### 14.2.1. Interactions - Euler/Lagrange - Introduction

The Euler/Lagrange interaction (Fluid-Structure Interaction) in Autodyn couples the Eulerian domain to the Lagrangian structural domain.

This coupling algorithm in Autodyn finds the intersection between the external faces of the interacting structures and the cells and faces of the Euler grids. These intersection calculations give an exact calculation (within numerical precision) of the covered fraction of the Euler cells and faces, as shown in the left hand figure below. These properties are used in the 3D Euler solvers for coupled simulations.

The coupling can be chosen to be fully automatic (2D/3D) or by means of polygons (2D).

Polygon coupling

This coupling technique is available in 2D to set up an Euler/Lagrange interaction where one or more Polygons are defined and the polygon points are manually attached to nodes on the surface of the Lagrange Parts that interact with Euler. All structured Lagrangian element types can be coupled to Euler using polygons. Unstructured parts cannot be coupled to Euler using polygons; in that case the automatic coupling needs to be used.

Automatic coupling

This coupling technique is available in 2D as well as in 3D. Autodyn will automatically set up the Euler/Lagrange interactions and the only thing you have to define is which parts interact with Euler. Automatically all outside surface segments of all selected Lagrangian parts are generated and coupled to Euler.

In 2D the automatic coupling is only available for structured and unstructured Lagrangian volume elements.

In 3D the automatic coupling is available for structured and unstructured Lagrangian volume and shell elements. For shell elements an artificial thickening of the shell parts is used.

Coupling and Erosion

Erosion in combination with Euler/Lagrange coupling can be used for automatic coupling in 2D as well as 3D.

When elements that are connected to the coupling surface are eroded the coupling surface automatically will be updated for the new uncovered Lagrangian element faces.

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