Induced flow model-Boundary Conditions
 

Hello to everyone,

I'm trying to do a CFX simulation of the airflow through an AFPM electric generator. My model consists of two Rotors (circular disks) each of one having 32 magnets located at their periphery (cuboid boxes) and a Stator in the middle of the Rotors. The Stator is a resin disk which contains 24 coils inside of it. There is a small air-gap (4mm) between each Rotor and the Stator and the Rotors are rotating at about 200RPM. So the air comes in from the front face of the Rotor, moves radially and goes out through the air-gap at the periphery (like a centrifugal fan).

What I want to simulate through CFX is the flow that is induced by the rotating discs. The problem is that in contradiction with most of the tutorials that I have come up with, the mass flow at the inlet or outlet is not a known variable for me. So the Boundary conditions that I have set to the model are 0 MPa relative total pressure at the inlet and 0 MPa relative static pressure at the outlet. However, the solver stops with an error message in this case and only if I give a mass flow rate at the inlet or a outlet I can have a non-zero velocity vector at the results.

Is there an explanation to this? How can I ask from CFX to compute by itself the velocity vector? Should I change my BCs?

P.S.My purpose is to do a CHT analysis, but as a first approach I just want to make sure that my model includes the rotating disks' induced flow, so for the time being, no heat transfer takes place at the setup.

Thnx for your time,
Alexandros Rontogiannis

Sounds like you need to use pressure boundaries, probably set to atmospheric pressure.

This FAQ should help: http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F

thnx ghorrocks for your answer. I didn't find a helping FAQ to be honest..

I changed my BCs a litllte bit.

What I did is to define the boundary conditions by defining the static pressure at the Inlet and Openings of the fluid stationary domain. This domain encloses the stator and a smaller rotating fluid domain that encloses the rotors.

So, at each domain I have a relative pressure=1 atm and for the big stationary fluid domain I have a static pressure=0 atm at inlet and 0 atm static pressrue at the openings, too. I choosed to make a transient analysis (instead of a steady state one that I had before) and it seems to work! At the results I have a velocity distribution which seems to take the rotation under consideration.

However, as I noticed at the CFX Modelling guide there is an issue of ''robustness'' when defining the static pressure both at inlet and openings. What exactly does ''robustness'' mean? Is there any trouble for me with this set of BCs?

Robustness in this context means the solver is liable to diverge and/or crash.

You will note that the transient approach is a common fix to these sort of issues and is discussed in the FAQ :)