Outlet pressure boundary condition upside down?
 

I have set a static pressure outlet for my simulation in the form of the following function:

1.474[atm]*exp(-g/R*(17/12*0.029)[kg m K^-1 mol^-1]*ln(12[m^-1]*y+4589))-0.98[atm])

where Y is the vertical position.

As you can see in the picture attached, it is a decreasing function so the relative pressure at higher hights is lower (as it should be). The thing is when i run the simulation and then see a pressure countour on that boundary on post processing, the function inverts itself, in other words, the relative pressure is lower at LOWER hights. I tried multiplyng the whole function by -1 to see what happens and it gave me function behaviour, but with higher pressures overall.

Anyone know why this happens and/or how can i fix it?

In buoyancy simulations CFX uses a modified pressure which has the static head component removed. That means you applied pressure boundary conditions will need to take into account the static head as well.

If you look at the absolute pressure you will see the absolute pressure as usually defined. But for buoyancy simulations, p+p(ref) will not equal the absolute pressure.

And in case you are wondering, the buoyancy reference location and density are used to calculate the static head component, so you will need to take into account density*gravity*height from these reference conditions.

Thanks for the explanation!

So in other words, as well as the absolute pressure distribution makes sense you are good to go?

Hopefully now both the absolute pressure and the pressure fields make sense now. I would not proceed until all variables are correctly set.

I couldnt fix it. I set relative pressure = 0 Pa on the outlet and my inlet is 0 velocity and a temperature profile (the other boundaries are walls).

I set the buoyant reference density to air density evaluated at an average temperature in my domain, and the reference location in the origin. Despite all that, the relative pressure in my domain still is higher at higher hights, which makes no sense.

How can i fix this?

The problem will be in the details of how you have accounted for the static head, what you are modelling and your material properties. But you are going to have to look into it and work it out, we cannot help you unless you post more details (your CCL and your geometry).