241 research outputs found
Applying a phase field approach for shape optimization of a stationary Navier-Stokes flow
We apply a phase field approach for a general shape optimization problem of a
stationary Navier-Stokes flow. To be precise we add a multiple of the
Ginzburg--Landau energy as a regularization to the objective functional and
relax the non-permeability of the medium outside the fluid region. The
resulting diffuse interface problem can be shown to be well-posed and
optimality conditions are derived. We state suitable assumptions on the problem
in order to derive a sharp interface limit for the minimizers and the
optimality conditions. Additionally, we can derive a necessary optimality
system for the sharp interface problem by geometric variations without stating
additional regularity assumptions on the minimizing set
Segmentation and Restoration of Images on Surfaces by Parametric Active Contours with Topology Changes
In this article, a new method for segmentation and restoration of images on
two-dimensional surfaces is given. Active contour models for image segmentation
are extended to images on surfaces. The evolving curves on the surfaces are
mathematically described using a parametric approach. For image restoration, a
diffusion equation with Neumann boundary conditions is solved in a
postprocessing step in the individual regions. Numerical schemes are presented
which allow to efficiently compute segmentations and denoised versions of
images on surfaces. Also topology changes of the evolving curves are detected
and performed using a fast sub-routine. Finally, several experiments are
presented where the developed methods are applied on different artificial and
real images defined on different surfaces
Segmentation of Three-dimensional Images with Parametric Active Surfaces and Topology Changes
In this paper, we introduce a novel parametric method for segmentation of
three-dimensional images. We consider a piecewise constant version of the
Mumford-Shah and the Chan-Vese functionals and perform a region-based
segmentation of 3D image data. An evolution law is derived from energy
minimization problems which push the surfaces to the boundaries of 3D objects
in the image. We propose a parametric scheme which describes the evolution of
parametric surfaces. An efficient finite element scheme is proposed for a
numerical approximation of the evolution equations. Since standard parametric
methods cannot handle topology changes automatically, an efficient method is
presented to detect, identify and perform changes in the topology of the
surfaces. One main focus of this paper are the algorithmic details to handle
topology changes like splitting and merging of surfaces and change of the genus
of a surface. Different artificial images are studied to demonstrate the
ability to detect the different types of topology changes. Finally, the
parametric method is applied to segmentation of medical 3D images
Analysis of a Cahn--Hilliard system with non-zero Dirichlet conditions modeling tumor growth with chemotaxis
We consider a diffuse interface model for tumor growth consisting of a
Cahn--Hilliard equation with source terms coupled to a reaction-diffusion
equation, which models a tumor growing in the presence of a nutrient species
and surrounded by healthy tissue. The well-posedness of the system equipped
with Neumann boundary conditions was found to require regular potentials with
quadratic growth. In this work, Dirichlet boundary conditions are considered,
and we establish the well-posedness of the system for regular potentials with
higher polynomial growth and also for singular potentials. New difficulties are
encountered due to the higher polynomial growth, but for regular potentials, we
retain the continuous dependence on initial and boundary data for the chemical
potential and for the order parameter in strong norms as established in the
previous work. Furthermore, we deduce the well-posedness of a variant of the
model with quasi-static nutrient by rigorously passing to the limit where the
ratio of the nutrient diffusion time-scale to the tumor doubling time-scale is
small.Comment: 33 pages, minor typos corrected, accepted versio
Nonlinear stability of stationary solutions for curvature flow with triple junction
In this paper we analyze the motion of a network of three planar curves with
a speed proportional to the curvature of the arcs, having perpendicular
intersections with the outer boundary and a common intersection at a triple
junction. As a main result we show that a linear stability criterion due to
Ikota and Yanagida is also sufficient for nonlinear stability. We also prove
local and global existence of classical smooth solutions as well as various
energy estimates. Finally, we prove exponential stabilization of an evolving
network starting from the vicinity of a linearly stable stationary network.Comment: submitte
On an incompressible Navier-Stokes/Cahn-Hilliard system with degenerate mobility
We prove existence of weak solutions for a diffuse interface model for the flow of two viscous incompressible Newtonian
fluids in a bounded domain by allowing for a degenerate mobility. The model has been developed by Abels, Garcke and Grün for fluids with different densities and leads to a solenoidal velocity field. It is given by a nonhomogeneous Navier-Stokes system with a modifed convective term coupled to a Cahn-Hilliard system, such that an energy estimate is fulfilled which follows from the fact that the model is thermodynamically consistent
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