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Old 01-20-2013, 03:31 AM   #29
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Quote:
Originally Posted by ayau View Post
So in theory, if you install panels underneath the vehicle and speed up airflow, then the net result should be an increase in lift. However, if you have a rear diffuser, then that should reduce lift. Is this correct?
Faster air is lower pressure.

What we want is the ability to transfer high pressure under the car to above.

What a diffuser does is speed up the air going under the car in the front, then slows the air down as it tapers up in the back, but in the back is a big low pressure section. So the pressure difference also 'pulls' the higher pressure under-car air to 'fill' the low pressure behind it. The reduction of high pressure under the car reduces lift, and the reduction of low pressure behind reduces drag.
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Old 01-20-2013, 03:41 AM   #30
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Originally Posted by CSG Mike View Post
Ok so we have one saying CL ~-25kgf at just under 200km/h, and another saying CL ~+24kgf at 40m/s (144km/h).

Were there any changes through S2k models that could account for the differences?
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Old 01-20-2013, 05:08 AM   #31
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Here's a picture of the BRZ's rear diffuser.


From the picture above, it appears that the BRZ's rear diffuser has a small angle to increase air pressure/decrease in velocity. However, it's not clear whether or not it's functional in producing downforce.
You mean this...?
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Old 01-20-2013, 05:10 AM   #32
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You mean this...?
Not the optional diffuser. I was referring to the rear oem bumper/faux diffuser.
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Old 01-20-2013, 06:19 AM   #33
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Old 01-20-2013, 06:26 AM   #34
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I'm just gonna put this out there...that rear diffuser wing thing is actually just for looks. The leading edge is almost flat, it breaks up any of the potential of the rest of the "diffuser" by blocking airflow rather than guiding it. I have it on my car and love the look but it's function is purely cosmetic.
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Old 01-20-2013, 09:48 AM   #35
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Quote:
Originally Posted by Dimman View Post
Ok so we have one saying CL ~-25kgf at just under 200km/h, and another saying CL ~+24kgf at 40m/s (144km/h).

Were there any changes through S2k models that could account for the differences?
AP1 vs AP2 bumper
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Old 01-20-2013, 03:47 PM   #36
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Diffusers and under body panels are cool because they're one of the few mods/designs that can simultaneously reduce lift (or create down force) and reduce drag. By giving the air under the car a clean path and smooth exit, the pressure differential caused by the shape of the car can be reduced. Depending on the car, it could be negative. Not likely with the bubbly roof line of our cars.

Overall these cars are pretty solid aerodynamically compared to many others, but the traditional coupe shape isn't as inherently down force friendly as something like a McLaren 12C or other wedge like supercar.
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Old 01-20-2013, 10:42 PM   #37
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So in the spirit of DIY, has anyone done seen the guys at Ecomodder and their "tuft testing"?

LINK to AutoSpeed


Similar to the wind tunnel tests summarized above, it's a really simple way to visualize flow directions and turbulent spots. Painter's tape + yarn/string + video camera = data. From there, I'm sure we could pinpoint the easiest gains.


Alternatively, I know that we could develop a solid model and run a CFD analysis to get those beautiful clown puke pictures... but at this point, tuft testing is much more accessible to the DIY community. If someone wants to volunteer for CFD duty, I'd be glad to help, but it's beyond my experience.
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Old 01-20-2013, 11:07 PM   #38
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Originally Posted by Shankenstein View Post
So in the spirit of DIY, has anyone done seen the guys at Ecomodder and their "tuft testing"?

LINK to AutoSpeed


Similar to the wind tunnel tests summarized above, it's a really simple way to visualize flow directions and turbulent spots. Painter's tape + yarn/string + video camera = data. From there, I'm sure we could pinpoint the easiest gains.


Alternatively, I know that we could develop a solid model and run a CFD analysis to get those beautiful clown puke pictures... but at this point, tuft testing is much more accessible to the DIY community. If someone wants to volunteer for CFD duty, I'd be glad to help, but it's beyond my experience.
It's the old F1/racing way, too.

Another is paint streaks, where a series of dots are painted on the car which then makes a few runs, they then look at the trails left on the car.

Analysts still use a similar technique when they can get pictures of race cars that run in wet conditions. They have very distinct streamlines left on them. They are particularly useful for analyzing inlet effectiveness.
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Old 01-20-2013, 11:19 PM   #39
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Does anyone have more pictures of the TRD Griffon underbody work? I'd like to try and replicate their work since it's probably the most extensively tested stuff for our cars so far.
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Old 01-20-2013, 11:29 PM   #40
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@serialk11r Don't know if you've seen this thread. Thought you might be interested.
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Old 01-21-2013, 12:31 AM   #41
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Quote:
Originally Posted by Dimman View Post
It's the old F1/racing way, too.

Another is paint streaks, where a series of dots are painted on the car which then makes a few runs, they then look at the trails left on the car.

Analysts still use a similar technique when they can get pictures of race cars that run in wet conditions. They have very distinct streamlines left on them. They are particularly useful for analyzing inlet effectiveness.
New plan. Somebody needs to splatter paint their car for science! Volunteers? I'd bet other substances would do just fine too (like mud, ice cream, motor oil, liquid soap, etc).

I still prefer string since you can see flow separation more easily. For a given frontal cross-section (assuming it's unavoidable and constant), we can lower the drag force on the car by keeping the flow laminar and connected.

Good opportunities for drag reduction:
  • undercarriage smoothness (flat is good)
  • airflow around tires (redirect flow around them)
  • front bumper smoothness (boxy is bad)
  • exiting airflow from engine bay (smoothly reintegrate it)
  • transitions at the cowl and side mirrors
  • transitions at rear windshield and trunk (big target for flow separation)
  • reconnection of the flow paths (minimize the virtual cross-section for exiting air)
As others have suggested, under-car air speed should be as high as possible without restricting flow. High speed = low pressure. This serves to decrease drag and provide the differential pressure for Bernoulli's downforce.

Wheels will generate turbulence around them, for alot of reasons (too many to name). It's usually best to guide the airflow away from them to avoid unnecessary drag. Obviously race cars will have dedicated ducts to sent air to the brakes, and that's a different ballgame.

The rest of them are about maintaining laminar flow as much as possible. The body panels will never resemble an airfoil/teardrop, but similar concepts are still relevant. Flow should stay connected with the body panels, and body panels should never create pockets.

Side note: I realize the golf-balling a car is somewhat legit science, but good aero can generally trump bad aero+dimples. Use turbulence only when necessary.
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Old 01-21-2013, 12:50 AM   #42
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Quote:
Originally Posted by Shankenstein View Post
As others have suggested, under-car air speed should be as high as possible without restricting flow. High speed = low pressure. This serves to decrease drag and provide the differential pressure for Bernoulli's downforce.
Besides installing underpanels, how else can you increase airflow speeds below the car?

Higher air speed = lower pressure = less lift/more downforce

What are the effects of lowering the car in terms of lift/downforce?
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