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flow_CH June 29, 2013 11:35

pressure problem
 
2 Attachment(s)
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

mqasimali July 1, 2013 03:10

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

flow_CH July 1, 2013 06:24

Quote:

Originally Posted by mqasimali (Post 436978)
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?

Gimlas July 1, 2013 06:42

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?

hossein65 July 2, 2013 11:59

first, tell me your model size, boundary conditions and the size of your meshes

hossein65 July 2, 2013 12:06

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

flow_CH July 2, 2013 12:17

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

hossein65 July 2, 2013 13:52

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.

flow_CH July 2, 2013 15:12

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)

hossein65 July 2, 2013 18:26

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

flow_CH July 3, 2013 00:57

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.

hossein65 July 3, 2013 15:10

send your mesh file to my email, I wanna check it

flow_CH July 4, 2013 07:53

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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