Learning Forum

[Chenxi Wang]

Hi all,

 

I am trying to simulate the pressure wave in a pipe with surge tank (below pic). The simulation uses two phases (air and water) VOF model. The pressure wave is generated by a sudden velocity increase at the inlet boundary. Right now, I am facing two problems. 

(1). For air phase, should I use compressible liquid or idea gas law considering an isothermal condition?

(2). As the speed of pressure wave is much faster than the inlet velocity, how can I ensure the courant number below 1 as tiny time step can slow down simulation?

 

Any suggestions would be appreciated.

[DrAmine]

Yes either Tait Equation or ideal gas but not solving energy equation can work.

Using adaptive time stepping with physics constraint to keep CFL number smaller than one and take into account acoustic time scale.

[Chenxi Wang]

Thanks for your reply. I have tried both compressible liquid and idead gas law (without solving energy equation). The solutions of pressure profile are different while the same setting was employed. The result using compressible liquid looks more realistic. Do I need to do any special setting for ideal gas?

By the way, what does acoustic time scale mean? Do you mean the acoustic CFL?

Thanks in advance. 

[DrAmine]

Please check the documentation to know more about it. Check the densities using Tait or Ideal Gas (Energy OFF). Pretty sure the there are density diffrences. Ideal gas with switching off energy equation is generally just a worakround. Tait Equation better suitable for liquids. Just now use the best for your setup corresponding to reality.