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

[JulianEdgar]

That sounds good. Normal speed not slow motion.

[freebeard]

I'd like to ask what areas on the car that you all would like to see on video in the test.

The hubcaps, please.

[JulianEdgar]

Originally Posted by Vman455

I still don't have anywhere to host these, and it looks like they aren't available online. This is one of them, but full-text isn't available for some reason.

I have received the full text - just request it from the author at that link and you'll probably get it too.

I've only skimmed it but it appears that ventilating box cavities is a potentially viable option to reduce drag.

So, Cd, your idea was a good one!

[Cd]

Julian, I'm glad to read that !
I did the test yesterday, and plan to upload some quick video to Youtube later today or tomorrow.

As always seems to happen, the video that the chase car took is not very useful.
There was a lot of traffic, and they were not happy to have us there ( tailgating us the whole time, despite driving the speed limit )

I have some some still images from the test, but they aren't very useful either, since the tufts appear to be laying a certain direction in the still image, yet are in fact spinning like mad.

I added a quick makeshift VG to the A pillar, and see disturbance in the tufts all along the side of the car.

The tes also (thankfully) revealed that the area just behind the passenger side rear tire is flapping heavily.
I would have never know about it though, because the piece is still attached firmly, and goes back to its' forner shape once there us no wind load.
I plan to radius the edge today for my return drive.

The first image is with the NACA ducts open. The next image has them sealed.

[Cd]

https://youtu.be/YSxon1xuP0Q
https://youtu.be/bCBRxXFGeUg
https://youtu.be/9HzoYs8cqsY

Here is some video.
Also, this is a still shot of the flapping piece.
Imagine the drag it was causing !

[JulianEdgar]

I think you're really only going to be able to see the upshot by measuring base pressure ie the pressures on the rear panels of the car with / without the extensions, and with / without the ducts in the extension. (Or throttle stop testing?)

[Vman455]

The combination of your parents' commentary and cinematography is hilarious, especially that last video.

That said, the shakiness and quick shots make it hard to glean anything useful. I can't make out anything around the ducts at all, but I did notice from some angles it looks like the flow coming over the diffuser may be separating before reaching the edge.

[freebeard]

Need advice ...

Have them wash their windshield.

Consider a selfie stick bungied to the car.

[Cd]

Originally Posted by Vman455

The combination of your parents' commentary and cinematography is hilarious, especially that last video.

That said, the shakiness and quick shots make it hard to glean anything useful. I can't make out anything around the ducts at all, but I did notice from some angles it looks like the flow coming over the diffuser may be separating before reaching the edge.

I haven't watched the videos yet.
Whuups. I didn't think to mute them.

On my return trip, I was able to observe the behaviour of the tufts in my rear view mirror.

I added a quick piece of cardboard to one side of the duct, and had it angled downward at a steep slope.
I added a VG.
Despite the curve, flow is quite good back there as it comes off the NACA duct.
Airflow looks to be just as good as that from the edge of the top portion of the extension.

On the other side, I added sawtooth VGs.
Flow was not as good, and I could see the pattern of swirling air on the tufts below it.
The tufts swirled to the center of the car, while the duct with the large VG on the drivers side had very little motion to the tufts below it.
I'll see if I can get video from inside the car later this week.

Regarding the diffuser, there certainly is sepetation. I don't have any panels installed underneath yet.
That was one of the things I had hoped to get done on the trip, but oh well.

[JulianEdgar]

I've read that paper now. The most interesting thing is this:

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

If we take that as an average of height and width at (say) 1600mm, that would suggest a cavity depth of 600mm is best!

But, with the cavity ventilated (only on the sides and top), the equation drops to 0.27 * D, which would give a depth of 430mm.

Obviously, do your own calculation of 'D' for your car.

Note that the testing was done on only a scale model squareback 'shape', not a real car. On the shape, the best reduction in drag was up to 10 per cent - substantial.

For reference, the paper is: SAE 2012-01-0171, Bluff Body Drag Reduction with Ventilated Base Cavities



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