Computational fluid dynamics and computational structure mechanics
are two major areas of numerical simulation of physical systems.
With the introduction of high performance computing it has become
possible to tackle systems with a coupling of fluid and
structure dynamics. General examples of such fluidstructure
interaction (FSI) problems are flow transporting elastic particles
(particulate flow), flow around
elastic structures (airplanes, submarines) and flow in elastic
structures (haemodynamics, transport of fluids in closed containers).
In all these settings the dilemma in modeling the coupled dynamics is
that the fluid model is normally based on an Eulerian perspective in
contrast to the usual Lagrangian approach for the solid model.
This makes the setup of a common variational description difficult.
However, such a variational formulation of FSI is needed as the basis
of a consistent approach to residualbased a posteriori error estimation
and mesh adaptation as well as to the solution of optimal control
problems by the EulerLagrange method. This is the subject of the
this project.
The work and results of this project were financed by the
Deutsche Forschungsgemeinschaft (DFG) through the
Project P2 of
the joint
Research Unit 493
'FluidStructure Interaction Modelling, Simulation, Optimisation'.
One of the goals is the developement of a general
FSI benchmark 'FLUSTRUKA'
(PDF)
and the quantitative comparison of simulation results.
The unit consists of nine groups, which
have implemented a wide array of approaches to simulating FSI problems.
The focus of the simulations always being the benchmark.
The results of the first phase of the joint research group
are availible in
FluidStructure Interaction
Modelling, Simulation, Optimisation.
Combining the Eulerian and the Lagrangian setting for describing FSI
involves conceptional difficulties. On the one hand the fluid domain
itself is timedependent and depends on the deformation of the structure
domain. On the other hand, for the structure the fluid boundary values
(velocity and the normal stress) are needed.
In both cases values from the one problem are used for the other,
which is costly and can lead to a drastic loss of accuracy.
A common approach to dealing with this problem is to separate the two
models, solve each separately, and so converge iteratively to a solution,
which satisfies both together with the interface conditions
Solving the separated problems serially multiple times is referred to
as a 'partitioned approach'.
A basic partitioned approach does not contain a variational equation for the
fluidstructure interface. To achieve this, usually an auxiliary unknown
coordinate transformation function is introduced for the fluid
domain. With its help the fluid problem is rewritten as one on the
transformed domain, which is fixed in time. Then, all computations are
done on the fixed reference domain and as part of the computation
the auxiliary transformation function has to be determined
at each time step. This fully coupled approach to solving the whole problem on an
(optionally arbitrary) reference domain is generally referred to as the
'arbitrary LagrangianEulerian' (ALE) method.
In this project, we follow an additional (to the authors knowledge new) way of posing the fluid as well
as the structure problem in a fully Eulerian framework.
In the Eulerian setting a phase variable is employed on the fixed mesh to
distinguish between the different phases, liquid and solid. This approach
to identifying the fluidstructure interface is generally referred to as
'interface capturing', a method commonly used in the simulation of
multiphase flows.
Examples for the use of such a phase variable are the Volume of Fluid (VoF)
method
and the Level Set (LS) method.
In the classical LS approach the distance function has to continually
be reinitialized, due to the smearing effect by the convection velocity in
the fluid domain. This makes the use of the LS method delicate for
modeling FSI problems particularly in the presence of cornered structures.
To cope with this difficulty, we introduce a variant, the 'Initial Position set method' (IP set), of the LS method
that makes reinitialization unnecessary and which can easily cope with
cornered structures.
Based on the Eulerian variational formulation of the FSI system, we
use the 'dual weighted residual' (DWR) method
to derive 'goaloriented' a posteriori error estimates. The evaluation of
these error estimates requires the approximate solution of a linear
dual variational problem. The resulting a posteriori error indicators
are then used for automatic local mesh adaption.
The methods have been (successfully) tested for both the ALE as well as the Eulerian framework
for a wide range of tests, stationary and instationary.

driven cavity with elastic base problem
ALE framework, adaptively refined mesh
Eulerian framework, adaptively refined mesh

