Quiz: Qualitative Flow Analysis

[aerohead]

Originally Posted by Vman455

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.

* You lowered pressure acting at a distance on the body overhang. According to Hucho, that's a fail.
* You lowered base pressure and increased pressure drag. According to Hucho, that's a fail.
Just sayin'

[Vman455]

Originally Posted by aerohead

* You lowered pressure acting at a distance on the body overhang. According to Hucho, that's a fail.
* You lowered base pressure and increased pressure drag. According to Hucho, that's a fail.
Just sayin'

By 5 Pa on average, which means realistically: inconclusive. 30 Pa on the roof: probably more trustworthy, especially since it was consistent. And that's an increase in pressure forward of the rear axle, acting on a surface that's close to horizontal.

[aerohead]

Originally Posted by Vman455

By 5 Pa on average, which means realistically: inconclusive. 30 Pa on the roof: probably more trustworthy, especially since it was consistent. And that's an increase in pressure forward of the rear axle, acting on a surface that's close to horizontal.

True, but in a position of no mechanical advantage, while simultaneously developing a deleterious torque aft.
You'll notice Indy cars and F1 moving wings as far forward, and as far aft of the axles, to get the force-at-a- distance leverage as Hucho advocates for in his chapter on high performance sports, and racing cars.

[JulianEdgar]

Originally Posted by aerohead

True, but in a position of no mechanical advantage, while simultaneously developing a deleterious torque aft.
You'll notice Indy cars and F1 moving wings as far forward, and as far aft of the axles, to get the force-at-a- distance leverage as Hucho advocates for in his chapter on high performance sports, and racing cars.

Nothing like complete inconsistency.

Yesterday Aerohead was complaining that rear spoilers create front lift; now he's complaining when a rear spoiler is placed so that it doesn't create front lift!

[Vman455]

All right, more testing yesterday. This time, I took readings at 10 locations on the factory spoiler, rear glass and roof, up to just behind the B-pillar, at an inboard position (not centerline) and outboard.

One important difference between yesterday and Wednesday's testing: wind was out of the south both days, so there was crosswind in my earlier East-West testing. I used a different route yesterday, an exactly North-South county road (everything here is conveniently on a one-mile grid) and, with winds again out of the south, that means lower yaw, maybe even zero yaw. All testing was done at 80 km/h indicated with a pitot tube giving reference static pressure and readings averaged over 45 seconds or so each run.

No add-on spoiler:


Add-on lip spoiler:


Difference:

(all values given in Pa)

Observations:
-On the roof, pressure is lower toward the middle (faster flow speed).
-Over the window, this flips; pressure is lower toward the edge. This may indicate vortex formation there.
-Adding the lip spoiler produced positive pressure difference from ambient just in front of it, on the factory spoiler, and ambient pressure on part of the window.
-Adding the lip spoiler increased pressure a significant distance up the roof.

In the past I've not thought much of small spoilers such as this, believing that they wouldn't do much of anything. But based on these two tests so far, I think I was wrong!

[JulianEdgar]

Great testing - well done!

[JulianEdgar]

So, you may have thought of all of this already, but if not...

The increase in pressures on the rear hatch and roof from the little lip spoiler will decrease lift (the upwards component of the resolved force) and decrease drag (the horizontal component of the resolved force on the hatch). The decrease in drag will be very small, however, and probably more than offset by the larger wake.

That's why I ended up going with a spoiler than increased pressures on the hatch without an increase in wake area.

[Vman455]

Yeah, and I'm not sure if I have a way of testing the increase in drag with any accuracy--since the Prius can't do throttle-stop testing. I might stick the spoiler on for my weekly hour-long drive (I teach one day a week this semester at a school an hour south of here) and see what happens; I've been getting just under 60 mpg depending on wind. Or, I might try the method from your book using a long hill; there's a section of interstate near here with a moderate grade for 1/2 mile. Or what I ended up doing with the air curtain ducts, measured fuel economy over a several-mile section of flat road with 3 or more runs in each direction in each configuration.

I tried reading engine load and horsepower on the Scangauge to see if rolling the windows down would show up, but there was no correlation--the problem, I think, is the variable torque from the electric motors. There's a MAF x-gauge I could use too but again, I think that would suffer from the same problem.

I might not get to do the Green Grand Prix this year because of Illinois' coronavirus positivity rate, which would have been at least a comparison of a long drive on a closed course with the two years I've done it already, the car in a different state each time.

If anyone has other ideas for testing drag changes...?

[JulianEdgar]

The downhill test might work if you can do it in neutral - not sure that is possible in the Prius. Otherwise, regen braking issues, etc.

Depending on your speed limits / interest of the law, I'd probably try to test fuel consumption over a short distance (eg 1-2km) on a flat, straight road at the highest possible constant speed eg 130 km/h+.

With either, do windows up/down to ensure your are getting real data.

[aerohead]

Originally Posted by JulianEdgar

Nothing like complete inconsistency.

Yesterday Aerohead was complaining that rear spoilers create front lift; now he's complaining when a rear spoiler is placed so that it doesn't create front lift!

* You know that Hucho advocated for an ' aerodynamic lever arm ' on page 231. And he provides all the context.
* I don't complain, I just post information germane to a complete discussion of a particular issue for which you've left holes big enough to drive a Titanic through.