[pierresandre]

Hi,

I am modelling natural ventilation from a point source in an open space using Fluent with the Boussinesq approximation. The model includes pressure inlets at the bottom and at the sides and a pressure outlet at the top. The source is at the bottom of the domain and is modelled as a wall with a constant heat flux.

Following Fluent’s manual recommendations for natural ventilation I have set the pressure of the pressure inlets and outlets to zero as I am not interested in having an external flow. However, when I run the case, I have inflows from the top and side boundaries that are not expected.

I have checked the pressure stratification with height and found out that the pressure is not zero throughout the domain but decreases linearly with height. Thus the pressure of the gas at the same height of the pressure outlet is less then zero. The end result is that the pressure outlet is at a higher pressure than the gas at the same height and this is causing air to flow into the domain from above in places where it should not. I have tried to decrease the Rayleigh number but there was no improvement. I even tried to run a case in which I have three walls with a pressure outlet at the top so that the Rayleigh number would be zero and still had the same problems.

The difference between the pressure of the pressure outlet and that of the gas at the same height is not given by rho*g*h but it is about 1% of that value. However, since I am considering very small velocities, this is enough to make my simulation go haywire. On the good side, when I correct for this 1% pressure difference, then I get the expected results.

Thus I would like to ask if anyone else has encountered this difficulty and if you think that my solution to the problem is adequate.

Thank you.