Bonnet air-flow characteristics of "MY" car

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WideOpenThrottle
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Bonnet air-flow characteristics of "MY" car

Post by WideOpenThrottle » Tue May 21, 2013 1:41 pm

This is the result of 110km/hr wind flow over the bonnet.

As you can see from the pics,there is always downward and backward pressure on the strings.

The strings across to the centre of the bonnt from the 2 black stripes on the guard faced backward but i could see theyhad very little wind speed passing by them.

The far outer lower string either went to the wheel or got sucked into the gap around the light cover.

The 3rd lower string sometimes got sucked into the headlight gap.

At no point did any of the strings lift vertically.

This indicates that there is always positive pressure over all this area but it will be less than directly in front of the car.
Attachments
Bonnet air flow test 001.JPG
At rest
Bonnet air flow test 003.JPG
110km/hr
Bonnet air flow test 004.JPG
110km/hr
Bonnet air flow test 005.JPG
110km/hr
Last edited by WideOpenThrottle on Tue May 21, 2013 7:32 pm, edited 1 time in total.
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Post by MrGrey » Tue May 21, 2013 1:42 pm

great idea for a test, thanks for sharing the results :D

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Post by WideOpenThrottle » Tue May 21, 2013 1:45 pm

This picture may be correct but is mis-leading as it seems to indicate negative or upward pressure but may in reality indicate less positive pressure
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Post by Timmo » Tue May 21, 2013 4:35 pm

Pressure acts along the surface normal (i.e. perpendicular to the surface) so that's why the pressure vectors are shown that way- they don't represent air flow

Remember that even at rest, pressure is still being applied to a surface (albeit equally) due to atmospheric pressure (standard pressure = ~1013 hPa at 15 degrees C and close to sea level).

As soon as the car starts moving, pressure will begin to be act non-uniformly with areas of relatively higher pressure occurring where air is forced to change direction (i.e. at the front of the car). The area of low pressure at the base of the windscreen is due to the boundary layer lifting further off the bonnet due to the roof, leaving space where air is 'sucked up'
Ex: 1991 BRG NA6, 1998 White RS, 1997 Wine Merlot M Edition

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Post by WideOpenThrottle » Tue May 21, 2013 5:20 pm

Hi Timmo...am bit confused on your analogy so in my case i will disregard pressure and only concentrate on wind flow such as indicated by my string test going on and in the headlight cover....which is what i was hoping for.
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Post by SkilletKid » Tue May 21, 2013 7:17 pm

Just an observation, but aren't the strings a bit too long? Tuft tests usually use around 2 inches so that the flow that it will show is the more local flow where they are mounted. When it's too long the larger influence of the tail will always pull it back.

The strings on the FSAE car from auckland uni when they went tuft testing were no longer than 3 inches.
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Post by WideOpenThrottle » Tue May 21, 2013 7:30 pm

All the pics i looked at show lengths vary enourmously but mine do seem to be at the longer end mainly to be be able to see whats going on at the front of the bonnet which slopes away & down out of view.
As for ideal length...well! thats a tuft one.
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Post by Timmo » Tue May 21, 2013 8:55 pm

SkilletKid wrote:Just an observation, but aren't the strings a bit too long? Tuft tests usually use around 2 inches so that the flow that it will show is the more local flow where they are mounted. When it's too long the larger influence of the tail will always pull it back.

The strings on the FSAE car from auckland uni when they went tuft testing were no longer than 3 inches.
Fair enough :) Long story short, the picture of the RX7 shows air pressure, a string test shows airflow/direction.
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Post by SkilletKid » Wed May 22, 2013 4:23 pm

WideOpenThrottle wrote: As for ideal length...well! thats a tuft one.

Hehehehe :lol:
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Post by Snapfrozen » Mon Jun 10, 2013 10:21 pm

Sorry to bump an old thread, but if you want to see something interesting, do the back window of the hard top. You'll find some crazy shit going on there (Hence why you should probably mount your wing higher if it is 2D)

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Post by WideOpenThrottle » Tue Jun 11, 2013 8:26 am

I probably would mount it higher for a dedicated track car but it is more for aesthetics really.

If I had a GoPro I could mount it on the wing & see what the hardtop tufts do!
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Post by SkilletKid » Tue Jun 11, 2013 11:04 pm

What about a ghetto-pro? =P

Lots of masking tape and an old digital camera. It survived being driven around a small albany suburb in my case haha
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Post by Snapfrozen » Wed Jun 12, 2013 6:49 am

To spoil the fun, you will find the string blows the wrong way, back towards the front of the car over most of the rear window. Hence running a hardtop spoiler with a wing ;)

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Post by Mad Kiwi » Wed Jun 12, 2013 8:40 pm

Yeah but what does a hardtop spoiler do to the airflow...

Is it a definate improvement or one based on logic (i agree it should help).....but does it?

Reason I ask is that the U.S race guys (not miata cup etc but open class) dont seem to use them. I would have thought that if it was a proven thing, they would....?

I see plenty on club roadster but very few (if any) on Miata turbo or other more practical based forums :lol: :evil:

Especially if it makes the rear wing operate more effiicently......

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Post by SkilletKid » Thu Jun 13, 2013 3:09 am

I think they don't run it because it is easier to just put higher mounts on the wings to keep it above the turbulent flow of air caused by the roof (probably easier to prove anyway). Look at Shaikh's Miata from Fat Cat Motor Sports. His wing is well above the roof line.

