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Old 02-26-2007, 05:23 AM   #1
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Question using raindrops to analyze airflow?

I've been experimenting with the something that perhaps might be called "turbulators"

I've attached a series of small rubber cones (originally intended as stick-on feet for internal computer cards) at regular intervals on my rear roofline. They're approx 10cm ahead of the point where the roofline "really" starts to curve down, although the transition is pretty smooth... more like a coup? than a hatchback though perhaps not that gracefull.

I hope that they might promote the airflow to better follow the decline of the roofline. much like vortex generators do. Although they wont make actual vortexes but just cause the boundary layer to become a bit turbulent. sound theory or wishfull thinking on my part??

It's been lightly raining most of the time so perhaps I could draw some conclusions out of the drop patterns on my roof and rear widow (the later can be observed at different speeds while driving)
What I have noticed is that when I stopped, there seemed to be stretched drops over the entire length of the roof, including in between the cones but not in their "wake" although big drops tend to accumulate at the rear root of the cones.

On my last drive I've taken off the 2 outer cones (one by accident checking how firmly It was attached and the other to preserve symmetry) and there seemed to be more water going down the side of my rear window rather than the center .

Could this mean the cones are repelling the airflow altogether or rather that they trail a turbulent boundary layer in which drops are not flowing as much as in a thin layer?
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Old 02-26-2007, 07:29 AM   #2
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I think you are right.... It is indeed creating little areas of turbulance. On question...are you increasing the frontal area of the car? This would be increasing your drag.

That being said, it's a neat fluid approximation of the classic "tell tail" lines. Tape a few peices of yarn in strategic places and see where they point to see the flow pattern.
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Old 02-26-2007, 09:32 AM   #3
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lunarhighway -

Great reuse of existing rubber feet. Can I seem some pictures?

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Old 02-26-2007, 02:14 PM   #4
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here are some pictures...

it was really dark outside so i'd have to use the flash. I apologize for the state of my car... i've washed it last week, but it's been raining a lot since,my fingers left greasy prints and on top of that some stupid bird has pooped on it.

there used to be 5 cones roughly 7 cm apart (the distance between the roof rack attachment strips is about 60cm) but i pulled the outer ones of...yet the glue seems sufficient to hold them in place even when they get wet, certainly for testing, once you pull them off however they wont stick again...

here's how they look:




the dirty streaks near the edge seem to be a result of the cones.
having driven with them for 2 days and only having my standard fuel gauge as i guide i'd say they're not boosting my FE a great deal. it's either normal, or slightly worse so maybe i'll just pull them off tomorrow.

the car does feels a little different though.
it somehow feels shorter when taking a curve at speed.

i think the cones might be acting more like a spoiler. actually creating a ridge of turbulence. the smooth oncoming air might hit this ridge and detach altogether. this might generate some downforce. good for racers, but not really what i need, especially not because it's bound to come with added dragg.

after a 10% gain from EOC this is somewhat disapointing but it was to be expected as there was no real sound theory behind the whole thing... i just pictured these little things would look cool on the roof... wich they do.

perhaps i should find a spot where i actually want to deflect the air... in front of my wheelwells perhaps?
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Old 02-26-2007, 02:31 PM   #5
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lunarhighway -

Thank you for the pictures. I wish we could put these things into an Aerodynamics calculation program and see what happens!!! Unless you are willing to cover your car in "silly string" and drive around with a pace car (that doesn't influence your aerodynamic wash or wake or whatever it's called!!!) taking pictures of the string, it's hard to prove an effect either way.

There are other GasSaver threads on vortex generators that talk about this stuff, but no solid conclusions .

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Old 02-27-2007, 01:19 AM   #6
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-cfg83

yes i agree... from what i can tell from my fuel meter they're not helping at all in fact they could be making things worse... i still get the feeling my car was a bit more stable in te curves but at the curent fuel prices i'd rather slow down a bit than install somemething that causes aditional drag.

seeing the fuel prices had gone up this morning (now it's more than 1,264?/L that's about 6.277$/us gallon if my calculations are correct) i took them off as soon as i got to work. if i ever buy a new petrol engined car i'll be looking for one with an LPG installation.

anyway i agree suppose it would be nice if we could test them in a virtual windtunnel, but than we'd have to include a model of the entire car.

i suppose i'll better stick to proven mods than waste fuel on these things.
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Old 02-27-2007, 04:44 AM   #7
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Quote:
Originally Posted by lunarhighway View Post
i suppose i'll better stick to proven mods than waste fuel on these things.
Don't give up so easy, how did the proven mods get to be that way?

Testing vortex generators is probably going to be tricky though. On a shape that is already reasonably aerodynamic, I suspect that the gain from them will be small enough to make it hard to detect at the tank.

But, a few comments anyways. First, I think the usual VG theory says they should be approx 25mm tall, which yours look shorter than. You might see more effect if they were taller.

If you honestly feel that the rear is gripping more, this would indicate an increase in air pressure on the rear surfaces of the car. More pressure on the rear glass = less drag (since a low pressure area there tries to suck you backwards) Your increased fuel consumption could be caused by the rain or any number of variables.

Try some tuft testing. The car I drive has a 'formal' sedan roofline with a nearly vertical rear window (rounded transition at the top of the roof though). In the morning, I notice that the dew will be pushed down the rear window untill I reach a speed around 40-50mph at which point any remianing droplets just hang there untill they evaporate.

Anyways, you could put some pieces of string back there and watch them in the mirror to see at what speeds they show laminar airflow, turbulent airflow, and detached flow. Testing with and without the turbulators would help back up any conclusions about their effect on the airflow around the car.
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Old 02-27-2007, 06:12 AM   #8
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You?re right of course?

Perhaps i?m a bit frustrated because the whole idea behind the raindrops would be to get a clear view of what?s going on? but so far the rain let me down a bit? despite being more pressent then i?d like to, the rain hasn?t been consistent enough to serve as a stable indicator.
And afterall there?s a lot of factors that might couse my tank to be slightly worse than usuall.

Anyway I think it?s time to put on some strings to the back of my car? afterall it might turn out I don?t need any aero aid on my roof . the high power GSI version of the kadett had a spoiler on the trunk edge which even reduced the Cd by 0.01 ! not bad for a spoiler? perhaps I should move my cones there.

Here?s a drawing of what I think the cones might be doing but it?s all speculative of course.

the turbulent wake might be seen as a solid object by the oncoming air (it might be smaller than what i drew but it's still going to be far bigger than the cones themselves i assume)

I?ve also been thinking putting some kind of powder on the roof might reveal flow patterns.

Anyway for now the car?s ?clean? again, so lets see if my preceived downforce has gone now or not.

At least I get the ?impression? they?re doing something?
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Old 03-13-2007, 09:59 AM   #9
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airflow

Lunarhighway:

I have found the a short trip after the morning dew is the best. Fast and short so the surface water has been moved but not completely dried from the surface.

Maybe spray the car with a mister then make a trip, stop and take note of the water patterns.

Foggy weather is good too.

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