/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.

Minimal input is possible with only 3 parameters: surf_ID, grbric_ID, and V_ref

Format

(1)	(2)	(3)	(5)	(7)	(8)	(9)	(10)
/INTER/TYPE18/`inter_ID`
`inter_title`
	`surf_ID`	`grbric_ID`	`I`_gap	`I`_pres	`I`_del		`I`_stf
`Stfval`		`V`_ref	`Gap`	`T`_start		`T`_stop

Definition

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`_gap	Gap formulation flag. 11 = 1 Variable gap = 1000 (Default) Constant gap (Integer)
`I`_pres	Pressure correction flag. = 0 No pressure correction. = 1 Reduce pressure by median pressure. = 2 Reduce pressure by mean surface pressure. = 3 Reduce pressure by mean estimated volumetric pressure. (Integer)
`I`_del	Node and segment deletion flag. = 0 (Default) No deletion. = 1 When all the elements (4-node shells, 3-node shells, solids) associated to one segment are deleted, the segment is removed from the Lagrangian side of the interface. Additionally, non-connected nodes are removed from the Eulerian or ALE side of the interface. = 2 When a 4-node shell, a 3-node shell or a solid element is deleted, the corresponding segment is removed from the Lagrangian side of the interface. Additionally, non-connected nodes are removed from the Eulerian or ALE side of the interface. (Integer)
`I`_stf	Interface stiffness definition flag. 2 = 1 (Default) Stiffness is a user constant and defined with `Stfval`. = 2 Stiffness is automatically calculated from `V`_ref which must be input. Then `Stfval` is an optional scale factor. (Integer)
`Stfval`	Interface stiffness value or stiffness scale factor. 2 3 4 If `I`_stf=1 Interface stiffness value. Default = 0.0 If `I`_stf=2 Interface stiffness scale factor. Default = 1.0 (Real)	$[\frac{N}{m}]$ or None
`V`_ref	Estimated reference velocity between ALE materials and Lagrangian surfaces (only use if `I`_stf=2). 4 (Real)	$[\frac{m}{s}]$
`Gap`	Interface gap. 5 = 0 A constant global gap is automatically calculated. (Real)	$[m]$
`T`_start	Start time. (Real)	$[s]$
`T`_stop	Time for temporary deactivation. (Real)	$[s]$

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)
$S t f v a l = \frac{ρ \cdot V_{r e f}^{2} \cdot S_{e l}}{G a p}$

Where,

$ρ$

The (highest) fluid density.

$V_{r e f}$

Reference velocity.

$S_{e l}$

Average surface area of the Lagrangian elements.
If I_stf=2, the stiffness value of the interface is automatically calculated from V_ref and a scale factor Stfval with:(2)
$S t f = S t f v a l . \frac{ρ \cdot V_{r e f}^{2} \cdot S_{e l}}{G a p}$
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 ( $1.5 Δ x_{f l u i d}$ ) along the normal direction to contact. If gap is not defined, it is computed using:(3)
$g a p = \frac{\sqrt{3}}{2} d i a g_{\max}$

Where, $d i a g_{\max}$ is the maximum diagonal of fluid element, which is exactly like $1.5 Δ x_{f l u i d}$ 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 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 reference 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).
When gap is variable, the Gap value is ignored. When gap is constant, the Gap value is optional since it can be automatically estimated.