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Old 07-18-2014, 01:17 AM   #21
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NACA duct only works with air flowing over it in the correct direction. The air inside the fender well should be mostly static or just a complete jumbler mess.
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Old 07-18-2014, 01:47 AM   #22
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Fair enough - that makes sense. Thanks.

Not being facetious at all: What about adding pancake (computer-style) fan to the underside of the louvered fender to help create a low-pressure area? It could be coupled to a temp sensor to turn it on or off, as required.

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Old 07-18-2014, 04:25 AM   #23
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You don't need fans or anything complicated, just pressure differentials.

-Ryan
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Old 07-18-2014, 04:45 AM   #24
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Quote:
Originally Posted by DeerHunter View Post
Perhaps I have the wrong end of the stick, but I think kiwi was suggesting that the inside of the fender contain the NACA duct (think of the picture above being the underside of the fender). Would that still break the functionality of the duct?
A NACA duct works on air flow, not a difference in pressure, so it has to be exposed to a fairly smooth air stream. You normally see them on hoods or other large body panels with good airflow. Inside a fender the air flow is chaotic and not really suitable for a NACA duct.

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Old 07-18-2014, 10:59 AM   #25
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They're designed as a low drag sunken inlet on a surface with laminar airflow. If you don't have laminar airflow, they won't work.

It's a great design, very effective when implemented correctly. But it's not The Ultimate Hole for all possible ways to move air. People like to use them badly because they look like race car parts.
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Old 07-18-2014, 02:50 PM   #26
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While inlet/outlet design is one factor for this topic, let's not let it get derailed too far off!

I just purchased a pair of magnehelic gauges, looking forward to playing with them and maybe we can get some more data together on the front fender issue for different set ups.

Keith - for you 'control' tube inside the cabin, were you running with windows up?

-Ryan
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Old 07-18-2014, 03:54 PM   #27
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Windows down, but the end of the reference hose was tucked in near a seat so it wasn't seeing moving air. Since I wasn't mapping pressure changes against speed, I figured the cabin pressure would be consistent at a given speed and it would work well as a reference.
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Old 07-18-2014, 04:10 PM   #28
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Windows down, but the end of the reference hose was tucked in near a seat so it wasn't seeing moving air. Since I wasn't mapping pressure changes against speed, I figured the cabin pressure would be consistent at a given speed and it would work well as a reference.
It could be an interesting test. Put the reference end in the normal spot, then put the other end next to it and plug the measurement end when the car isnt moving and repeat your test to see of the air pressure at your reference point compares to ambient, of course try to keep yourself to the same elevation during this test.
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Old 07-18-2014, 04:41 PM   #29
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I'm thinking the most relevant test, since air moves according to pressure differentials, is to place the two hose ends where you'd like for air to be moving between.

Or, for testing if you have pressure creating lift in the wheel well, one tube above the fender, one tube up near the top of the underside above the tire.

Placing one tube in an arbitrary control location works fine to just see relative changes when trying different things, but I think to see the bigger picture you want to test where that pressure is moving things to/from. In the targa car's case, I'd look at underhood pressure vs. wheel well pressure (up top and down by control arm) vs. pressure at/behind vent.

The gauge I got is a zero-center style, I haven't played with these before but I figured that that would allow me to place the hoses in two locations I want to see the pressure differential between, and then the gauge can tell me the differential either way, I won't have to get out of the car and switch tubes around if the differential is in the opposite direction from what I had expected.

-Ryan
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Old 07-18-2014, 04:58 PM   #30
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Well, yes. I was testing the differences from various fender modifications, so I just needed a stable reference. I took all my numbers on the same piece of road, so altitude (!) was not a factor. Neither was wind direction, of which there was none. The numbers on the gauge were rock steady, with just a bit of needle quiver.

If I was looking for the best place to put a vent, I'd be testing the two locations in question relative to each other. If I test for the best place for fender top vents, it'll involve a reference in the wheel well and a probe attached to a stick to map out the relative pressures across the surface. That's how we mapped the hood.
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Old 07-18-2014, 05:01 PM   #31
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I was actually kind of interested in you doing my test not only to see if you were getting good data but to see if the entire cockpit below the beltline is a low pressure zone like I think it is.
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Old 07-18-2014, 05:03 PM   #32
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Ah, then what you're saying makes more sense I'll play with that next time I'm playing around. Low pressure compared to ambient, right?
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Old 07-18-2014, 05:06 PM   #33
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Yes.
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Old 07-18-2014, 06:00 PM   #34
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Now, I did this testing because 1) I've seen the back of a lot of wheels uncovered on race cars and 2) I was looking for that improved flow. I figured pressure testing would tell me something. But looking at this GIF that someone forwarded to me, there's obviously a lot going on at the back of those wheels. So cutting out the rear of the fender may have a real effect on airflow down the sides of the car.

