# Basics : Weak FSI — Welcome to LS-DYNA Examples

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This LS-DYNA simulation shows a simple FSI coupling problem using weak/loose/explicit FSI. A rigid cylinder has a prescribed displacement in the Y direction. After five seconds, the inlet velocity boundary condition is triggered pushing the cylinder to the right. Since the coupling is loose, the two solvers retain their own independent timesteps and the solid mechanics solver runs in explicit. The FSI uses a boundary fitted approach. The fluid mesh must have a boundary with the structure. The meshes do not have to coincide but the geometries must remain close. When FSI is triggered, the fluid mesh moves in a Lagrangian way while the fluid flow evolves in a Eulerian way.

## Animated Result

Fluid velocity fringes

## Keywords

```*BOUNDARY_PRESCRIBED_MOTION_RIGID
*KEYWORD
*TITLE
*CONTROL_TERMINATION
*CONTROL_TIMESTEP
*DATABASE_BINARY_D3PLOT
*DEFINE_CURVE_FUNCTION
*DEFINE_CURVE_TITLE
*ELEMENT_SHELL
*ICFD_BOUNDARY_FREESLIP
*ICFD_BOUNDARY_FSI
*ICFD_BOUNDARY_PRESCRIBED_VEL
*ICFD_BOUNDARY_PRESCRIBED_PRE
*ICFD_BOUNDARY_NONSLIP
*ICFD_CONTROL_FSI
*ICFD_CONTROL_TIME
*ICFD_DATABASE_DRAG
*ICFD_MAT
*ICFD_PART
*ICFD_PART_VOL
*ICFD_SECTION
*INCLUDE
*MAT
*MESH_BL
*MESH_SURFACE_ELEMENT
*MESH_SURFACE_NODE
*MESH_VOLUME
*NODE
*PARAMETER
*PART
*SECTION
*END
```

## Reduced Input

```\$-----------------------------------------------------------------------------
\$
\$ Example provided by Iñaki (LSTC)
\$
\$ E-Mail: info@dynamore.de
\$ Web: http://www.dynamore.de
\$
\$ Copying for non-commercial usage allowed if
\$ copy bears this notice completely.
\$
\$X------------------------------------------------------------------------------
\$X
\$X 1. Run file as is.
\$X    Requires LS-DYNA MPP R8.0.0 (or higher) with double precision
\$X
\$X------------------------------------------------------------------------------
\$# UNITS: Dimensionless.
\$X------------------------------------------------------------------------------
\$X
*KEYWORD
*TITLE
ICFD Cylinder flow - Weak FSI
*INCLUDE
mesh.k
*INCLUDE
struc.k
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
\$                                                                              \$
\$                             PARAMETERS                                       \$
\$                                                                              \$
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
*PARAMETER
R    T_end      40.0
R  dt_plot      0.20
\$
\$--- Fluid
\$
R  v_inlet       1.0
Rrho_fluid       1.0
R mu_fluid     0.005
R dt_fluid     0.050
\$
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
\$                                                                              \$
\$                           ICFD CONTROL CARDS                                 \$
\$                                                                              \$
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
*ICFD_CONTROL_TIME
\$#     ttm        dt
&T_end &dt_fluid
*ICFD_CONTROL_FSI
\$#     owc
0
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
\$                                                                              \$
\$                       ICFD PARTS/ SECTION/ MATERIAL                          \$
\$                                                                              \$
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
*ICFD_SECTION
\$#     sid
1
*ICFD_MAT
\$#     mid       flg        ro       vis
1         1&rho_fluid &mu_fluid
*ICFD_PART
\$#     pid     secid       mid
1         1         1
*ICFD_PART
\$#     pid     secid       mid
2         1         1
*ICFD_PART
\$#     pid     secid       mid
3         1         1
*ICFD_PART
\$#     pid     secid       mid
4         1         1
*ICFD_PART_VOL
\$#     pid     secid       mid
10         1         1
\$#   spid1     spid2     spid3     spid4
1         2         3         4
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
\$                                                                              \$
\$                    ICFD BOUNDARY/INITIAL CONDITIONS                          \$
\$                                                                              \$
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
*ICFD_BOUNDARY_PRESCRIBED_VEL
1         1         1         1
*ICFD_BOUNDARY_PRESCRIBED_VEL
1         2         1         2
*ICFD_BOUNDARY_PRESCRIBED_PRE
\$#     pid      lcid        sf     death     birth
2         2
*ICFD_BOUNDARY_FREESLIP
\$#     pid
3
*ICFD_BOUNDARY_NONSLIP
\$#     pid
4
*ICFD_BOUNDARY_FSI
\$#     pid
4
*DEFINE_CURVE_TITLE
Velocity inlet
\$#    lcid      sidr       sfa       sfo      offa      offo    dattyp
1
\$#                a1                  o1
0.0                  0.
5.0                  0.
6.0            &v_inlet
10000.0            &v_inlet
*DEFINE_CURVE_TITLE
Pressure outlet
\$#    lcid      sidr       sfa       sfo      offa      offo    dattyp
2
\$#                a1                  o1
0.0                 0.0
10000.0                 0.0
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
\$                                                                              \$
\$                            ICFD MESH KEYWORDS                                \$
\$                                                                              \$
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
*MESH_VOLUME
\$#   volid
10
\$#    pid1      pid2      pid3      pid4
1         2         3         4
*MESH_BL
\$#     pid     nelth
4         2
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
\$                                                                              \$
\$                             DATABASE (OUTPUT)                                \$
\$                                                                              \$
\$---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8
*ICFD_DATABASE_DRAG
\$#     pid
4
*DATABASE_BINARY_D3PLOT
&dt_plot
*end```