# Pressure-volume coupling, ETFE cushion

Hello,
I would like to create a pressure-volume coupling for a two-layer ETFE cushion, however I don’t know how to do it.
So far, for the ETFE cushion in Kiwi, I can do the formfinding and static analysis for the foil layers separately.
But for the calculation of the cushion, for example for a wind load, I would like the bottom and top foil layers to be coupled by the enclosed air layer or air volume. Since the internal pressure of the cushion cannot be adjusted for the wind load, which only acts briefly, I have a closed system for the cushion in which the internal pressure x volume = constant (Boyle Mariotte law: p*V = constant).
Does anyone have an idea how I could solve this in Kiwi?

Furthermore, I was wondering what unit the internal pressure PRES_FL has for the surface load? Since I would like to have e.g. 250 Pa as internal pressure for my cushion.

I have added the file of my ETFE cushion in case it should help. However, the calculation for the foil layers is still done separately here.
ETFE-Kissen_2-lagig.3dm (46.3 KB)
ETFE-Kissen_2-lagig.gh (35.1 KB)

Hey,

pressure-volume-calculation is unfortunately not directly available in Kiwi. However, there could be a workaround using e.g. the plugin Anemone. It can use Kiwi iteratively. So you compute the deformation for the initial pressure, get the deformation and compute thereof the new volume and respective pressure. The new pressure value and the deformed membrane are then again input for the following iteration. In order to keep the resulting stresses and prestress aligned to the initial direction, you should use the reference element component. The deformed element is the updated one, whereas the other stays the same initial one. This approach is probably time-consuming but it should work.

1Pa = 1N/m². You can use the unit converter for computing the value using the chosen units in the analysis. 250Pa are e.g. 0.25 kN/m².

Hi,
thank you very much for your answer! I will try it out.