This velocity stack has only a single, minimally designed fin to support the center cone. It doesn't seek to direct the air, but rather to allow the swirl of turbulent air as much as possible on the way through.
The gradual curve of the outside lip and cone gradually reduce the cross-sectional volume down to only the width of the actual fan blades. I haven't done any flow modeling, but I noticed an anecdotal increase in fan output and decrease in fan noise when adding this.
You may also like my other related listings:
Velocity stack, 40mm
Fan duct, 40mm, 65 degrees