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-   -   Pressure drop in laminar flow (https://www.cfd-online.com/Forums/fluent/45838-pressure-drop-laminar-flow.html)

carno August 24, 2007 07:26

Pressure drop in laminar flow
 
I have solved a laminar flow problem. The Re number is 0.2. The objective is to calculate the pressure loss in the system. But the CFD simulation is giving around 17% errors. Sometimes the errors are positive and sometimes negative. I tried different mesh densities but in vain.

What could be the reason? Has anybody faced such problem?


AAA August 24, 2007 21:17

Re: Pressure drop in laminar flow
 
Hi

Can you give more detail about the system? And what did you use for the length when calculating the Reynolds number. Are you using double presicion? Did you check/adapt the velocity gradients?

Regards

salam August 25, 2007 08:58

Re: Pressure drop in laminar flow
 
AAA can you tell us what the relation between adapting the velocity gradient and predicting the pressure drop?


AAA August 25, 2007 09:32

Re: Pressure drop in laminar flow
 
Salam salam

In general, having smooth gradients for all parameters (p, v, etc.) by using the adaption tool offers better results.

carno August 27, 2007 08:17

Re: Pressure drop in laminar flow
 
Initially I calculated the Re number at inlet by hydraulic diameter. Then I checked the Re number at various points calculated by Fluent (from display->contours->velocity->Re).

I have not tried grid adaptation. I have used velocity inlet and pressure outlet. Now I am trying pr-pr BC with target mass flow rate. Let us see.

Let me know if you have any inputs.

Thanks for your comments.


salam August 27, 2007 09:48

Re: Pressure drop in laminar flow
 
OK can you guide us to how can did this with more details?


AAA August 27, 2007 12:42

Re: Pressure drop in laminar flow
 
Salam salam

Just grab any CFD book and see how pressure and velocity are coupled. The smoothness of the staggered grid dictates how accurate the final solution is. If you have a more specific question, I'll try my best to dig up the answer for you or at least refer you to a reference.

Kind regards

AAA

AAA August 27, 2007 20:38

Re: Pressure drop in laminar flow
 
Hi

Please try adaption and tell me if you obtain better results.

carno August 28, 2007 07:27

Re: Pressure drop in laminar flow
 
Hi,

I tried the grid adaptation method. But the error has has not gone and there is not much difference in results in this case. Now I am doubting the fluid properties at that temperature.

Let me have your comments.

AAA August 28, 2007 11:11

Re: Pressure drop in laminar flow
 
Hi

When you mentioned a 17% error, are you comparing your results with other people? What are your BCs? Is it an article you are comparing your results with?

If the case is simple 2D, I'll try to simulate it, If you like. Maybe I can find what is wrong.

Regards

carno August 29, 2007 02:51

Re: Pressure drop in laminar flow
 
Hi AAA,

The 17% error is with experimental results. The case is 3D. I have used the 4 different meshes and grid adaptation to track the error. Flow is laminar Re=0.2. Fluid is very viscuss. mu ~ 1 kg/m-s, rho ~ 0.8.

I have taken the fluid properties from its data sheet. This can be one source of error. Actual properties might be slightly different.

What do you think?

ak September 9, 2007 11:46

Re: Pressure drop in laminar flow
 
hi, should we try with outflow as BC? & as you say fluid properties can be one of the reasons. Is it assumed the walls to be smooth?


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