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December 17, 2006, 00:57 
"Pressure Inlet" Boundary Setup

#1 
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Hi All,
I have a prolem in defining the correct boundary condition for my case. If I used the "pressure inlet" boundar condition; I never got the exactly the same as the "static pressure" as put after completed the iteration. But FYI, after iteration completed, the total pressure is the same as I put. So what should I do? to get result as I expect (same static presure). I have done it with several boundary type of out let condition. But still not the same at the inlet. hopefuly someone could help me. Thanks & regards Rahmat 

December 18, 2006, 06:23 
Re: "Pressure Inlet" Boundary Setup

#2 
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hi: i am also interested in these quesiton and all pressure boundary conditions in fluent. the "pressure" (in fluent)is P'=P ru*g*r (where g & r are vector) ,according to these definition, and help doucument of fluent, the hydrostatic pressure in a fluid at rest is then P'=0. but you could have a try,make a square zone ,all boundarys are wall.and use 'VOF' method ,upper half of zone is air down is water,(it is not a question to give analysis resolution),or you could think a cup with a lid in the cup there is a half cup of water, finally all fluid should be at rest,but finally you will get votex or other motion in FLuent. (g=9.8kg/m*s y )


December 18, 2006, 10:43 
Re: "Pressure Inlet" Boundary Setup

#3 
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If you're modeling subsonic flow, then the total pressure is a fixed value, and the static pressure is only used when initializing the flowfield (see "Defining Static Pressure" in Chapter 7.3.1 in the Fluent Users Manual).
The rho*g*r that wayne described is for the hydrostatic head, and only comes into play if you have gravity turned on. Also, if you're working with a gas, then rho is really small, therefore the hydrostatic portion is small. For gases, it's only really related to bouyancy driven flows. It can be a big player for liquids though. If you post more info on your problem then you might get some suggestions for changes. Describe the "knowns" about your problem, and maybe a different set of BCs can be described. Hope this helps, and good luck, Jason 

December 19, 2006, 01:21 
Re: "Pressure Inlet" Boundary Setup

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HI Jason the following is from Chapter 7.3.1 of fluent User mannual
The pressure field (Ps` ) and your pressure inputs (Ps` orP0` ) include the hydrostatic head , rho*g*r . That is., the pressure in FLUENT is defined as: Ps`=Ps  rho*g*r (7.31) what is the "pressure field (Ps` ) and your pressure inputs (Ps` orP0` )" here? is it static pressure in display panel ? Gauge Total pressure in the pressure inlet input panel?Gauge pressure in the pressure outlet input panel? or All of three?i donot konw how to input the Gauge Total pressure in the pressure inlet input panel&Gauge pressure in the pressure outlet input panel In a problem which i want to turn on the gravity. do i need to subtract the rho*g*r from the really Pressue(the pressure i get from experiment),for example, if i calculate piple flow which i donnot want to neglect gravity from momentum equation,and inlet, which i know the really pressure distribution (suppsoing it can be obtained from a real experiment,Pinlet),is pressure inlet the outlet is open to the air(Patm),and the calculation will start from pressrue in let.so how can i decide the Gauge Total pressure& Intial Gauge pressure in the pressure inlet input panel & Gauge pressure in the pressure outlet input panel? is it like it: pressure inlet: Gauge Total pressurePinlet_input=("Pinlet"rho*g*r1)Poperating_pressure Intial Gauge pressurePstatic=("Pinlet"1/2*rho*v*v)Poperating_pressure Preesure Out let: Gauge pressure Poutlet=(Patmrho*g*r2)Poperating_pressure if it does.how can i decide "v" and "r1" "r2"? 

December 19, 2006, 02:00 
Re: "Pressure Inlet" Boundary Setup

#5 
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go on: i have tried before:make a square zone(XY plane,gravity is in the direction of y) ,and define all four boundary as "wall" boundary condition in Fluent,turn on the gravity in the operating Conditions panel.so there is no motion of fluid and calculation is converged with only 1 iteration. so the static pressure distribution is "0" which has subtract rho*g*r from the really static pressure distribution. so i guess the Gauge Total pressure (in the pressure inlet input panel ) i input should subtract rho*g*r which the gravity is open in the operation conditions panel.the question is how can i define the r,where is reference '0' for 'r'?is it origin point of coordinate?


