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

[freebeard]

"Don't criticize what you can't understand." — Dylan

I gave it my best shot at Permalink #14, got a little exasperated at #16, and pushed back at #18.

....and you made a point which puzzled me. I asked you to support what you'd said but instead, we got basically nothing.

Could it possibly be the case that we're both puzzled by the same thing? Hmmm?

We good now?

Vman455 — TIA

[Cd]

Where did this argument come from ?

Just a minor update - I took a 136 mile trip to see.my parents today.
I taped some tufts inside the box cavity, and around the NACA vents.
The good news, is that the ducts seem to be working.
The airflow is blowing to the rear of the car, rather than the wake reversing the flow.
The apparent windspeed at the edge of the duct looks to be the same as that coming from the edge of top of the box cavity.

I'll take video later in the week.

Going by 'seat of the pants' I'd guess that the car doesn't coast like it used to.
However, I just got the highest mileage I have ever seen with the car, and this was at speeds of up to 80 mph, with an average speed of probably 68 mph.


So more testing is needed.

[Vman455]

Originally Posted by freebeard

Vman455 — TIA

Well, shoot--I have a couple papers, but they're both too big to upload here. I'll see if I can figure something else out.

[aerohead]

Originally Posted by Cd

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. )

By making holes in the panel you're destroying some of the facility for pressure recovery, which is the underlying premise for the box-cavity's performance. It needs to be as air-tight as you can make it.

[JulianEdgar]

Originally Posted by aerohead

By making holes in the panel you're destroying some of the facility for pressure recovery, which is the underlying premise for the box-cavity's performance. It needs to be as air-tight as you can make it.

If it were just pressure recovery caused by longer flows down the sides/top, it wouldn't need to be a cavity would it?

[aerohead]

Originally Posted by JulianEdgar

If it were just pressure recovery caused by longer flows down the sides/top, it wouldn't need to be a cavity would it?

By longer, do you mean, that if the body were elongated, and diminished in cross-section along a streamlined path?

[JulianEdgar]

Originally Posted by aerohead

By longer, do you mean, that if the body were elongated, and diminished in cross-section along a streamlined path?

Your point seems to apply to any lengthened / contracting body, not one specifically with a cavity. I don't think that a box cavity works as you appear to assume it to.

I doubt that holes in the cavity extensions will make much difference, either way. But I am just guessing.

[aerohead]

Originally Posted by JulianEdgar

Your point seems to apply to any lengthened / contracting body, not one specifically with a cavity. I don't think that a box cavity works as you appear to assume it to.

I doubt that holes in the cavity extensions will make much difference, either way. But I am just guessing.

* ' the drag reduction from an elongated tail varies almost linearly with the reduction in cross section area.' SAE 2020-01-0673
* Any porosity would only compromise the effect. Voids cannot sustain pressure.

[JulianEdgar]

Originally Posted by aerohead

* ' the drag reduction from an elongated tail varies almost linearly with the reduction in cross section area.' SAE 2020-01-0673

Sure

* Any porosity would only compromise the effect. Voids cannot sustain pressure.

Only if you think that's how box cavities work. I don't think that - because they reduce drag even when the extensions are not angled inwards and so when there's no reduction in cross-sectional area.

It's the old thing - to a hammer everything looks like a nail. Or, once you have a theory in your mind, you want to apply it to everything.

[freebeard]

I doubt that holes in the cavity extensions will make much difference, either way. But I am just guessing.

You find yourself down here in the mud with the rest of us?

I agree with aerohead. I've seen some shallow boxed cavities.



If you can't trust Gaylard, who can you trust? My favorite example would be the Cobra Coupe.


http://www.motornature.com/wp-conten...by-Coupe-2.jpg

It's almost just a Faschenfeld tearing edge.