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Old 09-20-2020, 04:53 PM   #11 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
What were you trying to do with the ones on your insight? Was the main goal just for something to mount the rear wing to?
The first version of fins was to provide better straight line stability by moving the lateral centre of pressure backwards. The fins also mounted the wing (that kept the fins upright).



The second version supports the fins by a steel frame behind the car and uses a spoiler instead of a wing. When throttle-stop testing this version I found lower drag with the fins, and the fins and spoiler, in place. My guess is that the fins are disrupting the trailing vortices off the pillars.


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Old 09-20-2020, 05:21 PM   #12 (permalink)
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Do we get the answer soon?
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Old 09-20-2020, 06:30 PM   #13 (permalink)
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Right! I almost forgot:



It's the same lip spoiler, 21mm high, at the end of the factory spoiler.

There were a couple of employees waiting in their cars near where I parked to swap arrangements, for their shifts to start I suppose. I bet they wondered why the hell I kept coming back there.
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Old 09-20-2020, 06:34 PM   #14 (permalink)
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Thanks - it's a good example of how flow changes can occur upstream of the modification.
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Old 09-22-2020, 09:20 AM   #15 (permalink)
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You just need to get your Vape God friend to ride with you and tape a go pro to the outside of the car now to see the entire wind flow pattern. lol
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Old 09-24-2020, 08:02 AM   #16 (permalink)
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I took some pressure readings yesterday for another picture of what's going on. These were centerline readings. First, no add-on spoiler:



Next, with the lip spoiler in place at the top of the window:



And finally, with the lip spoiler at the rearmost edge of the stock spoiler:



The pressure readings decreased by 25-30 Pa over the rear window with the roof spoiler, but pressure increased on the roof ahead by 45 Pa.

Pressure on the window with the edge spoiler varied from baseline by + or - 5 Pa, but again the roof pressure was higher by 30 Pa.

I'm going to do some more testing of the rear lip spoiler today between teaching, with more locations on the window and roof.
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Old 09-24-2020, 02:45 PM   #17 (permalink)
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pressure distribution

Quote:
Originally Posted by Vman455 View Post
I took some pressure readings yesterday for another picture of what's going on. These were centerline readings. First, no add-on spoiler:



Next, with the lip spoiler in place at the top of the window:



And finally, with the lip spoiler at the rearmost edge of the stock spoiler:



The pressure readings decreased by 25-30 Pa over the rear window with the roof spoiler, but pressure increased on the roof ahead by 45 Pa.

Pressure on the window varied from baseline by + or - 5 Pa, but again the roof pressure was higher by 30 Pa.

I'm going to do some more testing of the rear lip spoiler today between teaching, with more locations on the window and roof.
Looks like the 1st-condition, with no add-ons had the best performance.
The other two increased lift and drag.
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Old 09-24-2020, 06:02 PM   #18 (permalink)
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Quote:
Originally Posted by Vman455 View Post
I took some pressure readings yesterday for another picture of what's going on. These were centerline readings. First, no add-on spoiler:



Next, with the lip spoiler in place at the top of the window:



And finally, with the lip spoiler at the rearmost edge of the stock spoiler:



The pressure readings decreased by 25-30 Pa over the rear window with the roof spoiler, but pressure increased on the roof ahead by 45 Pa.

Pressure on the window with the edge spoiler varied from baseline by + or - 5 Pa, but again the roof pressure was higher by 30 Pa.

I'm going to do some more testing of the rear lip spoiler today between teaching, with more locations on the window and roof.
Interesting. But what I'd like to see you do is to use a large spoiler - when pressure testing, I always go for something much larger than I intend using, see the results, and work backwards from there.

For example, we can see that the upper spoiler position increases pressures over the curve at the top of the hatch (from -170 in std from to -125) but puts the rest of the hatch in separated flow (lower pressures than standard) and also decreases wake pressure. So you'd say less lift but more drag.

The spoiler at the base of the hatch increases average pressures on the hatch, but because the spoiler is so small (and as we saw with the tufting) the effect is also quite small. A 25mm high lip is the smallest I'd ever test in the lower hatch position - I'd start with 75mm and then go smaller in 25mm increments.

So I'd not bother doing any more measurements with these configurations, but go larger in the modification so the effects are clearer. Some of these measurements may be in the range of noise - another reason I start with major changes first. Just use cardboard and tape to make the different height spoilers.
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Old 09-24-2020, 06:09 PM   #19 (permalink)
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I don't agree with aerohead's assessment. The lip spoiler at the rear edge of the factory spoiler increased pressure by 30 Pa on the rear part of the roof! These pressures are the average of E-W runs; the data for no lip spoiler and rear lip spoiler are:

Roof:
No lip spoiler: -140 (E)/ -200 (W)/ -170 (avg)
Lip spoiler: -110 (E)/ -170 (W)/ -140 (avg)

Upper window:
No lip spoiler: -50 (E)/ -80 (W)/ -65 (avg)
Lip spoiler: -70 (E)/ -70 (W)/ -70 (avg)

Lower window:
No lip spoiler: 0 (E)/ -20 (W)/ -10 (avg)
Lip spoiler: 0 (E)/ -10 (W)/ -5 (avg)

Hatchback base:
No lip spoiler: -20 (E)/ -50 (W)/ -35 (avg)
Lip spoiler: -30 (E)/ -50 (W)/ -40 (avg)

I'm hesitant to ascribe significance to a difference in average pressure of 5 Pa. However, on the roof where the reading with the lip spoiler was consistently +30 Pa--a much larger difference--I think it's safe to say it is reducing lift. So is the spoiler placed at the top of the window, but at the expense of decreased pressure over the window behind it; the question is, how much further forward does the pressure increase over baseline?

I forgot to bring the spoiler with me so I didn't get to test it today, but I have some time tomorrow evening. I'm going to measure pressures further forward on the roof, and outboard of the centerline to see if I can get a better picture.
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Old 09-24-2020, 08:02 PM   #20 (permalink)
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Have you thought about trying to put it midway through the window? That's not an educated guess, I'm just curious as to how it might change things.

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