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Old   February 11, 2008, 08:52
Default Urgent need please help me
  #1
rosol
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Dear all ,

I need to solve the flow and heat transfer in channel under constant pressure drop and Re .How can I specify the pressure and velocity at the inlet of channel to specify the pressure drop along channel and Re .

any help will appreciat

Thanks
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Old   February 11, 2008, 10:27
Default Re: Urgent need please help me
  #2
Dong Fu
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for pipe flow you can calculate the pressure Loss due to Frication (Major Loss) by using Darcy-Weisbach Equation

http://en.wikipedia.org/wiki/Darcy-Weisbach_equation

hope it helps

==== Dong Fu
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Old   February 11, 2008, 11:18
Default Re: Urgent need please help me
  #3
rosol
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Dear Dong,

Thank you for your response. But what I need is to fixing the pressure drop along channel and hence there are no need to calculate the pressure distribution along channel.since fluent solve the pressure -velocity coupling by a certain algorthim such as simple .I need to introduce the value of constant pressure drop and Renold No.and solve the momentum and energy equations along channel.How to do that ??

with thanks
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Old   February 11, 2008, 11:26
Default Re: Urgent need please help me
  #4
Dong Fu
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Dear rosol

It is a pressure driven flow, isn't it? If so ,the pressure lose is due to friction if it is straight pipe.

you might specify different pressure in the inlet and outlet. Will it works?
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Old   February 11, 2008, 11:29
Default Re: Urgent need please help me
  #5
rosol
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The pressure which I mean is the pressure required to drive the flow i.e from pump .what is the B.C must be used to specify the pressure at inlet in addition to velocity

Regards
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Old   February 11, 2008, 21:40
Default Re: Urgent need please help me
  #6
Dong Fu
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This is pressure-inlet in FLUENT
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Old   February 12, 2008, 02:30
Default Re: Urgent need please help me
  #7
rosol
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Yes is the pressure inlet but when I chose pr.inlet B.C that is mean the fluent also will slove the coupling of pressure-velocity and calculate the velocity according to pr. intoduced. I need to introduce the value of both pr. and velocity or i.e in addition to pr. I wand to fixing the value of velocity at the inlet.

Regards
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Old   February 12, 2008, 05:49
Default Re: Urgent need please help me
  #8
randomwalker
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giving both pressure and velocity at the same inlet sounds like overspecifying the problem.

What are you really trying to determine? If looking for the wall roughness or pipe diameter that will give you the 'correct' pressure drop at a given velocity, you may need to do a series/parametric study for the given velocity or pressure and manually iterate your way to the answer you want.
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Old   February 12, 2008, 08:19
Default Re: Urgent need please help me
  #9
rosol
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My question in brief is:

How to simulate the flow "calculate the velocity distribution along 3D channel " at constant pressure drop along this channel and constant Renold No.

With thanks
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Old   February 12, 2008, 09:26
Default Re: Urgent need please help me
  #10
-----------
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YOU REPEAT SEVERAL TIMES FIXING VELOCITY & RENLODS BE AWARE IF YOU FIXE THE VELOCITY REYNOLDS ALWAS IS FIXED I M RIGHT? FOR SETTING PRESSURE AT INELT TRY THE FOLLOWING: 1)SET THE OPERATING PRESSURE TO ZERO THEN USE PRESSURE INLET BC TO SET THE PRESSURE VALUE YOU WANT . 2)FOR SETTING THE INLET VELOCITY JUST SET ITS VALUE IN THE THE INITILIZE PANEL BY GOING TO SOLVE>INITILIZE THEN INITILIZE THE SOLUTION.

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Old   February 12, 2008, 10:42
Default Re: Urgent need please help me
  #11
rosol
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Dera friend,

Thank you very much for your replay

But I did the steps you mentioned but the velocity did not fixed since the fluent calculate the inlet velocity according to inlet pressure contrariwise what I need .

Regards

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Old   February 13, 2008, 00:50
Default Re: Urgent need please help me
  #12
JSM
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Hello,

From your posts, I understood that you need to specify the pressure and velocity in the inlet. It is not possible only using by single boundary condition. Specify the pressure (say 3 bar) in the operating pressure panel and use velocity inlet boundary condition to fix the velocity based on the reynolds number.

Since the water is incompressible, if you specify the operating pressure, you can achieve both.

Hope it will help you

with warm regards, JSM
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Old   February 13, 2008, 02:46
Default Re: Urgent need please help me
  #13
rosol
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Dear JSM,

Thank you very much for your nice feeling.

But the problem remain the same when I define the value of veocity at the inlet and use the velocity inlet B.C fluent will calculate the pressure at inlet according to the value of velocity.

Best regards
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Old   February 13, 2008, 02:49
Default Re: Urgent need please help me
  #14
JSM
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Hi rosol,

once you changed the operating pressure, then you should check the absolute pressure at the inlet. It will give the correct pressure value.

hope it will help you

with warm regards, JSM
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Old   February 13, 2008, 05:15
Default Re: Urgent need please help me
  #15
rosol
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I think the changing of operation pressure is not useful becaues the operating pr. effect on the all channel i.e inlet and outlet therefore is don't affect the pr. drop along the channel
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Old   February 14, 2008, 00:17
Default Re: Urgent need please help me
  #16
JSM
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Hi,

If it is the case, then you are simulating unrealistic one. Use pressure are velocity inlet boundary condition based on your reynolds number. And cross check the pressure drop achieved in the simulation with your experimental data. It is the right procedure.

Check the case file once again, for missing parameters like surface roughness etc..

with regards, JSM
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Old   February 14, 2008, 11:05
Default Re: Urgent need please help me
  #17
Amadou Sowe
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JSM, What you are saying makes sense. But I think what rosol wants is the solution to a fully developed channel or pipe flow where th inlet and outlets are translationally periodic planes. This way rosol will specify the mass flow rate and fluent will calculate the pressure gradient or he will specify the pressure gradient and fluent will calculate the mass flow rate. I hope this suggestion is helpful if not I am sorry for disturbing your dialogue. Regards, Ammadou
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Old   February 15, 2008, 02:45
Default Re: Urgent need please help me
  #18
rosol
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Dear Amandou thank you very much

I thing the problem is with me since my question is simple but I didn't to formulated or expressed it in right manner .And I will repeat my problem here :

I want to simulate the flow in channel under constant pressure gradient and Renold No. and as I understant from fluent it solve the coupling of pressure and velocity .When you specify the pr. fluent will calculate the velocity and when specifying velocity it will calculate the Pr.

I want to specify both the pressure and velocity at the inlet of channel .

I will dont express best this Regards
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Old   February 15, 2008, 12:01
Default Re: Urgent need please help me
  #19
dc
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hi,

you can not specify both velocity and pressure at the same boundary. for getting a constant pressure gradient you would require a fully developed flow. this means simply giving velocity will not work. what is the best bet for you is to specify a velocity profile (say, parabolic in case of laminar flow) at the inlet and pressure at the outlet. you can obtain analytically the velocity distribution from the reynolds number, fluid used and pipe size. this will give you all the quantities. difference in total pressures at the inlet and outlet will give you loss.

dc
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Old   February 16, 2008, 12:33
Default Re: Urgent need please help me
  #20
rosol
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But I know the value of pressure drop (loss) along the channel and velocity at the the inlet and want to introduce it and calculating the distribution of only velocity along channel .
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