Originally Posted by
nsusammyeb any explanation for this?
The transition between the plenum and the runners should be a proper, raised "velocity stack" bell mouth, raised above the plenum floor approx. 1". This is proven time and again to be the least restrictive way to transition from a large volume of air to a smaller pipe. They promote laminar flow and the pressure gradient around a bell mouth (sorry can't find a good picture right now) shows there to be an area under the lip of the bell mouth where air will travel up and around into the pipe as easily as it would from directly in front of the bell mouth.
If I find the pressure gradient image I'm thinking of it would be very clear.
Edit: This is not what I was looking for, but it's decent. It shows there are high pressure zones below the inlet plane that air is just as likely to enter the pipe from as anywhere else.