Need advice on NACA duct placement to airflow on curved surface.
 

I'm adding some NACA ducts to my cars' box cavity.
I'm running into some placement issues with the ducts, since they need to line up with the airflow around curved surfaces.
Luckily for me, I had previously done a tuft test, and I am able to overlay the image onto a photo and guess at the correct placement.
When lined up into the direction of the airflow, the ducts look odd. Nothing matches the angles of the other ducts, and they look crooked.
I have done two ducts on top, but will wait to cut out the others till I get advice.
Please have a look at the images and let me know what angles you think the ducts should be at.
Thanks !

What are the ducts for?

To create drag. ( most likely :D lol )

But the hope is that it will channel some air into the wake.

I plan to try and channel some underbody airflow into the wake as well.

Since the airflow is so much greater inside the cavity, it may actually push out air, rather than suck air in.

We shall see.


They will also function to expel some air out in case of a windstorm. The area where the 3rd brake light is is open on the sides, and seems to have done well in the last windstorm we had here.

I also like the look, even if they have no actual function.

Match them to the local airflow direction, as indicated by the tufts. That is, what you are doing.

Here are the two NACA ducts I have so far.
You can't see how rough they are here in the pictures.
They are purely function over form at this point.
The paint was still wet at this point.

I thought that for sure they would intrude down into my line of sight out the back, but they are placed just far forward enough to not even show up below the extension I have already added.
I really like the fact that to test them, all I have to do is look in my rear view mirror. ( I'm placing some tufts inside the box cavity. )

I'd keep any interaction between the wake and the air passing around it to a minimum, because any transfer of momentum between them should (according to Newton) lower base pressure.

I don't think that is correct in the way Cd is trialling it.

If you can raise the pressure in the wake by adding airflow to it, base pressure will be increased and so drag reduced. A la a 'blown wake' in fact.

I don't actually think that Cd's approach will achieve that, but it's worth trying. And it's not an approach (ie with NACA ducts) that I have seen tried anywhere else at all.

If I saw this being done in an SAE tech paper, I'd be very interested in the results.

What pattern of airflow do you have in mind that will do this?

I don't understand the question.

Cd — perforating a boxed cavity seems counterproductive.