The density value is set to assist at the domain outer flow boundaries. If rho_operating – rho_outside is zero you can set the pressure boundary as 0 Pa. If you don’t get a zero difference you need to account for boundary height against the difference so pressure becomes a profile and varies with height.
For a couple of metres the pressure variation on the boundary isn’t much, but in natural convection flows the driving pressure for the flow is also very small.
Hopefully https://courses.ansys.com/index.php/courses/heat-transfer-modeling-in-ansys-fluent/lessons/how-to-model-natural-convection-in-ansys-fluent-lesson-4/ will explain more clearly. The search tool was upgraded overnight, so we can look everywhere rather than just in the Forum.
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