[boffin5]

After working with some heat transfer cases, just dealing with templates and tutorials, regarding the pressure initial condition, it struck me:


For chtMultiRegion cases, the pressure units are kg/(meter second^2), and the default value in the tutorials is 10000.


Whereas, in incompressible simpleFoam cases, the pressure units are meter^2/sec^2 and the default value is 0.


Can someone help me to understand the differences in these units, and the default values?

[joshwilliams]

Hi,



In incompressible simulations, we are only interested in pressure gradients, i.e. if pressure at inlet is 10020 Pa and pressure at outlet is 10000 Pa in real life, we can just express it as 0 at the outlet and 20 at the inlet. It is much easier. But of course in compressible simulations, there is a change in density that depends on total pressure, thus the 'default value' is generally around 10000 Pa. In incompressible simulations, the 'default values' will be relative to some baseline (usually 0 m^2/s^2 at the outlet).



Considering the units, the density is constant, so we divide the pressure on the right hand side of the momentum equation by density (making the units go from Pa to m^2/s^2).


Best,
Josh

[boffin5]

Josh,


Thanks, I really appreciate your help. This helps me to understand the basics, after I been thrashing through the minutia.


Alan w