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-   -   Fluent Basic Doubt (http://www.cfd-online.com/Forums/fluent/48281-fluent-basic-doubt.html)

Fluent learner May 19, 2008 23:52

Fluent Basic Doubt
 
Hi Friends,

I am very new to fluent software and just now started learning fluent...

In that i have one doubt i modeled an horizontal pipe and given 2 bar as pressure inlet boundary condition in that i want to see the presssure drop in the pipe..

What i did is Operating Pressure 1 bar and pressure inlet is 2 bar is my boundary condition...

Suppose if i go to pressure results and see means i am getting static pressure some arround e-02..But how it is possible to get, generally there should be negligible pressure drop inside the tube..

And for viewing the pressure reults which pressure i have see whether static, absolute pressure or total pressure...

Please clear my doubt friends..Thanks in advance...

With regards, Fluent learner

mAx May 20, 2008 00:43

Re: Fluent Basic Doubt
 
waht is you outlet BC? if it is pressure Outlet... you give to fluent the pressure drop as BC, it means you become massflow as result. If you want to compute the pressure drop you should set the massflow (or velocity) at the Inlet and pressure Outlet at the Outler

Fluent Learner May 20, 2008 00:53

Re: Fluent Basic Doubt
 
Hi Max,

Thanks for ur reply.....

I kept pressure outlet as my boundary condition... Whether am i correct or not.... And for viewing the pressure results which pressure i have to see whether static, total or absolute pressure...

With regards, Fluent Learner

mAx May 20, 2008 01:45

Re: Fluent Basic Doubt
 
then you are wrong. If you already specify the pressure drop, you cannot expect another drop pressure else than the one you specified. Switch with massflow inlet and pressure outlet. Then for computing your drop pressure, go to surface report panel (Area Weighted Average), and choose Total pressure for inlet and outler. Pressure Drop is simply the difference between both values

Fluent Learner May 20, 2008 02:06

Re: Fluent Basic Doubt
 
Hi Max,

Thanks once again, then in results what is that Static, Absolute, Total pressure means please can u explain me that one, so that i may get cleared max...

Thanks.

Fluent Learner.

mAx May 20, 2008 02:25

Re: Fluent Basic Doubt
 
absolute_pressure = static_pressure + operating_pressure

total_pressure = static_pressure + dynamic_pressure

dynamic_pressure = 0.5*rho*u**2

Fluent Learner May 20, 2008 02:41

Re: Fluent Basic Doubt
 
Hi max,

Can u tell me why it is not possible to calculate the pressure drop by using pressure inlet as my Boundary condition and preesure outlet as my outlet condition.. And that too i am not having mass flow rate or velocity data with me as my inlet condition max...

So please can u tell me what can be done for this..

Thanks, Vijay

mAx May 20, 2008 05:46

Re: Fluent Basic Doubt
 
Sure you can calculate the pressure drop with the same procedure I mentionned earlier. But if you set your Pressure inlet at 2 bar, and the outlet at 0 bar (free), you will have a pressure drop (inlet-outler) from 2 bar. in this case, it is interesting to know the massflow for this pressure drop

Fluent Learner May 20, 2008 06:24

Re: Fluent Basic Doubt
 
Hi max,

Still i have doubt please can u give me ur contact no. So that i may call u to clarify my doubts... Please dont mind for this... And thanks for ur co operation....

With regards, Fluent Learner

mAx May 20, 2008 09:19

Re: Fluent Basic Doubt
 
please do some tutorials to get more familiar with the software. you can also make a model of your pipe (2d-axi) and observe what fluent computes with your BC. This is the best way to get your own opinion.

Ramesh.K May 20, 2008 13:55

Re: Fluent Basic Doubt
 
Hi, The flow I think ur simulating is a subsonic flow (think for sake). so for this case u need to specify two quantities perssure and density at the inlet out of the three unknows pressure, density and the velocity. from the given pressure boundary condition at inlet the code will approximate velocity at the inelt where at the out let u need to specify only one quantity either pressure or density the other two will be approximated.


Fluent Learner May 20, 2008 23:56

Re: Fluent Basic Doubt
 
Hi Ramesh,

Sorry i am not getting you..... Can u give ur mail id so that i may send my cas and data file to ur mail.. And please help me how to do this basic procedure....

Thanks

With regards, Fluent Learner

Ramesh. K May 21, 2008 01:15

Re: Fluent Basic Doubt
 
Hi, we are trying to solve 3 equations in any problem of fluid mechanics, this is known to u. If you rewrite the N-S equations to solve any problem in terms of primitive variables the three variables are density, pressure and velocity. so when we fix boundary conditions we fix according to the type of the flow. so in subsonic flow we fix two quantities either pressure or velocity or density which ever is known to us according to the problem. so look at ur problem and see what u know at the inlet either pressure or velocity the known quantity is prescribed and the other will be extrapolated by the code. So at the outlet u should not specify the same quantity as us specify at inlet and at the outlet two quantities will be extrapolated and the other one will be specified by you. the specified boundary at the oulet should be different from the quantity u specify at the inlet.

by the way we dont use packages we write our own code for the problems we solve any way try pressure inlet and for this u should use outflow look into the fluent manual for the type of boundary conditions to be used.

bye


Fluent Learner May 21, 2008 01:57

Re: Fluent Basic Doubt
 
Hi ramesh,

Thanks for ur reply and spending some time to me... But still i have some doubts in that ramesh... I had given 2 bar as by boundary condition and kept 0 as pressure outlet to find the pressure drop inside the pipe but i am getting static pressure result as some e-02... How is it posssible becuase there should be very negligible pressure drop and that too i am using only laminar flow....

So please can u tell me were i am wrong please ramesh... So get the results properly...

With regards, Fluent Learner

Ramesh. K May 30, 2008 00:48

Re: Fluent Basic Doubt
 
Hi, first know that whether u are specifying gauge pressure or absolute pressure as the boundary conditions. because 0 pressure cant be there try giving 1 instead of zero. remember one thing when ever u are testing a code for a given problem u are doing experiments virtually think in this manner and try again with other pressure values.

Fluent Learner May 30, 2008 01:52

Re: Fluent Basic Doubt
 
Hi Ramesh,

I kept operating pressure as 1 bar then pressure inlet as my boundary condition that as 2 bar(gauge pressure).... And kept my pressure outlet as 0 because i have seen some of the tutorials files regarding this one.... Is that procedure correct or not.....

With regds, Fluent Learner

Ramesh. K May 30, 2008 04:09

Re: Fluent Basic Doubt
 
make the operating pressure to 0 and remaining at 2 bar at inlet and 1 bar at oulet



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