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-   -   About compressible microchannel flow! (http://www.cfd-online.com/Forums/main/5608-about-compressible-microchannel-flow.html)

Sander Nagoo January 6, 2003 09:19

About compressible microchannel flow!
 
Dear sir:

Hi! I'm new to CFD. I would like to simulate the compressible microchannel flow (by using dimensionless compressible Navier-Stokes and energy equation). The working fluid is Nitrogen,through microchannel 1.2e-6m height and 3000e-6m long for a pressure range of 1.34 to 2.5 atmospheres (outlet pressure is fixed at atmosphere pressure, 100.8 kpa). The boundary conditions are the following:

1) u=u0,v=0 ====> inlet

2) u=0,v=0 ====> upper and bottom wall

3) p=100.8 kpa => outlet

After several tests I found that the centerline u velocity jump sharply in the entrance region and then levels off with a slight linear increase at the exit. I don't know this result is correct or not! Would you please give me some suggestion!! Thank you very much!!

Best regard Sander Nagoo

versi January 7, 2003 02:25

Re: About compressible microchannel flow!
 
do you use u,v extrapolation at oulet?

Sander Nagoo January 7, 2003 04:06

Re: About compressible microchannel flow!
 
Dear Dr. Versi:

Thank you for your reply. Yes! The boundary value at oulet are obtained by extrapolation. Did you ever calculate this problem (compressible microchannel flow)? Because my centerline u velocity jump sharply in the entrance region (only in entrance region) and then levels off with a slight linear increase at the exite. Would you please give me some suggestion! Thank you for your help!!!

Best regard

Sander Nagoo

Alton Reich January 7, 2003 10:48

Re: About compressible microchannel flow!
 
Sander,

Are you using a constant velocity at the inlet boundary?

Consider what a fully developed velocity profile in the channel should look like.

Does this provide any insight to what's going on?

Regards, Alton

Sander Nagoo January 8, 2003 00:36

Re: About compressible microchannel flow!
 
Dear Dr. Alton:

Thank you for your reply. I'm so sorry to disturb you again!

1)Yes! I using a constant velocity at inlet.But the centerline u velocity still jump sharply in the entrance region(only in retrance region).

2)Actually, I thought about fully developed velocity(such as entrance region) before, but the velociuty jump don't happen to analytical solution. Did you ever calculate the compressible microchannel flow. Please help me!

Best regard

Sander Nagoo

versi January 8, 2003 20:12

Re: About compressible microchannel flow!
 
There are many discussions about inlet & outlet BCs before. In your compressible NS case, temperature or total pressure like varibables may be required on BCs. A constant U at inlet is not consistent with NS.

Sander Nagoo January 9, 2003 01:09

Re: About compressible microchannel flow!
 
Dear Dr. Versi:

Thank you for your help! Sorry to disturb you again. I obey your instructions that a constant U at inlet is not consistent with NS. Therefore,by using fully developed velocity at inlet. The centerline u velocity sudden jump in the entrance region never happen again. But I don't know this procedure(not constant velocity at inlet) is correct or not. Please help me!!Thank you very much!

Best regard

Sander Nagoo


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