[arjdk]

I read in the Fluent manual that the outlflow boundary condition is unsuitable "if you are modeling unsteady flows with varying density, even if the flow is incompressible."



Is anyone able to explain or know why that is?

[kisssinger]

outflow bc is close to a fully developed condition, as the outflow boundary conditions assumes a zero streamwise gradient for all flow variables except pressure.

[arjdk]

Well even thought the flow is unsteady with varying density, can't the outflow not still have zero streamwise gradients for each time step?