@thantik Oh, I see. Since you say output port that means the air flow is in the opposite direction, from a smaller output to a much larger pipe. So the potential problem I was seeing doesn't exist in that use case.

@tophee The rectangular end is the stock size of the output port on the K40 without modification. There's no getting around this without actual hardware modification of the K40 or dumping it into a new case.

I've been working on a very similar part (not yet published) and one of the constraints for me was that the part should not reduce the air flow coming from the 150mm tube, i.e. the area of the cross-section should be constant throughout the part. In your model, this is not the case (the rectangular end is much smaller than the round one). I just wanted to mention this for anybody who may not be aware of it. I'm also wondering whether you happen to know what the actual effect of such a bottle neck part is on the air flow. I am specifically thinking of an exhaust pipe for a mobile a/c. Those often use 150 mm tubes and it seems evident to me that a/c's efficiency would be reduced by such a bottle neck part, but I have no idea how big the difference would actually be in practice. Any air-flow engineer around?