Hydraulic diameter for multiple ducts
 

Hi,
I am making a simulation of gas diffusion in room.
In which continuous air is coming from 4 ducts and 4 exhausts are there and gas is introduced from some place inside the room.

I have used k-e model with realizable and with buoyancy effect. Is this correct for this simulation?
And what should be the hydraulic diameter for 4 ducts? Do i need to make 4 individual inlets and give BC to them OR can i give BC to all at single BC,by multiple selection. and what should be the hydraulic diameter that case?

It is advised to keep your inlet and outlets individual and not bunch them in to groups. So you can keep the 4 inlets and 4 outlets separate and calculate hydraulic diameter separately for each inlet and outlet.

It depends on the mesh model. If your inlets are not connected before entering the room then you should set BC for each inlet. If you are using circular cross section of pipes then pipe diameter is the hydraulic diameter, else there is a good formula to calculate it.