# pressure problem

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June 29, 2013, 10:35
pressure problem
#1
Senior Member

Saeed
Join Date: Jan 2013
Posts: 177
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Hi.
I am modeling a channel on Fluent
my results for pressures in Compared with laboratory experiments are very large. may be i should calculate other pressure (out of static or absolute). Is there difference between static and hydrostatic pressure? ( What are relations between pressures)
see attached image
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 pr.jpg (61.5 KB, 16 views) vf.jpg (58.2 KB, 12 views)

Last edited by flow_CH; July 5, 2013 at 05:23.

 July 1, 2013, 02:10 #2 New Member   Qasim Ali Join Date: Mar 2013 Location: Islamabad, Pakistan Posts: 18 Rep Power: 6 why dont you try boundary conditions of pressure inlet and outlet if both of these are known to you, i hope it might work out for you

July 1, 2013, 05:24
#3
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Saeed
Join Date: Jan 2013
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Quote:
 Originally Posted by mqasimali why dont you try boundary conditions of pressure inlet and outlet if both of these are known to you, i hope it might work out for you
I have pressure outlet boundary with gauge pressure=0 but for pressure inlet boundary i can not define velocity for inlet. Can you tell me what relations are between static pressure and other pressures?

July 1, 2013, 05:42
#4
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Join Date: Nov 2011
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You will find all the equations here:

http://www.sharcnet.ca/Software/Flue...et-hydrostatic

But I've got another question concerning the pressure issues. According to this manual:

Quote:
 An important consequence of this treatment of the gravitational body force is that your inputs of pressure (now defined as ) should not include hydrostatic pressure differences. Moreover, reports of static and total pressure will not show any influence of the hydrostatic pressure.
In that case, is it possible to display the hydrostatic head or include it in the static pressure?

 July 2, 2013, 10:59 #5 Member   Hossein Join Date: Oct 2010 Location: Greensboro, NC, USA Posts: 30 Rep Power: 8 first, tell me your model size, boundary conditions and the size of your meshes __________________ Hossein Amini PhD student in Biochemical Engineering; Computational Science and Engineering department; North Carolina Agricultural and Technical State University

 July 2, 2013, 11:06 #6 Member   Hossein Join Date: Oct 2010 Location: Greensboro, NC, USA Posts: 30 Rep Power: 8 there is no difference between static and hydrostatic but static and dynamic pressures are different (remember something like bernouli's equation) v^2/2g is dynamic pressure, P0 is the atmosphere pressure, and Z is the static pressure since you have movement in your system, you should consider the total pressure __________________ Hossein Amini PhD student in Biochemical Engineering; Computational Science and Engineering department; North Carolina Agricultural and Technical State University

 July 2, 2013, 11:17 #7 Senior Member   Saeed Join Date: Jan 2013 Posts: 177 Rep Power: 6 My model lenght is 2 meters, and 1 meter Width that it is going gradually small.( 1 meter in inlet and 0.9 meter in outlet) I have two inlet boundaries for water and air ( velocity inlet for water=4 m/s and air=0.001 m/s) and one pressure outlet with gauge pressure=0. Up boundary is pressure inlet with gauge pressure=0. Others are wall. I tested meshes with 0.008 interval size (Hex/wedge) and bigger and with pressure inlet for air inlet but all of them are approximately similar. I know that negative pressure is normal in fluent but why in exactly on inlet? Why is big difference between my results of laboratory pressure and fluent pressure? thanks

 July 2, 2013, 12:52 #8 Member   Hossein Join Date: Oct 2010 Location: Greensboro, NC, USA Posts: 30 Rep Power: 8 in such problems, mesh size really changes the results. for such size, you need to at least use a mesh size of 1-3mm. boundary conditions seem fine, but try to put the outlet as out-flow and the top as symmetry. I have used symmetry for open channel flow before, at it's really better that pressure BCs. __________________ Hossein Amini PhD student in Biochemical Engineering; Computational Science and Engineering department; North Carolina Agricultural and Technical State University

 July 2, 2013, 14:12 #9 Senior Member   Saeed Join Date: Jan 2013 Posts: 177 Rep Power: 6 My flow regime is super critical and it is better i use velocity inlet. Outflow and pressure inlet are not compatible together. It used just with flow rate inlet boundary. (section 7.10 user guide)

 July 2, 2013, 17:26 #10 Member   Hossein Join Date: Oct 2010 Location: Greensboro, NC, USA Posts: 30 Rep Power: 8 that's ok, I know (I meant you could use velocity inlet for air and out flow for both air and water, basically, there is not much difference between pressure outlet and outflow) so, for water inlet-->velocity inlet air inlet --> pressure inlet and for both outlets-->pressure outlet for the top, pressure inlet is OK, but as I have experience, symmetry is better (although it is said it should be used for symmetrical problems) the BCs are OK now. let's move on to your other options tell me about the model you're using, transient or steady state?, if transient, what is the time step? and number of iterations per time step? residual limits, and solution controls all these parameters really affect the solution __________________ Hossein Amini PhD student in Biochemical Engineering; Computational Science and Engineering department; North Carolina Agricultural and Technical State University

 July 2, 2013, 23:57 #11 Senior Member   Saeed Join Date: Jan 2013 Posts: 177 Rep Power: 6 I am using steady state. VOF - implicit - k-epsilon RNG I did not active implicit body force in multiphase panel. All absolute criteria for residual are 1e-3. Pressure velocity coupling is SIMPLE. Under relaxations are between 0.2-0.5. Pressure discretization is PRESTO. Last edited by flow_CH; July 3, 2013 at 01:24.

 July 3, 2013, 14:10 #12 Member   Hossein Join Date: Oct 2010 Location: Greensboro, NC, USA Posts: 30 Rep Power: 8 send your mesh file to my email, I wanna check it __________________ Hossein Amini PhD student in Biochemical Engineering; Computational Science and Engineering department; North Carolina Agricultural and Technical State University

 July 4, 2013, 06:53 #13 Senior Member   Saeed Join Date: Jan 2013 Posts: 177 Rep Power: 6 The lowering of mesh interval size could not help me too. Nobody can not tell me why my static pressure is high?

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