Learning Forum

[Antonin HIPPOLYTE]

Hello everyone,

I tried to simulate gravity driven fluid flow in Fluent (gravity=9.8m.s^-2, water, 1m heigh, two outlets with same pressure) and the velocity resulting was 12 orders of magnitude too low.

So I closed the outlets to see the hydrostatic pressure profile and it is also ridiculously low.

Further I have a good hydrostatic profile (10^4 Pa of difference for one meter of water) with a gravity of 10^14 m/s^2.

Do you have any idea why the effects of gravity exist but are far too weak ?

 

Thank you in advance

[Rob]

 

https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_bcs_sec_bound_cond.html%23flu_ug_sec_pinlet_hydrostatic

Also, please confirm what material is set in the fluid cell zone panel. 

 

[Antonin HIPPOLYTE]

Thank you for your answer but I'm sorry, it seems I don't have acces to this link.

As I said, the material I use is water but, even with the lightest gas, a pressure of 1e-11 Pa is unexplainable.

[Rob]

Click on Help in Fluent. Then paste the link into the browser. 

Please post a screen grab of the cell zone panel: many copy water into the available materials and then don't get them into the model. Easy error, hence me wanting to confirm. 

[Antonin HIPPOLYTE]

Thank you for the help.

First, as you can see I use water in the cell zone panel.

 

Then, thank you for the link, I understood the plotted pressure doesn't contains the hydrostatic term. But in that case I have two questions :

• Even if it is very low I have still a hydrostatic pressure profile, and it is too clear to be a machine zero error. And multiplying the gravity by 10^13 I obtain the right hydrostatic profile. Why ?
• Why is the velocity so low ? you see I have water but even with air it should be greater.

[Rob]

What operating density did you set?

[Antonin HIPPOLYTE]

Thank you very much, I didn't know about operating density and I had the one by default.

With an operating density of 1000kg/m^3 I have a much better pressure profile yet still not the 10000 Pa we would expect from 1m of water.

[Rob]

Keep reading! Also note, water in Fluent is 998.?? kg/m3 unless you altered it. 

[Antonin HIPPOLYTE]

I wrote 1000 because it was faster but I'm not sure a difference of density of 0.2% would induce a pressure gradient 1000 times too low...

[Rob]

Read the section covering (rho - rho_operating).