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

[freebeard]

The depth of an unventilated box cavity for lowest drag was 0.37 * D, where D was 'effective body diameter'.

So that would be a planning parameter rather than a r*le of th*mb.

I don't know what the above depth was, but nowhere near the 560mm I calculate from the above equation (400mm with vents).

Sixteen inches, more or less? Seems like a good candidate for perforating. Did you save the pieces?

Lots of experimental possibilities with slot/hole size and location.

[JulianEdgar]

Originally Posted by freebeard

Sixteen inches, more or less? Seems like a good candidate for perforating. Did you save the pieces?

Lots of experimental possibilities with slot/hole size and location.

No, I didn't keep the pieces. There is no way such an approach would be legal in this country, even if they were made of clear plastic.

You'd probably get away with it here on a box trailer or caravan though.

[freebeard]

The Clark Cortez would be a good candidate, if I'd been able to hold onto it. I just found out in this interview that the Byrds had a Cortez when they came back from their England tour.

[Cd]

Since it seems like the NACA ducts provide a surprising amount of good velocity air, I plan to try and channel that air into the wake to reduce its' size.

Where would you all think that the ideal target area would be ?

Should I angle the airflow towards the license plate ? ( light blue lines )

Right now, they blow straight back ( yellow lines )

I also am going to go ahead and add two additional NACA ducts on each side of the box cavity now that I know their optimal placement.

And of course, I am getting a huge amount of air spillover around the lights, but I plan to have the area open for legal reasons.
I currently just have trip strips on the lights.

[Cd]

I was thinking that perhaps I need to have the airflow directed outwards instead of inwards.
If I have the airflow directed towards the middle of the wake, like i have in the image, this might actually just add more air to the already swirling air.
This may even make the vortexes even stronger and create more drag than if left alone.
So perhaps have the ducts angled to blow air outwards ? What do you all think ?

[freebeard]

I think that one of the most helpful voices got themselves banned because reasons...

As to the question, it depends on what you mean by outwards. Toward the tail lights? I doubt you can do that without moving the opening. They would [migrate/morph] toward the slots that Bertone and Jaray used.

[Vman455]

Originally Posted by Cd

I was thinking that perhaps I need to have the airflow directed outwards instead of inwards.
If I have the airflow directed towards the middle of the wake, like i have in the image, this might actually just add more air to the already swirling air.
This may even make the vortexes even stronger and create more drag than if left alone.
So perhaps have the ducts angled to blow air outwards ? What do you all think ?

Anything we can offer is purely speculative. Does throttle-stop testing work on your car? That would give an immediate indication of whether the drag force is changing and how much in each configuration.

[freebeard]

It will be interesting to see if throttle stop testing remains going forward. I'm already seeing push-back: ecomodder.com/forum/showthread.php/rule-thumb

[aerohead]

Originally Posted by freebeard

So that would be a planning parameter rather than a r*le of th*mb.



Sixteen inches, more or less? Seems like a good candidate for perforating. Did you save the pieces?

Lots of experimental possibilities with slot/hole size and location.

I'm looking at Figure 8.67, on page-333, of Hucho's 2nd-edition. It's the constant-section box-cavity behind the Volkswagen Transporter.
I'm not sure if this is what's being discussed.
If so, then Hucho is using the total length ( l ) of the vehicle in relation to the depth ( x ) of the extension.
Only two data points are given.
At x / l = 0.22, the drag reduction is Cd 0.06.
Cd 0.43 - 0.06 = Cd 0.37 ( bingo! )