Old (really old) formula cars did it this way.

What would be an interesting experiment is if someone tried putting vortex generators on the hard tops. Much like alot of Evos and a few Commodores have; they might help a bit more than the roof spoiler.
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Post by Snapfrozen » Thu Jun 13, 2013 7:16 am

SkilletKid wrote:I think they don't run it because it is easier to just put higher mounts on the wings to keep it above the turbulent flow of air caused by the roof (probably easier to prove anyway). Look at Shaikh's Miata from Fat Cat Motor Sports. His wing is well above the roof line.

Old (really old) formula cars did it this way.

What would be an interesting experiment is if someone tried putting vortex generators on the hard tops. Much like alot of Evos and a few Commodores have; they might help a bit more than the roof spoiler.
They'll help if it is a 2D spoiler, 3D are designed to run in turbulent air. 3D spoilers don't need to be mounted as high.

For example, the best result that WoT could have would be to mount his wing at roof line, add vortex generators and a hardtop spoiler.

Usually they don't run them for 2 reasons.

1. Cage requirements stop them from fitting
2. Points based systems throwing them into another class.

All the time attack cars I've seen have run them, AutoX they don't.

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Post by Furai » Thu Jun 13, 2013 7:44 am

Garage Hundred doesnt run one. Hardtop spoilers must be useless :lol:

In sayin that though the only companies who make them all only make cosmetic parts with no aero testing. Are you sure they arent just creating more turbulent air?

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Post by SkilletKid » Thu Jun 13, 2013 1:42 pm

Snapfrozen wrote: They'll help if it is a 2D spoiler, 3D are designed to run in turbulent air. 3D spoilers don't need to be mounted as high.

For example, the best result that WoT could have would be to mount his wing at roof line, add vortex generators and a hardtop spoiler.

Usually they don't run them for 2 reasons.

1. Cage requirements stop them from fitting
2. Points based systems throwing them into another class.

All the time attack cars I've seen have run them, AutoX they don't.

Oh ok, didn't know that about the 3D wings. Thanks snap. =]

To be honest I haven't seen someone use a MX5 as a time attack car. Maybe I'm just looking at the wrong places?

I can't imagine a cage would stop someone running a roof spoiler or vortex generators unless the cage was designed to not have a roof on? I've seen a few spec miata boys run hard tops.
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Post by Snapfrozen » Thu Jun 13, 2013 8:11 pm

2D wing on left, 3D wing on right to clarify.

2D is a flat wing that remains the same shape the entire width, 3D wings tend to work better in poor wind conditions than 2D but can also do the opposite as what's intended if they're in a poor position.
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@Furai

How many "race car parts" are made by companies for a car. 99% of the fast Miata's/MX5's are home built. Look at the likes of the following 2.. All home researched, all DIY, all trial and error making small tweaks. Unless you have $300k, you will be DIY'ing shit...

The Pass from Clubroadster (God I hate hotlinking to it, but his work is amazing, so much dedication to being faster)
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Moti - owner of Blackbird Fabworx in the states, as above, dam awesome
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Post by Furai » Thu Jun 13, 2013 8:49 pm

It has no hardtop spoiler? That was sort of what I was getting at. Race companies make wings and splitters for these but no hardtop spoilers?

I would have thought the blackbird car would have one if its important as you say? :?:

Edit: Can you link me these time attack cars with them?

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Post by Snapfrozen » Thu Jun 13, 2013 9:17 pm

Furai wrote:It has no hardtop spoiler? That was sort of what I was getting at. Race companies make wings and splitters for these but no hardtop spoilers?

I would have thought the blackbird car would have one if its important as you say? :?:

Edit: Can you link me these time attack cars with them?
The first car runs a 2D Nascar COT wing so hardtop spoiler is pointless, the wing is high enough to get clean air without it. The reason people run them is to change the air pressure on the rear window of the hard top. The other solution is to remove the rear window. Otherwise, fastback ;)

As for Moti's car. Don't even bother. It has no windscreen, the entire airflow characteristics of the car are changed and that's not to mention that he is beating turbo car's in supermod classes with a factory engine.

Time attack rules also mean you may be penalised for extra aero, so depending on the country/rule set you may lose points for it/not have enough points to run it/change classes by having it.

Image

Image

Can't find the image/video's of tuft testing I was looking for

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Post by WideOpenThrottle » Thu Jun 13, 2013 9:23 pm

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Post by WideOpenThrottle » Thu Jun 13, 2013 9:25 pm

And a bit more!...both care of a mate! :D

http://robrobinette.com/S2000Aerodynamics.htm
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Post by Furai » Thu Jun 13, 2013 9:30 pm

Snapfrozen wrote:All the time attack cars I've seen have run them, AutoX they don't.
No offence(There isnt many ways I can put this without sounding like a prick) but can you link me some with them or some solid testing showing they work?

I understand what your saying and see the purpose but havent seen something to back up your theory yet(Be a car or testing).

Wouldnt want to spend a couple hundred on one to find they dont work IMO

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Post by Snapfrozen » Thu Jun 13, 2013 9:34 pm

Big waste of time and money for you. You've got bigger drag problems...

I'll get you some proof tomorrow. For now, I've got a headache. Take my 300+ hours of aero research with a grain of salt, but here is a good place to start. Also said I would find you the tuft tests but having trouble right now locating the videos. If you want more info Kieran, PM me your email address and I'll send you some light reading. Aero is an art. You can't have right and wrong and theory only gets you half way...

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