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Volume in Monitored volumes

Hello,

I wanted to know how Radioss calculates at every time step the volume of a monitored volume, i did not find the answer in the theory manual. I'm using four monitored volumes type COMMU1, comunicated by dummy surfaces, but the geometriy is a little bit complex and during the simulation, the common surfaces are sometimes outside the monitored volumes as you can see in the image, the green surface on the top. How this impact What would be the difference between using multiple monitored volumes COMMU1 and only one monitored volume type AIRBAG?

Thank you

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Altair Forum User

Hi,

The AIRBAG and AIRBAG1 are uniform pressure airbag. Hence, regardless of state of inflation or shape, the pressure inside the chamber remains uniform.

Whereas COMMU monitored volumes are used where user expects a progressive deployment.

The basic energy equation of the airbag can be written as dE = -PdV + dHin - dHout

Where:
E is the internal energy
P is the pressure
V is the airbag volume
Hin is the incoming enthalpy
Hout is the outgoing enthalpy

This equation above allow the current airbag volume to be determined. Attaching a pdf document from RADIOSS Reference Guide on Monitored Volume which explains about the calculations.

The model behavior seems to be bit strange. Attaching a sample model file on COMMU monitored volume type for your reference and ensure that you have followed similar steps for modelling.

Unable to find an attachment - read this blog

Altair Forum User

Post modified: Quoting George message

Altair Forum User

Hi,

The AIRBAG and AIRBAG1 are uniform pressure airbag. Hence, regardless of state of inflation or shape, the pressure inside the chamber remains uniform.

Whereas COMMU monitored volumes are used where user expects a progressive deployment.

The basic energy equation of the airbag can be written as dE = -PdV + dHin - dHout

Where:
E is the internal energy
P is the pressure
V is the airbag volume
Hin is the incoming enthalpy
Hout is the outgoing enthalpy

This equation above allow the current airbag volume to be determined. Attaching a pdf document from RADIOSS Reference Guide on Monitored Volume which explains about the calculations.

The model behavior seems to be bit strange. Attaching a sample model file on COMMU monitored volume type for your reference and ensure that you have followed similar steps for modelling.

Hello George,

Thank you for your answer.

I do not understand yet why volume is calculated using the thermodynamic equation. So, how is calculated the initial volume if we only know the initial pressure and temperature but we do not know the initial mass inside the control volume?

I though that it was calculated using a geometric approach, using the position of nodes and elements, and then using some algorithm to calculate the enclosing volume. Then this volume would be used to calculate other thermodynamic variables such as pressure or temperature, and not the reverse. That is why I supposed having common surfaces outside the volumes will cause an error in the volume calculation.

Unable to find an attachment - read this blog

Altair Forum User

Hi,

You can check the initial volume from the starter out file. Also,please check the element normals as the shell elements must orient outward.