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Old   January 30, 2014, 05:28
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  #21
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Anvesh
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Solution is converging at the end. For a channel with smooth surfaces either side, pressure drop is matching with theoretical drop. But if I use channel with lower surface has steps (such as 25 x 25 microns), we have to stop the simulation after fluid fills the channel (except the pockets), and get the pressure drop. But immediately after the start of flow, reversed flow occurring at outlet. Will it effect the result (in my case, its pressure drop)? If affects, how to avoid reverse flow?

P.S. I am new to FLUENT
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Old   January 30, 2014, 05:51
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Ganapathy Iyer
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HI,
Suppose I have a duct. In the duct, the inlet occupies only 25% of the cross section a the top. The outlet occupies the complete cross section of the duct.
In this case, if the duct length is small, the outlet will have reverse flow. This will also happen in the real world.

If you want to avoid this, you must know the outlet pressure profile and apply it using a UDF or by dividing the outlet in appropriate number of parts.
All this is tedious, so simply pull the outlet much further away from its current location, i.e. if the outlet is at say 100 micron from the inlet, (this may be your actual case), pull it till say 500 micron( this is not real, but this will avoid reverse flow)
You can always measure dP across a plain representing the real outlet location
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Old   February 18, 2014, 00:30
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Thank you very much sir.

And one more problem I want to pose. Since I am imposing const. velocity condition at the pipe inlet I am getting a greater pressure drop, so I extended the upstream length. It is causing more computational time. So how to impose parabolic velocity inlet at the pipe inlet in FLUENT. Pl help me.
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Old   July 17, 2014, 03:58
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hi all,

ı have an rectangular geometry (lenght:76, width:27 mm). heat flux is applied from one of the vertical walls. ı have velocity inlet BC at lower part and also pressure outlet BC at upper part. velocity magnitute is zero (0) (no forced convection), initial gauge pressure is also zero and temperature is 298 K at velocity inlet BC. also, for pressure outlet BC gauge pressure is 0 and backflow temperature is set 298 K. initial temperature is also 298K.

the problem is that reversed flow occured.
"reversed flow in 34 faces on pressure-outlet 17"

the solution is converged but ı am not sure it is right or not.

any suggestion. thanks for you time.
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Old   July 25, 2014, 16:17
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hi all,

ı have an rectangular geometry (lenght:76, width:27 mm). heat flux is applied from one of the vertical walls. ı have velocity inlet BC at lower part and also pressure outlet BC at upper part. velocity magnitute is zero (0) (no forced convection), initial gauge pressure is also zero and temperature is 298 K at velocity inlet BC. also, for pressure outlet BC gauge pressure is 0 and backflow temperature is set 298 K. initial temperature is also 298K.

the problem is that reversed flow occured.
"reversed flow in 34 faces on pressure-outlet 17"

the solution is converged but ı am not sure it is right or not.

any suggestion. thanks for you time.
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Old   July 28, 2014, 08:47
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i don't know if it helps, but if possible: try to use smaller time steps and with multiphase flows you should also enable specified operating density. it solved my problem with the reversed flow.

Last edited by svefu14; August 22, 2014 at 14:58.
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Old   July 6, 2016, 04:23
Default Long pipe (laminar flow) gets reversed flow - not converging
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My model is a u-shape pipe which is pretty long (220 cm total). There is heat flux from walls. The error is repeated continuously while solving. Also the answers are not converging.
Can anyone help me please?
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