### 14.3.5. Interactions - Euler/Lagrange (3D) - Weak Coupling

Within Autodyn-3D, there is a facility for the application of a pressure field in 3D space to Lagrange type parts as a boundary condition. The pressures in 3D space are contained in a sequence of Field files, the format of which is described below. Also included is an additional feature that enables you to create these fields when using either the Ideal Gas or the Multi-material Euler processors. Weak Lagrange Coupling can be seen as equivalent to the current Euler-Lagrange coupling but without the feedback from the Lagrange type parts back into the Euler part. This weak coupling assumes that any deflections in the Lagrange model are small, and should only be used in such cases.

The current implementation only considers the possibility of mapping pressure data, and applies this to the Lagrange boundary surfaces as nodal forces. The pressures will be applied to all wetted surfaces of Lagrange type subgrids within the volume described by the pressure files. The advantage over the current Euler-Lagrange coupling is that multiple Lagrange calculations can be carried out on one coupled Euler calculation, thus saving analysis time. Also, some speedup would be expected over a normal coupled solution for a single Euler and then Lagrange calculation.

Creating the Field files

The Field files are created as a field edit when an Ideal Gas or Multi-Material Euler type simulation is being carried out. You should click Output on the navigation bar and open the Field output part of the dialog panel. Under this menu there are three options, namely Cycles , Times and Set_part . The cycle and time options work in the traditional manner of all edits in Autodyn and enables you to select the frequency of the Field file output. The third menu, set_part, enables you to select which part the pressure data will be taken from, and the I, J and K range. The file format only allows one block of data to be written. If you do not supply this information, then the entire range of the first Euler part found will be written to the Field file. A geometry data field is only produced when a problem is started or restarted, and this cycle coincides with a field edit.

Using the Weak Interactions

The coupling option Weak Coupling is included under the Euler/Lagrange Interaction tab after Interaction is selected from the navigation bar. The Euler-Lagrange/Shell interactions need to be set to chose which Lagrange type part are included in the coupling, and how shells interact. Under the weak coupling settings, the Ambient Pressure should be entered. This pressure is removed from the Field file pressure, i.e.

PAPPLIED = PFIELD - PAMBIENT

Also the Field file set need to be selected by using the file browser to select the first in the required set.

Notes on the Mapping

Autodyn-3D interpolates linearly in time the pressure data from two Field files, the current time of the simulation being in-between the times of the fields. When a field file is read into the solver a mapping is created. This is a lookup table for each cell face in the Lagrange model that is activated pointing to a cell in the pressure field. If a geometry data field is not present in the Field file then the existing mapping from the previous field is used. The mapping is created using one of two possible geometry choices for the Lagrange type parts:

1. The initial coordinates of the Lagrangian parts (i.e. at time equal to zero). With this option, a geometrical remap is performed only at cycle zero, and during the analysis each Lagrangian face will use the pressure of the same Euler element from the different Field files.

2. The current coordinates of the Lagrangian parts. With this option, a continuous geometrical remapping takes place during the analysis and therefore is more expensive.

The interpolation in space is the same as for the current Euler-Lagrange interactions; the nearest cells center to the face center is used.

Field file format (.ADF FILES)

The ADF (Autodyn FIELD) file format contains field information on a regular structured rectangular grid. Scalar data is cell centered. The naming convention of the files is:

IDENT_####.ADF FORMAT:'(A,A1,I4.4,A4)'

Where #### is an increasing integer number, starting at 0001.

Record 1 | FILEID (CH*256) | Character string which can be used to identify the file
Record 2 | NCYCLE, TIME, NTU, NMU, NLU | NTU is the time unit (1:μs 2:ms, 3:s) NMU is the mass unit (1:mg 2:g, 3:kg) NLU is the length unit (1:mm, 2:cm, 3:m)
Record 3 | NI, NJ, NK | Maximum number of nodes in each of the three directions
Record 3+(NS-1)*2 | ITEM_NAME (CH*256) | Name of the item to follow; that is, GEOMETRY or PRESSURE
Record 4+(NS-1)*2 | The data attached to the item | Item specific - see below.
Record (LAST) | "END OF SCALAR FILE" |

The data attached to the items are:

GEOMETRY |
--- | ---
XC (1:NI) | X positions, in line with the I axis
YC (1:NJ) | Y positions, in line with the J axis
ZC (1:NK) | Z positions, in line with the K axis
| 40000 + file counter
PRESSURE |
PN(1:NI-1, 1:NJ-1, 1:NK-1) | Pressure of each cell. I varies fastest

A geometry data record is only needed if the geometry has changed. The presence of this field enforces a new mapping. Therefore, if the geometry is only present in the first field, then the mapping is only done at the beginning of the calculation. On the other hand, if the geometry is present in every file, then a remapping is done every time a new flow file is read. If a file does contain a geometry data record then Autodyn expects that it will occur before the pressure record. A pressure record must exist in all files.

Errors, Warning and other Outputs

Errors encountered during the mapping cause a generic error, stop the current simulation. A summary of the error is written to the message window. The following table summarizes the error codes:

Too many scaler files | 1
File does not exist | 10000 + file counter
Expecting geometry data | 20000 + file counter
No pressure data | 30000 + file counter
Error in reading file | 40000 + file counter

Errors and Warnings in the Field File writing routines will not stop the calculation, but will write an appropriate message to the message window.

Release 18.2 - © ANSYS, Inc. All rights reserved.