/INTER/TYPE18
Block Format Keyword This interface manages indirect coupling between ALE and Lagrange for FSI applications. A penalty method is used and requires stiffness and gap values.
Format
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

/INTER/TYPE18/inter_ID  
inter_title  
surf_ID  grbric_ID  I_{pres}  I_{del}  I_{stf}  
Stfval  V_{ref}  Gap  T_{start}  T_{stop} 
Definitions
Field  Contents  SI Unit Example 

inter_ID  Interface
identifier. (Integer, maximum 10 digits) 

inter_title  Interface
title. (Character, maximum 100 characters) 

surf_ID  Lagrangian surface
identifier. (Integer) 

grbric_ID  Eulerian or ALE brick
elements group identifier. Used for 3D
analysis. (Integer) 

I_{pres}  Pressure correction flag.
(Integer) 

I_{del}  Node and segment deletion flag.
(Integer) 

I_{stf}  Interface stiffness
definition flag.
(Integer) 

Stfval  Interface stiffness value.
2
3
4
(Real) 
$\left[\frac{\text{N}}{\text{m}}\right]$ or None 
V_{ref}  Estimated relative
velocity between ALE materials and Lagrangian surfaces (only use if
I_{stf}=2). 4 (Real) 
$\left[\frac{\text{m}}{\text{s}}\right]$ 
Gap  Interface gap. 5
(Real) 
$\left[\text{m}\right]$ 
T_{start}  Start
time. (Real) 
$\left[\text{s}\right]$ 
T_{stop}  Time for temporary
deactivation. (Real) 
$\left[\text{s}\right]$ 
Comments
 V_{ref} is the reference velocity and can be considered as the estimated relative velocity between ALE materials and Lagrangian surfaces. It is advised to use the maximum value expected during the simulation. For instance, for ditching applications, the initial impact velocity of the structure can be used. For IED or mine blast applications, the maximum material velocity in the shock wave impacting the structure can be used.
 If I_{stf}=1, the recommended value of Stfval
is:
(1) $$Stfval=\frac{\rho \cdot {V}_{ref}{}^{2}\cdot {S}_{el}}{Gap}$$Where, $\rho $
 The (highest) fluid density.
 ${V}_{ref}$
 Relative velocity.
 ${S}_{el}$
 Average surface area of the Lagrangian elements.
 If I_{stf}=2, the stiffness value of the interface is automatically
calculated from V_{rel} and a
scale factor Stfval with:
(2) $$Stf=Stfval.\frac{\rho \cdot {V}_{ref}{}^{2}\cdot {S}_{el}}{Gap}$$  It is advised to carry out a sensitivity study of the model with the interface stiffness. For that purpose, with I_{stf}=2, the stiffness scale factor can be used and set to different values in each simulation (for instance the value can be changed from 1 to 2, 4, 10 and 100).
 Gap is the interface gap,
recommended value is 1.5 times of fluid element size (
$1.5\text{\hspace{0.17em}}\text{\Delta}{x}_{fluid}$
) along the normal direction to contact. If gap
is not defined, it is computed using:
(3) $$gap=\frac{\sqrt{3}}{2}dia{g}_{\mathrm{max}}$$Where, $dia{g}_{\mathrm{max}}$ is the maximum diagonal of fluid element, which is exactly like $1.5\text{\hspace{0.17em}}\text{\Delta}{x}_{fluid}$ for a regular mesh.
 Prior to version 2020, the fluid Eulerian or ALE nodes were defined using a grnd_ID_{s} in line 1 field 1. This input format is still supported for legacy models. This input format is still supported for legacy models with LAW5, 6 or 51.
 The accuracy of the interface is increased, if the mesh size of the ALE side is decreased. Thus, to increase the accuracy of the interface, it is recommended you refine the ALE mesh in the contact zone, and at least in the global normal direction of the Lagrangian surfaces.
 To increase accuracy, several TYPE18 interfaces with different gap and stiffness values can be used for different contact zones (for instance, for cases where the mesh size and/or relative velocity are varying substantially).
 If element deletion in case of material failure has been activated for the Lagrangian parts, it is highly recommended to use I_{del}=1 or 2.
 In the case of SPMD parallelization, each Lagrangian segment defined by surf_ID must be associated with an element (with a property assigned to it which can be /PROP/VOID).