Subaru Impreza WRX STI Forums: - View Single Post - GD: BlueScooby's 06 STI - 03/16/19 Track Day Synopsis
View Single Post
Old 10-13-17, 07:11 AM  
Silver Member
Fav Mod: Recaro Pole Position and Shifter Bushings!
My Subaru Parts for sale

Join Date: May 2009
Location: Cary, NC, USA
BlueScooby is a glorious beacon of lightBlueScooby is a glorious beacon of lightBlueScooby is a glorious beacon of lightBlueScooby is a glorious beacon of lightBlueScooby is a glorious beacon of light
Default Re: BlueScooby's 06 STI - Diffuser/Underbody RE-DO

revisiting the NACA Duct discussion relative to the underbody. I pulled out my aero book and it does have some info on naca duct design and usage but nothing specific about application in an underbody. I did, however, find an interesting CFD model of a NACA duct that I think we can extrapolate some information from. This picture is from an article comparing airflow through a scoop vs. a NACA duct but its still relevant here.

This is a profile shot of a NACA duct showing airflow through the duct as well as outside on the "exterior" of the vehicle. The far left of the vehicle falls away so that part isn't useful, but the "exterior" panel just past the entrance to the duct shows the disruption that the duct creates:

You can see a small turbulent airflow bubble beginning at the end of the duct, which appears to stabilize pretty quickly. So there is a disturbance in the airflow but it is small. However, the "exterior" flow velocity immediately drops at, and past, the naca duct which is the opposite of the desired effect of "ground effects" on a car. The whole point is to accelerate the airflow through a "venturi" shaped underbody to lower the air pressure. If you lower the velocity of the airflow by drawing some out that would yield a reduction in the venturi effect you are designing for.

Granted this is a lot of extrapolation based on one picture and the use of "mach number" (what I'm interpreting to basically show flow velocity) could be misleading. If this was a plot of pressure rather than flow, it would appear that the naca duct would be very beneficial for an underbody, but thats not what is being shown here.

What do you think? Does that make sense or do you think I'm way off here?

After doing more reading, I realized I misunderstood the relationship to pressure/velocity as it pertains the rear diffuser. The velocity under the car (before the diffuser) is accelerated, producing downforce, but as the airflow enters the diffuser and starts to expand, it actually slows down. Q=A1*V1=A2*V2.

In this scenario with the NACA duct immediatly before the diffuser, I'm not quite as clear how this would effect the overall aero performance, because it would be slowing the airflow down before the diffuser, where the air is still supposed to be high velocity, but at the same time, it could amplify the effect of the airflow through the diffuser itself. It could even be a wash with a (+) and a (-)...

So basically, this is a lot of words to say I don't know. And also, this may be a complete waste of all our time... haha
Lord, help me to be the kind of person my dog thinks I am...

Bluescooby's 06 STI Journal
IG: BlueScooby06

Last edited by BlueScooby; 11-05-18 at 06:09 AM.
BlueScooby is offline  
Likes 0 Dislikes 0
Reply With Quote