NACA Duct vs. Scoop

A few months ago, a fellow Pikes Peak driver David Kern, asked us a question about the difference between traditional style air scoops and NACA Ducts. There seemed to be some info that mentioned NACA ducts creating far less drag, and that they were specifically designed for aircraft use, but we were not able to find anything that actually quantified the two styles and compared them.

To do this, we drew up a CAD model of a NACA Duct and a traditional scoop with equal sized openings and other dimensions in a virtual wind tunnel. The difficult part here is that these are different designs and are meant for different applications, but by constraining the dimensions (even though in real life they may be a bit different) we are able to quantify efficiency results.

Initial setup to compare equally sized scoop and NACA Duct.

As expected, the two ducts behave rather differently in identical environments. This is because the traditional scoop more or less ‘rams’ air into the opening and then redirects it, while the NACA Duct ‘sucks’ air in by creating a low pressure zone with the unique geometry.

Comparison of air flow through traditional scoop and NACA Duct.

To answer David Kern’s question directly, “what flows more air?” The answer is that the traditional scoop has higher mass flow rates.

The NACA Duct flows 55% of the air that a traditional scoop flows under equal conditions. That doesn’t necessarily mean that the scoop is better though, it all depends on the application. While the scoop is able to force in more air, it leaves a nasty wake of turbulent air and drag.

Low pressure pocket formed behind the traditional scoop

In the end, one should take overall functionality into consideration. The Pikes Peak Audi S1 has a NACA Duct built into the roof so that airflow is optimized for the aerodynamic components of the rear wing. But if higher flow rates are necessary for added cooling for example, then a scoop may be the better option as long as it is not placed in a way that degrades the performance of aerodynamic components behind it.

Air density of flow though a traditional scoop and NACA Duct.

Air density of flow though a traditional scoop and NACA Duct.

Air density of flow though a traditional scoop and NACA Duct.

Surface plot of pressure and flow direction over a traditional scoop and NACA Duct.

Trajectory and pressure plot of flow around and behind a traditional scoop.

Flow velocity through a traditional scoop and NACA Duct.

Flow velocity through a traditional scoop and NACA Duct.

Flow velocity through a traditional scoop and NACA Duct.

Flow velocity through a traditional scoop, note the large blue region behind the scoop signifying drag.

Flow velocity through a NACA Duct.

Cross section plot of flow velocity through a traditional scoop and a NACA Duct. Note how the NACA duct leaves external flow virtually unaffected, while the traditional scoop leaves a large region of slow air behind it.

Flow velocity through a traditional scoop and NACA Duct.