# Porosity

- $A$
- Area of the specified surface
- $\lambda $
- Scale factor
- $P$
- Pressure of the gas
- $t$
- Time

It is also possible to define closure of the porous surface in the case of contact.

The isenthalpic model is also used for porosity. In this case, one can define the outgoing
surface by:(1)

$${A}_{OUT}=\lambda \text{\hspace{0.17em}}f(A)\text{\hspace{0.17em}}g(P)\text{\hspace{0.17em}}h(t)$$

Where,

- $A$
- Area of the specified surface
- $\lambda $
- Scale factor
- $P$
- Pressure of the gas
- $t$
- Time

It is also possible to define closure of the porous surface in the case of contact.

The second model integrated in Radioss is called the Chemkin
model, in which the mass flow due to the porosity is computed by:(2)

$$\dot{m}={A}_{vent}\text{\hspace{0.17em}}u\text{\hspace{0.17em}}{\rho}_{airbag}$$

Where,
$u$
is the user-defined function, represented as:(3)

$$u=f\left(P-{P}_{ext}\right)$$