Learning Forum

[warhammer975]

Hello! A small question about setting boundary conditions in Ansys Fluent. I am trying to model the air movement in the gap between a stationary outer and a rotating (revolutions are known) inner cylinder. Air is supplied with a fixed mass flow uniformly through the side surface (gap) between the cylinders. When setting the air flow through the Mass-Flow-Inlet, you need to specify the Radial, Tangential, Axial components of the flow direction. Please tell me how to correctly calculate these components if the geometry, mass flow and revolutions are known?

[Rob]

You calculate based on what vector you want the flow to take at the inlet. For swirl it'll be mainly axial with some tangential but you may find axial + angular velocity is more appropriate. 

[warhammer975]

Thanks for the quick reply! Could you please elaborate on this?

[Rob]

The mass flow boundary (and if it's constant density use a velocity bc) has direction vector options as you've seen. The component values are for you to decide, but for swirl I'd tend to favour axial and angular components over axial and tangential. Tangential velocity would need a profile whilst angular may be constant over the radial profile. 

[warhammer975]

I found information that the radial component of the flow direction is zero, axial component Vx = V * cos(alpha) and a tangential component Vt = V * sin(alpha). V - is calculated through the flow rate and the flow area? But I still don't understand how to correctly determine the alpha angle?