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Old 07-18-2014, 06:08 PM   #35
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Yeah, but you're getting into advanced under car aero where you're creating a boundary with vortices between the rocker and the ground that prevents air from getting pulled under the car which should get you lower pressures under the car. Thats the main point of the very complicated canards you see on some production cars. They effectively use air to create a side skirt. You can see the GTR doing that.
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Old 07-18-2014, 06:11 PM   #36
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Or you're simply seeing the tire spray dripping off the "side diffuser" That's what I see. It's what's going on along the middle of the door that interests me. You can try to make it complicated, or you can take what you can from what's available.

It doesn't really rain here. One of you guys in Oregon go take your Miata for a drive and post a video of what's going on around the front wheels
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Old 07-20-2014, 07:45 AM   #37
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Just made me think of another car I was looking at the other day...

OPTIMA Presents Corvette of the Week: 1988 Callaway Twin Turbo Corvette #57 | Corvetteforum

Regardless, Interesting fact finding your doing, Keith. If you go for vents on the top, are you going to make a short tube or something to draw from the wheel area? Its rather cramped up there, is it not? Or do you feel the vents will be enough to pull without needing tubes to direct it? I understand the whole topic pertains to high-low pressure air movement, but I assume directing/managing it could make it work better.
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Old 07-20-2014, 09:32 AM   #38
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Making holes at the top of the fender is easy, you are trying to use the semilaminar flow on the top/outside to pull air from the very turbulent wheel well.
With no liner in the way you almost have the wheel as undisturbed as with the ALMS etc (but not as close), but I assume you would see an effect even with the liner in place, the air will find it's way.

As mentioned before, make 1" holes on the top and form the edges a bit. Anyone with a pair of fenders with some patina?
Yarn-testing is the easiest.

I'm pretty sure there will be a flow out of such holes (my fenders is GF and I have a lot of other aero things to worry about, e.g. no top, so I don't feel like testing), but will it have a noticeable effect on anything?
Increased flow in the brake duct hose could be one thing to measure/notice.
If the aero balance is shifted and the drag reduced it would be fantastic (not that easy to measure).

Completly different animal of course, but it's just have the front wheels closer to the front than a Miata.
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Front fender aero testing-e36_gtr-s_vented_fenders_m3.jpg  
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Old 07-20-2014, 02:06 PM   #39
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Quote:
Originally Posted by chance91 View Post
If you go for vents on the top, are you going to make a short tube or something to draw from the wheel area? Its rather cramped up there, is it not? Or do you feel the vents will be enough to pull without needing tubes to direct it? I understand the whole topic pertains to high-low pressure air movement, but I assume directing/managing it could make it work better.
It's important to understand in what conditions tubes and hoses are appropriate vs. where they would be detrimental. In areas with lots of turbulence, control your pressures and the air will follow.

-Ryan
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Old 07-22-2014, 01:14 AM   #40
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Interesting topic this...

I decided to look at high dollar front engine GT cars to see what ideas can be "borrowed" since they have just a little more simulation resources than me. My reasoning was that GT cars are restricted look somewhat like the street cars, plus front engine cars have the double duty of aero for heat extraction and lowering drag/adding downforce.

The closest I saw to Keith's trial and lightyear's mod is the Bentley.



The difference I notice most is that the lower third of the fender is cut back to the door line, where the side skirt comes in to shape the flow.

AFAIR, Aston does a similar thing, except with no pulled fender but there are louvers on the side skirt that may be for clearing heat. Corvette does the 1/3 cut, no pulled fender, big vertical vents behind the wheel. BMW does a 2/3 cut that's about 6"(?) wide and louvers on the top of the fenders.
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Front fender aero testing-gt-bentley-continental-gt3-reveal-2013-bentley-continental-gt3.jpg  

Last edited by billybobster; 07-22-2014 at 01:36 AM.
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