Defining "Neutral" boundary conditions? (force=0)

MW

Hello,

Up to recently I have been using COMSOL but have changed to fluent for various reasons. In COMSOL I was using a boundary condition called, "Neutral boundary condition". Their definition of this boundary condition is - "The neutral boundary condition means that no forces act on the fluid." Also in respect to my model, "The neutral condition defines that you assume the computational domain extends to infinity."

My model is a small heater sitting on what can be described as an infinite base with and infinite fluid above it. The small heater is creating a convective flow in the fluid. Basically, I want to model a finite part of the infinite domain - i.e. mainly the convective stream. It was suggested that I model this with walls far away from the heater. In other words the walls are far enough away from the heater and the convection, that it is essentially the same as an infinite domain, because the no-slip boundary is not close enough to interact with the convection stream. I semi-successfully modelled this but I found the walls have still have a large influence. Also, there is the obvious increased computation time due to the increased size of the mesh when modelling my problem in this way.

Does anyone know how to model a "neutral" boundary condition? It seems like it would be commonly used, as part of the boundary condition set within fluent, but I can't find anything. I can post some more info on my problem if needed.

Regards,

MW