December 19, 2006, 09:52 
Re: "Pressure Inlet" Boundary Setup

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First, I never trust a solution that converges in 1 iteration. Means your convergence criteria is useless. You should turn off the "check for convergence" options in the Solve>Monitors>Residuals menu. Then create a surface monitor of static pressure.
Second, what was your fluid? Was it a gas or a liquid? If it's a gas, you wouldn't see much, if any, difference along the wall. If it's a liquid, then it's incompressible, and the "0" value for r doesn't matter, because for incompressible fluids the pressures are all on a relative basis (absolute doesn't matter, only gauge... that's why if you don't have a pressure inlet, setting the reference pressure location keeps the absolute pressure from running away... if you do have a pressure inlet, then this is going to fix your absolute pressure, and again the "0" value for r will be trumped by the constraints of the BC). Hope this helps, Jason 

December 20, 2006, 01:34 
Re: "Pressure Inlet" Boundary Setup

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THANKS! But,first,i try to turn off the "check for convergence" options. Then create a surface monitor of static pressure.and there is no change in the both monitr of static pressure and Residuals(continuity,x,y velocity. ) (iter continuity xvelocity yvelocity time/iter
1 0.0000e+00 0.0000e+00 0.0000e+00 0:00:00 9999 2 0.0000e+00 0.0000e+00 0.0000e+00 2:46:38 9998 3 0.0000e+00 0.0000e+00 0.0000e+00 2:13:18 9997 4 0.0000e+00 0.0000e+00 0.0000e+00 1:46:37 9996 ...........) i think it is because the question is the motion in all area is zero and the boundary condition is easy,so the the iteration goes so fast. so the another question is how to decide if the iteration goes convergence? usually we turn on the "check for convergence" options.my convergence criteria is from that. second.i do not quiet understand the pressure definition in the FLuent,which is define as P`=Prho*g*r according to the user`s mannual,so the quesiton goes with these definition is that what is the 'pressure' define in the pressure boundary condition panel?should the pressure in fluent pressure boundary input panel subtracte ' rho*g*r' from the pressure in the real world?for example the free surface boundary condition is Patm,if a question in fluent which turn on the calculation of gravity,the free surface boundary condition is still P0? or P0rho*g*r? another question is if a problem (gravity is turned on) with two presuure outlet boudary(A(x,y),B(x,y)),both A&B are open to the air,as the common knowledge,the boundary condition to A&B are both pressure boundary conditionP=Patm,if the direction of gravity is 'y', and A_y is not equal to B_y,According to definition of pressure in fluent P`_A=Patmrho*g*R_y and P`_B=Patmrho*g*R_B so we will input the different value of pressure for these two pressure outlet boundary,is that right? if it is right what is the reference point for r? 

December 20, 2006, 16:18 
Re: "Pressure Inlet" Boundary Setup

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Honestly, I've only used gravity for DPM simulations on a steady state model (it was injecting tracer particles into air... so we didn't use gravity for the steady state solution, just to see where the particles would go). Search the forum for anything with gravity in it and see if it helps. Other than that, try things and see what happens.
Good luck, Jason 

December 21, 2006, 00:12 
Re: "Pressure Inlet" Boundary Setup

#9 
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thanks!


December 21, 2006, 03:59 
Re: "Pressure Inlet" Boundary Setup

#10 
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by the way are you interested in hydrocyclone?or other cyclone problem? how do you define you boundary condition? for overflow and underflow? thanks!


December 21, 2006, 09:31 
Re: "Pressure Inlet" Boundary Setup

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Sorry, never worked on any cyclones. Most of my work has been external aerodynamics.
I've seen several posts on here about that type of work though, so I'm sure you can find what you need. Good luck, Jason 

December 23, 2006, 07:21 
Re: "Pressure Inlet" Boundary Setup

#12 
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hai all,
I have made no progress. I problem is so simple. For example, we have a flow throgh a long pipe. the boundary condition is expected to be Inlet static pressure and mass outlet flow. But somehow the inlet static pressure is not the one that i have setup, after complete the iteration. But the total pressure is the same as manuallu calculation. Do you know what should I do?? regards rahmat 

April 9, 2013, 21:26 

#13 
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Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,185
Rep Power: 16 
hi
whats the procedure of pressure inlet bc process in subsonic inlet? Which variable is calculated by zero gradient (from internal field) and which is calculated?at some arrangment? I have read the help but not grasp exactly. Could anyone help? 

April 11, 2013, 08:50 

#14 
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Ehsan
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Location: Iran
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Rep Power: 16 
could anyone guide?


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