially, the colloidal mass is distributed between different size clusters. We treat these clusters as different species involved in a diffusion-advection-reaction mechanism. This modeling procedure allows for different material properties to be varied between the different species, specifically the rates of diffusion, aggregation, deposition as well as the advection velocities. We apply the periodic homogenization procedure to give insight into the effective coefficients of the upscaled model equations. Benefiting from direct access to microstructural information, we capture by means of 2D numerical simulations the effect of aggregation on the deposition rates recovering this way both the blocking and ripening regimes reported in the literature.

math.AP↗

arXiv2013

Mathematical analysis of a discrete fracture model coupling Darcy flow in the matrix with Darcy-Forchheimer flow in the fracture

We consider a model for flow in a porous medium with a fracture in which the flow in the fracture is governed by the Darcy-Forchheimer law while that in the surrounding matrix is governed by Darcy's law. We give an appropriate mixed, variational formulation and show existence and uniqueness of the solution. To show existence we give an analogous formulation for the model in which the Darcy-Forchheimer law is the governing equation throughout the domain. We show existence and uniqueness of the solution and show that the solution for the model with Darcy's law in the matrix is the weak limit of solutions of the model with the Darcy-Forchheimer law in the entire domain when the Forchheimer coefficient in the matrix tends toward zero.

math.NA↗

arXiv2011

Rigorous Homogenization of a Stokes-Nernst-Planck-Poisson Problem for various Boundary Conditions

We perform the periodic homogenization (i.e. $\eps\to 0$) of the non-stationary Stokes-Nernst-Planck-Poisson system using two-scale convergence, where $\eps$ is a suitable scale parameter. The objective is to investigate the influence of \textsl{different boundary conditions and variable choices of scalings in $\eps$} of the microscopic system of partial differential equations on the structure of the (upscaled) limit model equations. Due to the specific nonlinear coupling of the underlying equations, special attention has to be paid when passing to the limit in the electrostatic drift term. As a direct result of the homogenization procedure, various classes of upscaled model equations are obtained.

math.AP↗