Simulation Forums

Find Simulation answers, share expertise, and connect with your peers

Simulation CFD


Outlet Condition - 0 Pressure not working properly

412 Views, 12 Replies
03/07/2018 10:39 AM

Hi there, 

 

I hope you can help me. I'm trying to simulate a free surface water flow over a weir. At the inlet condition, I have set up a volume with a fluid height and an inlet boundary condition (flow volume rate). At the outlet face, I have set a static 0 Pa gauge pressure. All water reaching the outlet boundary should leave the simulation model.

I have started the simulation with a small ans imple part of the weir, where the outlet boundary condition was working fine. 

However, as I tried to simulate the entire weir, the outlet boundary isn't working anymore. Water does not leave the computational area. It reacts as if hits a wall at the outlet face and flows right back into the model.

 

Any ideas what how to improve the simulation setup so that the water can leave the simulation?

 

Thanks

David


Report Inappropriate Content
Message 1 of 12 ( Views: 412 )

Re: Outlet Condition - 0 Pressure not working properly

03/07/2018 11:55 AM

I think we might need to see a model to help you here, could you share a CFZ file (support file) with us?

 

Strange behaviour for sure.

 

Thanks!

Jon


Report Inappropriate Content
Message 2 of 12 ( Views: 397 )

Re: Outlet Condition - 0 Pressure not working properly

03/08/2018 6:40 AM

Hi Jon, 

 

thanks for offering your help. I have uploaded the CFZ-file in the attachment. 

I guess the problem is linked to the mesh... Is it possible 0 pressure at the outlet for free surface simulations only works with a very fine mesh?

I understand free surface requires a fine mesh, but because the region of interest within the model is not the outlet boundary approx. 10m downstream of the weir, I tried to coarsen up the mesh at the outlet...

 

Best
David

ATTACHMENTS:
Weir.cfz

Report Inappropriate Content
Message 3 of 12 ( Views: 386 )

Re: Outlet Condition - 0 Pressure not working properly

03/08/2018 12:41 PM

Thanks for sharing.

 

I think you doubled up on gravity, which might have an effect here :)

I would also stick to a better mesh, or you can end up with fluid seeming to disappear.

 

How about adding an additional fluid volume where the real fluid will be so that you do not have to mesh the 'air' to finely?

 

Be careful with the time steps saved too, I would aim for no more than 50 or so for a solution. Perhaps 100 at a push. 

How many will you end up with?

 

Thanks,

Jon


Report Inappropriate Content
Message 4 of 12 ( Views: 376 )

Re: Outlet Condition - 0 Pressure not working properly

03/09/2018 3:08 PM
Hi Jon,

thanks for your suggestions. I redefined the region where flowing water is expected. Refined the mesh in general, coarsened the "air" and specified an extra region at the outlet. 0 Pa pressure works fine now. So it seems the problem was really the coarse mesh at the outlet.
And many thanks for the double gravity hint ;)

Best
David

Report Inappropriate Content
Message 5 of 12 ( Views: 366 )

Re: Outlet Condition - 0 Pressure not working properly

03/13/2018 6:43 AM

Hi! ;) I also want to do this, share the right CFZ file, please!


Report Inappropriate Content
Message 6 of 12 ( Views: 349 )

Re: Outlet Condition - 0 Pressure not working properly

03/28/2018 8:23 PM

I'm running into a similar issue - what do you mean by "doubling up" on the gravity?


Report Inappropriate Content
Message 7 of 12 ( Views: 320 )

Re: Outlet Condition - 0 Pressure not working properly

03/29/2018 6:02 AM

Gravity only needs to be assigned once :)

 

What settings do you have for gravity right now?

Also, what makes you think you have the same issue?

 

Thanks!

Jon


Report Inappropriate Content
Message 8 of 12 ( Views: 317 )

Re: Outlet Condition - 0 Pressure not working properly

08/02/2018 9:47 AM

Hi;)! 2G.jpgdouble gravity?


Report Inappropriate Content
Message 9 of 12 ( Views: 273 )

Re: Outlet Condition - 0 Pressure not working properly

08/02/2018 6:46 PM

Hi @svetashov,

 

Your image above implies you have 2 G in the postive Y direction and 1 G in the negative Z. I am not sure what planet you're on Smiley Wink Just use a 1 for gravity. If upwards is in the positive Y, then make gravity (0,-1,0). Check out the image below. 

gravity vector.jpg



Matt Bemis

Technical Support Specialist


Report Inappropriate Content
Message 10 of 12 ( Views: 255 )

Re: Outlet Condition - 0 Pressure not working properly

08/15/2018 5:28 AM

Hy! thx!  ;)  correctly?7.jpg


Report Inappropriate Content
Message 11 of 12 ( Views: 59 )

Re: Outlet Condition - 0 Pressure not working properly

08/15/2018 6:33 AM

What Matt shared is the correct setting (you have double gravity right now)


Report Inappropriate Content
Message 12 of 12 ( Views: 55 )