Learning Forum

[Dato]

Hi everyone,

I'm running a transient simulation and want to applied  a steady flow field result as initial condition for transient. It's a Multiphase simulation using VOF. One inlet and outlet. But I found that Static Pressure is quite different from steady flow when I read steady result into transient case. Image on the left is the Static Pressure in steady flow and the right is the one after enabling VOF model and reading steady data. 

Though it's a little strange but the increased pressure seemed to be reasonable in transient case due to the height of about 6.5 meters. 

How can we explain for the situation?

 

[Rob]

Assuming both are solved solutions? What fluid(s) are you using? 

[Dato]

I need to run transient simulation. I solved flow and turbulence equation in steady flow and used it as initial flow field in transient. I'm just using default water-liquid with temperature dependent density and it's a single phase flow.  

Sorry that I'm not exactly understand what "Assuming both are solved solutions?" means. Flow field has converged in steady flow. 

[Rob]

If you run single phase and then turn on multiphase you need to be very careful with the data because of the mixture threads and how operating density is accounted for. 

 

[Dato]

Yes! I disabled multiphase model to run steady flow and write .dat file, after which enabled multiphase model. Then read the .dat file. It is time that the situation occurred. I didn't change operating density. According to Fluent user guide, operating density needn't to be cared  because density is temperature dependent and hence Fluent would ignore it. 

It sounds like something unknowable had happened internally when setting multiphase model. Well, thank you Dr Rob. It's quite a noteworthy experience that cannot to get in Fluent manual. 

[Dato]

Plus: 

The reason why I writed steady data is that I found after enabling multiphase model, all data have changed. The static pressure turn to 0 Pa. 

[Rob]

Assuming the single phase was run with water you may be able to patch a result. Moving from single to multiphase is always complex as you move from domain to phase/mixture domain and not all data has an equivalent. You can run VOF with only one phase, so set any boundary conditions to the phase of interest and ensure you fill the domain with that phase. There are some more things to consider but it generally works. 

[Dato]

I got it. In order to have a single phase steady flow result to be patched as initial condition of transient multiphase flow, it's better not to switch off multiphase model and turn it on, which is what I have done. It's better to set the case under multiphase model. 

Thank you for your helpful advice. 

[Rob]

It's the easiest option in most cases. There are other ways, but I'm not sure they're technically public knowledge so I can't share on here. Keep looking in the Knowledge Materials as we release more tips over time. 

[Dato]

Okay, it's said that there are still a lot of techniques to explore with Fluent. It's not just about simply clicking the mouse. There is still a long way to go for me. 

Thanks, Dr Rob.