# Fluid-Structure Interaction (TYPE 1 Interface)

TYPE1 interface is used to model fluid-structure interactions, as shown in Figure 1.

This interface allows Lagrangian elements (structure) to interact with ALE (Arbitrary Lagrangian Eulerian) elements, which model a viscous fluid. Full slip conditions are applied at the boundary between the two domains.

The acceleration of the Lagrange node is computed by:

$${\overrightarrow{\gamma}}_{i}=\frac{{\overrightarrow{F}}_{i}+\overrightarrow{F}{}_{a}}{m{}_{i}+\text{\hspace{0.17em}}{m}_{a}}$$

The acceleration of the ALE node is computed by:

$${\overrightarrow{\gamma}}_{a}=\frac{\overrightarrow{F}{}_{a}}{{m}_{a}}$$

The grid velocity of the ALE node is equal to the material velocity of the Lagrange node:

$${\overrightarrow{w}}_{a}={\overrightarrow{v}}_{i}$$

The normal material velocities of Lagrange and ALE nodes are equal. Therefore:

$${\overrightarrow{v}}_{a}\cdot \overrightarrow{n}={\overrightarrow{v}}_{i}\cdot \overrightarrow{n}$$