# COMMU1 Type

The airbag simulation used by Radioss adopts a special uniform pressure airbag. Hence, regardless of the state of inflation or shape, the pressure is uniform.

Perfect gas law and adiabatic conditions are assumed. Injected mass and temperature are defined as a time function. A sensor can define the inflate start time.

Deflation of vent hole is available after reaching a pressure (${P}_{def}$) or time (${T}_{def}$) criteria.
The key assumptions are:
• Uniform airbag pressure $⇔$ kinetic energy is negligible
The airbag simulation must include:
• Injection of energy and mass
• Bag mechanics (i.e. unfolding, expansion, membrane tension, impacts, ...)
• Exhaust through vent holes

This option is used to simulate chambered airbags and may be used to unfold an airbag.

Each COMMU1 type monitored volume works like an AIRBAG type monitored volume with possible vent communication with some other monitored volume of COMMU1 type. A chambered airbag is therefore modeled with two or more COMMU1 type monitored volumes.

Each monitored volume can have an inflater and an atmospheric vent hole.

Monitored volume 1 can communicate with monitored volume 2 with or without communication from 2 to 1. Communicating area, deflation pressure or time from 1 to 2 can be different from corresponding values from 2 to 1. It is thereby possible to model a valve communication.

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## Thermodynamical Equations

Same equations as for AIRBAG type monitored volume are used. Refer to Thermodynamical Equations.

## Mass Injection

Same equations as for AIRBAG type monitored volume are used. Refer to Mass Injection.

## Venting

Same equations as for AIRBAG type monitored volume are used. Refer to Venting Outgoing Mass Determination.

The mass flow rate is given by:(1) ${\stackrel{˙}{m}}_{out}={\rho }_{vent}{A}_{vent}u=\rho {\left(\frac{{P}_{ext}}{P}\right)}^{1}{\gamma }}\text{\hspace{0.17em}}{A}_{vent}u$
The energy flow rate is given by:(2) ${\stackrel{˙}{E}}_{out}=\stackrel{˙}{m}\text{\hspace{0.17em}}\frac{E}{\rho V}={\left(\frac{{P}_{ext}}{P}\right)}^{1}{\gamma }}\text{\hspace{0.17em}}{A}_{vent}u\text{\hspace{0.17em}}\frac{E}{V}$

These mass and energy flux are removed from the current volume and added to the communicating volume at next cycle.

## Supersonic Outlet Flow

Same equations as for AIRBAG type monitored volume is used. Refer to Supersonic Outlet Flow.

## Jetting Effect

Same explanation as for AIRBAG type monitored volume is used. Refer to Jetting Effect.

## Reference Metric

Same explanation as for AIRBAG type monitored volume is used. Refer to Reference Metric.

## COMMU1 Type Examples

### Example: Communication between the 2 Volumes

Volume 1 communicates with volume 2 and vice-versa.

Monitored volume 1 communicates with monitored volume 2 with or without communication from 2 to 1. The communicating area, deflation pressure or time from 1 to 2 can be different from the corresponding values from 2 to 1. It is thereby possible to model a valve communication.

### Example: No Communication between 2 and 3

Volume 1 communicates with volume 2 and volume 2 with volumes 1 and 3, but there is no communication from 3 to 2.

Two COMMU1 type monitored volume communications can have common nodes or common shell property sets but this is optional.

To model a folded airbag, one COMMU1 type monitored volume is used for each folded part. The boundary between two folded parts is closed with a dummy property set (fictitious property). The pressure in each folded part will be different and the area of communication will increase during inflation. With this model, the volume with inflater will inflate first and before than folded parts (better than "jetting" model).

### Example: Monitored Volume with Communication Coefficient

Volume 1 and volume 2 with common property set.