1. Department of Computer Science, TU München
A Modular and Efficient Simulation Environment for Fluid-Structure Interactions
Miriam Mehl
Bernhard Gatzhammer
Department of Computer Science, TU München

2. Outline no fluid solver details FSIce motivation
application programming interface
modules
data transfer
coupling strategies
numerical results

3. Plug-and-Play for FSI MpCCI Data Mapping Coupling Strategy Fluid
Solver
Structure
Solver

4. Plug-and-Play for FSI
MpCCI
Fluid
Solver
Structure
Solver

5. Plug-and-Play for FSI Coupling Strategy Central Surface Mesh FSIce
Fluid
Solver
Structure
Solver

6. FSIce – Application Programming Interface
FSI_Init ()
FSI_Data_exchange ()
FSI_Finalize ()
FSI_Is_running ()
FSI_Is_new_interface_values ()
FSI_Is_implicit_converged ()
Fluid
Solver
API of FSIce
3 central communication functions
3 helper functions for querying coupled simulation state

7. FSIce – Application Programming Interface
while (more time steps)
Set time step length
Compute values of next time step
Store values of next time step
end while
Fluid
Solver
How to prepare a solver for coupled simulations with FSIce
Basic solver

8. FSIce – Application Programing Interface
while (more time steps)
Read coupling data from com.mesh
Set time step length
Compute values of next time step
Write coupling data to com.mesh
Store values of next time step
end while
Fluid
Solver
Necessary for all partitioned approaches is the mapping of data to some data structure communicated

9. FSIce – Application Programing Interface
FSI_Init ()
while (more time steps)
Read coupling data from com.mesh
Set time step length
Compute values of next time step
Write coupling data to com.mesh
FSI_Data_exchange ()
Store values of next time step
end while
FSI_Finalize ()
Fluid
Solver
FSIce communication functions
FSI init to initialize communication and exchange communication mesh and other initial information
FSI Data exchange to send and receive coupling data and the state of the coupled simulation
FSI Finalize to finish the coupled simulation, remove communication data structures

10. FSIce – Application Programing Interface
FSI_Init ()
while (FSI_Is_running())
if (FSI_Is_new_interface_values())
Read coupling data from com.mesh
Set time step length
Compute values of next time step
Write coupling data to com. mesh
FSI_Data_exchange ()
if (FSI_Is_implicit_converged())
Store values of next time step
end while
FSI_Finalize ()
Fluid
Solver
Functions to query state of the coupled simulation
Not all are necessary
FSI_Is_running to give the control about the length of the coupled simulation to the supervisor
FSI_Is_new_interface_values to avoid unnecessary mapping from the coupling mesh to the solver mesh, e.g. in a subcycle
FSI_Is_implicit_converged for implicit coupling scheme, when in interface iteration process

11. FSIce – Modules - Modular structure of FSIce source code
- FSIce is composed out of two main modules: Libraries and Supervisor
- module libraries contains functionalities used by simulation programs to be coupled and functionalities for the coupling supervisor in module Supervisor
- module Supervisor implements the coupling supervisor acting as client in the couple simulation
- Libraries consists of the packages: FSImesh, FSIcom and FSItools
- FSImesh provides the implementation of the coupling mesh and the FSIce internal communication interface (MPI or sockets)‏
- FSIcom defines and implements the API for solvers to be coupled
- FSItools contains optional additional functionalites for solvers such as octree based neighbor search or geometry creation for Cartesian grids
- Supervisor contains the packages CouplingUnit, GUI, Batch and TestDummies
- CouplingUnit encapsulate the coupling schemes and logic of a coupled simulation
- GUI and Batch are specific implementations of a coupling supervisor, using CouplingUnit
- TestDummies contains solver dummy executables to enable a test of the coupling supervisor and the ability of a solver to perform coupled simulations with FSIce

12. FSIce – Data Transfer
How to map data from solver mesh to coupling mesh ?
Octree for efficient neighbour search
Some projection / interpolation included
Custom interpolations possible
Provided to user in package FSItools
A question appearing in every partitioned FSI simulation is how to map data from the native solver mesh to the communication data structure
FSIce provides the functionality of an octree for this purpose, which allows to search for neighbouring points onthe two meshes
Also some interpolation mehtods are included
Users with specific requirements for interpolation schemes can write their own functions and plug them into FSIce
This functionality is bundled in the package FSItools library

13. FSIce – Coupling Strategies
Explicit (weak)
Implicit (strong)
Subcycling
Pre-computations
Many others possible:
just extend supervisor
A typical situation in partitioned FSI simulations is, that the fluid solver requires shorter time step lengths than the structure solver
To save computation effort on the structure side, subcycling is employed, which allows different time step lengths on both sides of the coupled simulation
Short explanation

14. Numerical Results – Implicit vs. Explicit

15. Numerical Results

16. Numerical Results – Data Mapping
17.4 sec
128,000
512
14.3 sec
32,000
10.1 sec
8,000
2.6 sec
256
runtime
# triang.
Cart.res.
neighbourhood search measured on a Pentium M 1.6 GHz processor with 2048 kB cache

17. Conclusion enhancements of FSI*ce
modular structure  extendability, flexibility
integration of various components
first examples computed
future: more solvers, more coupling strategies

18. Thank you for your attention!

19. FSIce – Application Programing Interface
Future goal: Multigrid
Multilevel coupling mesh
Multigrid scheme controlled from coupling supervisor
Allows V, W, ... schemes without modifying solvers
Fluid
Solver
A future goal will be to support partitioned coupled multigrid solution procedures
This can be optimally supported by a coupling mesh with several grid resolutions
The multigrid algorithm will be controlled from the coupling supervisor again
When a solver has the necessary multigrid functionality integrated (restriction, perlongation) arbitrary multigrid schemes can be realized through the coupling supervisor, without the necessatiy to further modify the solver codes