natural convection again

Jan Langebach

Hi, I have the same problem as two other users in this forum. Since answers are rare I try it once more: I try to model a heat sink in free convection. To test various geometries I want to cut one fin from the middle of the heat sink and use periodic boundary conditions for the left and right borders to keep the grid small. So far no problems. To get a free buoyancy driven flow I selected pressure-inlet at the bottom and pressure-outlet at the top of the domain. Both got the pressure value 0 Pa. The fins are at constant termperature. But the solution dont want to be stable. I get an upstream next to my isothermal fin wall but also an downstream inbetween two fins. That seems to be not realistic since the gravity is pointing downwards. Modelling the same problem in forced convection had no problems, is easy to converge and agree with physics. But I have no idea why free convection doesnt work. I would appreciate any advice or experience.

Thanks. Jan