# Hydrodynamic Viscous Fluid Materials (LAW6)

This law is specifically designed to model liquids and gases.

- ${S}_{ij}$
- Deviatoric stress tensor
- $\overrightarrow{V}$
- Kinematic viscosity
- ${\dot{e}}_{ij}$
- Deviatoric strain rate tensor

## Perfect Gas Model

`C`

_{0},

`C`

_{1},

`C`

_{2},

`C`

_{3}must be set to equal zero. Also:

A perfect gas allows compressibility and expansion and contraction with a rise in temperature. However, for many situations, especially very slow subsonic flows, an incompressible gas gives accurate and reliable results with less computation.

## Incompressible Gas Model

- $c$
- Speed of sound

Incompressibility is achieved via a penalty method. The sound speed is set to at least 10 times the maximum velocity.

This classical assumption is not valid when fluid and structures are coupled. In this case, set the sound speed in the fluid so that the first eigen frequency is at least 10 times higher in the fluid than in the structure.