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PB4 September 24, 2010 05:22

Pressure outlet backflow
Hi there, my first post, been reading this forum for quite a while now..

I ran through the DFBI tutorial, all went ok.
Now I want to do the same with my own hull, Hull created in Catia, very similar to the dfbi one, exported in Catia as iges, I then import it in star ccm+ (v4.04), and split my body by patch to get all my boudaries separated, in the end it all looks like the dfbi tutorial, now I do the meshing, surface and volume all look ok, probably will need a bit more work in some interest areas but for now will do.
I set all the rest as in the dfbi tutorial (except I use flat wave) and when I run the simulation I get this kind of message :
"reversed flow on faces limited in xxxx cells in region Outlet" for every inner step,
If I let it run I can see the flow around my body is as expected but at the outlet it looks like I have a backflow creeping in, forming like a wave, eventually the whole domain fills up...

Now what am I doing wrong ? I thought maybe my outlet face is reversed ? is that even possible ? How can I check it and correct it ?

Thanks all !

PB4 September 24, 2010 05:29

1 Attachment(s)
Attachment 4787

picture now uploaded.. :)

bengy September 24, 2010 07:28

try with extruder option in mesh continuum. you should "extrude" your outlet


PB4 September 24, 2010 09:04

1 Attachment(s)
extruding the pressure outlet face by 1m, 10 sections didn't solved the problem, here is a new shot..
Attachment 4789

bengy September 24, 2010 09:06

could you show me your mesh with extruded outlet?

PB4 September 24, 2010 09:33

1 Attachment(s)
sure :
Attachment 4790

bengy September 24, 2010 09:38

please explaine me something. I would thing that this trimmer part is your extrude part and brown "wall" is your outlet, am i right?

PB4 September 24, 2010 10:03

well, the red surface is the water iso surface, there is no geometry surface at that position, the water surface is initiated at 0.5 of the region vertical extend that's what you see, now since I extruded the outlet, the iso surface cut in the volume mesh shows the extension.. the brown outlet before the extenssion would be at the start of the extruded section..
hope this helps.. one of my first CFD as well..

bengy September 24, 2010 10:07

yeah, but I asked you about your mesh because I wanted to see this "extension" after extruding your outlet

PB4 September 25, 2010 10:23

has to be the way I define my outlet, here is what I tried:
- My 6 DOF boddy is only able to Z translate
- First I checked my volume mesh, all perfect cells
- I split my inlet so only the "front wall" is the inlet, the side wall and top+bottom are set as slip walls.
- I swapped the inlet and outlet, now my back wall is my inlet and my front wall is my outlet, off course I reversed the current and wind speeds

=> I have the same phenomena creeping on the outlet, water seem to disappear from bellow..

I also created another simulation, rectangular region with basic rectangle body, it works, no disappearing water...

bukm January 14, 2011 03:43

try the outlet as split outlet....

PB4 January 14, 2011 04:00

Well "reverse flow in.." meant just that, the flow at my inlet was negative.. Very embarassing.. Made me learn a lot though..

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