Quiz: Qualitative Flow Analysis

[aerohead]

Originally Posted by 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!

Where is the comparative underbody pressure profile.
What you're sharing has no context.

[Vman455]

Originally Posted by aerohead

Where is the comparative underbody pressure profile.
What you're sharing has no context.

Well, bless your heart.

[aerohead]

Originally Posted by Vman455

Well, bless your heart.

There's a reason I bring it up.
We're in subsonic flow and any change, anywhere, according to Hucho has implications to elsewhere around the vehicle.
And specifically as to spoilers, Schenkel's spoiler research ( also in Hucho's 2nd-Ed. ) revealed that a rear spoiler altered the pressure under the car as well as over the boot.
I'm not marginalizing your efforts, it's just that, without a full accounting, the total effect remains an unknown quantity.

[jakobnev]

Vman455 -

Is this the fancy Prius with the auto leveling headlights? I'm asking because I read it has a rear ride height sensor.

[Vman455]

Originally Posted by aerohead

There's a reason I bring it up.
We're in subsonic flow and any change, anywhere, according to Hucho has implications to elsewhere around the vehicle.
And specifically as to spoilers, Schenkel's spoiler research ( also in Hucho's 2nd-Ed. ) revealed that a rear spoiler altered the pressure under the car as well as over the boot.
I'm not marginalizing your efforts, it's just that, without a full accounting, the total effect remains an unknown quantity.

Forgive my exasperation, but I spent 4 hours, including the trip to the hardware store for materials to mount the pitot tube in freestream air above the car when my first apparatus failed, out of my very busy schedule (full-time job, half-time in school, and teaching part-time at another school) gathering this data only for you to chime in with your opinion that it is worthless, as you have criticized JulianEdgar on this forum before in an attempt to discredit any sort of on-road testing, even stooping to personal attacks as recently as this very afternoon. I don't know what axe you're trying to grind here; I don't think it is worthless, and if adding the spoiler lip has increased pressure under the car--well, I'll be measuring that soon and doing what I can to mitigate it.

Originally Posted by jakobnev

Vman455 -

Is this the fancy Prius with the auto leveling headlights? I'm asking because I read it has a rear ride height sensor.

Unfortunately, no--I added the LED headlights aftermarket, but not the auto-leveling. The OEM ride height sensor is $200+, a little rich for me, so I'll probably try to fab something up like JulianEdgar has done or use some other sort of (cheaper) distance sensor to measure ride height.

[JulianEdgar]

Originally Posted by Vman455

Forgive my exasperation, but I spent 4 hours, including the trip to the hardware store for materials to mount the pitot tube in freestream air above the car when my first apparatus failed, out of my very busy schedule (full-time job, half-time in school, and teaching part-time at another school) gathering this data only for you to chime in with your opinion that it is worthless, as you have criticized JulianEdgar on this forum before in an attempt to discredit any sort of on-road testing, even stooping to personal attacks as recently as this very afternoon. I don't know what axe you're trying to grind here; I don't think it is worthless, and if adding the spoiler lip has increased pressure under the car--well, I'll be measuring that soon and doing what I can to mitigate it.

Keep doing your testing - I think your results are great, and we will all be able to learn a lot from them. In fact, you might want to think of keeping a Word doc with all your results in it - so much easier to refer back to than miscellaneous forum posts.

As for Aerohead, I have avoided the personal belittlement he is fond of, but it's obvious he has some issues. There's a lot of confusion in what he writes, and as you have previously found, many of the references that he cites don't actually support what he claims. However, I think the worst thing is that he refuses to learn from developments of the last 20-30 years and so keeps on rehashing the same old (mis)information.

Of course he isn't going to like your testing, because (as with mine) it will so quickly show that much of what he says is simply wrong!

Pressure and tuft testing tell you so much, and when you add lift/downforce testing, you'll be looking at information radically better than any guesswork / rules of thumb / 1930s models / templates, etc. (And, I'd also argue, a lot better than crappy CFD.)

Re using other height sensing techniques, I have tried ultrasonic height detection but couldn't get the resolution needed for lift/downforce testing at normal road speeds. So back to a simple P38 Range Rover analog pot-based sensor. I think my latest smoothing circuit is better than the one in my Veloce book, and seemed to work really well on the Impreza rear wing / spoiler tests.

So keep testing - it certainly takes time and effort, but it's also like taking off a blindfold and seeing the world for the first time.

[Vman455]

Originally Posted by JulianEdgar

Re using other height sensing techniques, I have tried ultrasonic height detection but couldn't get the resolution needed for lift/downforce testing at normal road speeds. So back to a simple P38 Range Rover analog pot-based sensor. I think my latest smoothing circuit is better than the one in my Veloce book, and seemed to work really well on the Impreza rear wing / spoiler tests.

I'll probably end up going this route; I looked up other height sensors and most of them can be had much more cheaply than the Prius arm, for some reason (maybe because they aren't very prolific?). Thanks for the advice on the ultrasonic sensor; too bad, because TI makes one that could be logged directly on my calculator.

Originally Posted by JulianEdgar

So keep testing - it certainly takes time and effort, but it's also like taking off a blindfold and seeing the world for the first time.

Very much so. I've spent the last two years in calculus, physics, and engineering coursework, and it's the same sort of experience--especially physics labs pre-pandemic, where experiencing physical phenomena is worth 1000 times more than reading about them in books. One professor had us do a weekly write-up on important physicists of the past; one week I chose Emilie de Chatelet, a mistress of Voltaire who demonstrated (in the early 1700s, mind you) that kinetic energy is proportional to the square of velocity by dropping balls into sand and measuring how far they penetrated. I imagine a lot of people today would poo-poo that sort of experiment as low-tech and uncontrolled, yet it was instrumental to our current understanding of physics.

[JulianEdgar]

Originally Posted by Vman455

Very much so. I've spent the last two years in calculus, physics, and engineering coursework, and it's the same sort of experience--especially physics labs pre-pandemic, where experiencing physical phenomena is worth 1000 times more than reading about them in books. One professor had us do a weekly write-up on important physicists of the past; one week I chose Emilie de Chatelet, a mistress of Voltaire who demonstrated (in the early 1700s, mind you) that kinetic energy is proportional to the square of velocity by dropping balls into sand and measuring how far they penetrated. I imagine a lot of people today would poo-poo that sort of experiment as low-tech and uncontrolled, yet it was instrumental to our current understanding of physics.

Good stuff. My father was a scientist and physicist and he instilled in me from a very young age that doing an experiment is often far preferable to reading theory. And in any area of car modification, that applies in spades.

[freebeard]

I can't find a link, but Harry Miller was working (1931 4WD race car?) with German engineers who asked him about IIRC the diameter of the inlet to the supercharger.

He held up two fingers and the 'exasperated' German engineer used his caliper to measure the air. Turned out he was right on the money. That's what hands-on experience does.

[JulianEdgar]

Originally Posted by freebeard

I can't find a link, but Harry Miller was working (1931 4WD race car?) with German engineers who asked him about IIRC the diameter of the inlet to the supercharger.

He held up two fingers and the 'exasperated' German engineer used his caliper to measure the air. Turned out he was right on the money. That's what hands-on experience does.

I'd never known much about Harry Miller until I went to the Peterson Museum in Los Angeles. Then I was transfixed by this stunningly beautiful car - front-wheel drive, too!