Use of surfaceNormalFixedValue for inlet and outlet


Hello every one,

In many cases we know the flow rate into and out of the domain, So can’t we able to give constant fixed velocity at inlet and at the outlet like it is possible to define in fluent. In my case it is a heat transfer problem by using bouyantSimpleFoam. The area of the inlet and outlet both are different and they also have different velocity and temperature too. when I run the simulation at the first iteration it self it is giving mass inflow is not equal to mass outflow. by following the mass conservation principle I have try to change the temperature at the outlet, there by thinking as changing the density pass the conservation principle. Still I am not succeed in this, Please anyone who have try this they can help me or give me some suggestion to get me out of this problem.

Thanks & Regards


Hi @shadabdyn,

can you share an example with us to have something we can refer to? In general you have to make sure that the refValue is defined in your case. surfaceNormalFixedValue is a vector boundary condition requiring a minus sign if you want the flow to go inside the domain. Is that something you have taken into account inside your simulation? As I already said feel free to share your simulation here in the chat. @PowerUsers_CFD and I will then have a look at it, no problem.

Cheers and all the best!



Hi @jousefm,
I am solving a heat transfer problem in a room containing inlet and outlet and I want to specify velocities on both inlet and at outlet.

Generally at the outlet zeroGradient for velocity and fixedValue BC is applied at the outlet and my simulations are running fine. But, when I tried to specify the surfaceNormalFixedValue BC at the outlet with positive refValue and for pressure as a zeroGradient BC. This combination giving me the continuity error stating that inlet mass flow rate is not equal to outlet mass flow rate. To over come this problem I used fixedValue BC for pressure at the outlet instead of zeroGradient and my simulations are running fine for few Iterations. The temperature value on some cells near the outlet is increasing gradually and going into thousands range and then K and Omega residuals of my KOmegaSST model is started diverging. So It will be nice if you can suggest me how to specify fixedValue velocity at the outlet with combination of pressure BC which runs well and give comparable result with the fluent.


Hi @shadabdyn!

could you share the project with us please? Also tagging @Fluids_Support here who might give you more insight on how to overcome this issue effectively.




It would be helpful to have access to the project because the only thing I can say based on the description is that:

  • Specifying a fixed velocity at both the inlet and outlet is overconstraining the system of equations as the mass conservation principle would take care of the mass flow at the outlet.
  • In the CAD both surfaces may have the same surface, but when meshing some minor differences in total area could arise, specially when the faces are not aligned with the mesh. That could well be the reason for the continuity error —due to overconstraining.
  • A fixed velocity at the outlet will, as far as I know, impose a constant and uniform value over the whole surface which is unrealistic, more so when the outlet is surrounded by no-slip walls with a boundary layer.

Also, are you using the very same mesh as in Fluent?


Hello All @pfernandez @jousefm @Fluids_Support,

I am attaching one of my case, I have tried to specify surfaceNormalFixedValue boundary condition at the outlet. But at the outlet it always either may give high velocity on some cells are high temperature and from there my solution start diverging.

I am seeing the effects mention by @pfernandez. I am still finding a way to specify the velocity at the outlet patch, because in some case outlet flow rate will always be less than the inlet as in the case of ventilation and in thermal comfort studies. According to steady state analysis the mass inflow is equal to mass outflow, So shall we able to implement this kind of velocity BC at outlet ?

I am unable to upload my case file, therefore I am providing the g drive link.

Thanks in advance


Hi @shadabdyn!

I somehow cannot access the file you provided us with. Could you change the privacy settings please?