Questions about NACA ducts

[EngJW]

After reading several NACA papers on submerged ducts and ram entrances, I am finding that my understanding is still pretty shaky (my background is mechanical).

What determines how much area is needed for the opening? I think that it would depend on the air flow requirement of the engine and an assumed velocity. If so, how would you select the velocity? It must have something to do with the velocity ratio in the NACA test data.

Next, why would the velocity through the area be any different than the velocity of the vehicle or air stream? It seems like the air would flow through at the same speed and then pile up at the carburetor.

When you want to determine the ram pressure, would you use the total pressure based on the air stream, or the air flowing through the duct?

Sorry if these are basic. I'm ready to be educated.

 

[carockwell]

The velocity through the duct will be dependent on the pressure differential between the entrance pressure and the exit pressure. This means that the velocity will be dependent on both the vehicle speed and the engine's air consumption.I cannot follow the link where you get your data. NACA ducts should be sized based on air flow requirements and pressure differentials.With my limited and outdated experience with automotive air flows we installed a duct, measured the pressure differential, and then used that data to get closer to our optimum size.To start, I would measure the pressure differential across the area you propose to put the duct, and the engine intake area. Next, you have to start cutting in a duct.

 

[btrueblood]

There are irrecoverable pressure losses associated with fluid friction and turbulence. These losses occur whenever the flow touches a wall, or if the flow is turned. Both mechanisms are at work in any duct, and thus the total pressure (recovery pressure) available at the end of the duct will be less than the theoretical ram air pressure due to vehicle speed. The recovery pressure will also vary with the mass flow rate thru the duct (higher velocity thru the duct will cause greater pressure losses).