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 :confused:

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

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.

backflow
 

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.

reversed flow
 

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.

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.

Long pipe (laminar flow) gets reversed flow - not converging
 

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?

hello
I have same problem but the result converge
is the result will be right

It happens due to a poor quality of mesh, abnormal boundary condition at inlet/outlet, short downstream length etc.
Remedies are:
> if vortex formation/recirculation of flow near an outlet boundary, increase the downstream length.
> check the mesh quality, improve it.
> use higher order scheme
> At last reduce the relaxation factor if necessary.

Hello guys, (Doubt atmospheric conditions)

I am working on rectangular geometry with air as fluid. Actually i have one inlet at the one side face of the rectangle and one outlet is quite opposite to the other side face of the rectangle and reaming all are walls. I want atmospheric conditions at both inlet and outlet, for that i have taken pressure inlet and pressure outlet with default values (i.e. Gauge pressure is Zero).

But unfortunately when I am simulating I am getting reverse flow. Moreover my solution is converging. Could you please any one explain me. What kind of mistake I have done. I am attaching my Residual and geometry images.

Revesed flow heat transfer
 

Yes..Hybrid Initialization works most of the times for me...No reversed flow...

Reverse flow is on, this massage will be shown when your result will diverging.

To solve this problem, you should check the boundary conditions properly and most importantly change the under relaxation factor from solver settings. Lower the relaxation factor, get higher stability in convergence. And I think thus you can get more better simulation result.

Sorry, that is not correct. Reverse flow could be a normal flow behaviour at your boundary, if you not have a straight forward outflow. Especially at pressure outlets and buoyancy driven flows. There are some possiblitys which could help:

• If energy is on, set the backflow temperature to the average temperature at the outlet (via report definition and output parameteres)
• Turn on the Prevent Reverse Flow option, in this case Fluent will create artificial walls at the cells where the backflow occurs.
• Redesign the geometry of your outlet that the flow behaviour is more direkt.