Tue, 2008-10-07 14:55 — Anonymous

Year:

2008

Language:

English

Abstract in English:

The present thesis focuses on the visualization of vector fields with
an emphasis on flow fields. In connection with a DFG project, close
collaboration and discussions with fluid dynamicists raised new
questions concerning the detection and depiction of special features
in flows.
The contributions of the thesis in this context are the
following: First, we present and apply a method to extract the
contribution of a subregion of a flow to the global flow. To isolate
this contribution we decompose the flow in the subregion into a
potential flow that is induced by the original flow on the boundary
and a localized flow. Since the potential flow is free of both
divergence and rotation the localized flow retains the original
features and captures the region-specific flow that contains the
local contribution of the considered subregion to the global flow. We
discuss the application of some widely used feature extraction methods
on the localized flow and describe applications like reverse-flow
detection using the potential flow. Furthermore, an extension to
time-dependent fields is given.
Next, to complement animations that
provide only transient impressions of momentary flow, two approaches
to visualize time varying fields with fixed geometry are
introduced. We show how bundles of path lines running at different
times through one point in space yield an insightful visualization of
flow structure. As second approach, we use a simple measurement of
local changes of a field over time to determine regions with strong
changes.
Concerning the interaction between wall shear stress and
three-dimensional flows, we propose a method for the extraction of
separation manifolds originating from separation lines. We address the
problem by investigating features in flow cross sections around
separation lines. We use the topological signature of the separation
in these sections, in particular the presence of saddle points and
their separatrices, as a guide to initiate the construction of the
separation manifolds.
Further investigating wall shear stress and
three-dimensional flows, we present a method to visualize vortices
that originate from bounding walls of three-dimensional time-dependent
flows. These vortices can be detected using their footprint on the
boundary, which consists of critical points in the wall shear stress
vector field. In order to follow these critical points affected
regions of the surface are parameterized. Thus, an existing
singularity tracking algorithm devised for planar settings can be
applied. The trajectories of the singularities are used as a basis for
seeding particles. This leads to a generalized concept of streak lines
which visualize the particles that are ejected from the wall.
Finally, we apply the concepts of finite-time Lyapunov exponents to
enable the analysis of steady and unsteady flows in the immediate
vicinity of the boundaries of flow-embedded objects by limiting
Lagrangian analysis to surfaces closely neighboring these
boundaries. To this purpose, we present an approach to approximate
FTLE fields over such surfaces. Furthermore, we achieve an effective
depiction of boundary-related flow structures such as separation and
attachment over object boundaries and specific insight into the
surrounding flow using several specifically chosen visualization
techniques.

Appeared / Erschienen in:

Shaker Verlag

Pubdate / Erscheinungsdatum:

2008-10-01

Pages / Seitenanzahl:

176

Notes / Bemerkungen:

Zugl.: Leipzig, Univ., Diss., 2008

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2008-7.pdf | 47.68 MB |