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Old 06-08-2008, 09:01 PM   #51
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Originally Posted by SL8Brick View Post
Alright, I'd like to propose a possible scenario on what you've done. At their current height, your VGs are probably inducing effective vortices at lower speeds(guessing up to 45mph). But at higher speeds when the boundary layer is fully formed, they're creating vortices and a measurable amount of drag. At some point(highway speed) their height will become a disadvantage since the drag they create will nullify the drag alleviated behind the vehicle.

Its just a guess...does this make any sense?
I do understand what you're saying. I did test the foam ones at 55mph and the flagging tape reacted well at that speed. Most of my driving is under 60mph.

I'm going to see how they do on my next tank. Scangauge is a bit off,saying I'm out of gas when the needle shows enough for at least 30 miles more at 460miles.
The fins are not glued on, but a magnetic base with a thin strip of tape around the edges.I can remove them for highway trips if I find what you're saying is the case.
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Old 06-09-2008, 07:46 AM   #52
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Hateful, Here's an excerpt from the Mitsu PDF relating to VG size that's relevant here:



Your delta fins might be creating vortices, but they're also extending beyond the boundary layer and most likely creating unwanted drag.
Not sure I concur with the assessment that the VG should be equal in height to the thickness of the BL. I would think that an optimum VG would be slightly taller than the BL to bring in some high energy freestream air.

Hateful's VGs seem a little large (I made some "chevron" style VGs from meat tray foam that were similarly large), and I think scaling them down by 24 to 40% might yield the right size. Although if most of his driving is less than "highway speed," they might be just the right size.

Let's see what his MPG numbers look like, then discuss.
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Old 06-09-2008, 08:35 AM   #53
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Not sure I concur with the assessment that the VG should be equal in height to the thickness of the BL. I would think that an optimum VG would be slightly taller than the BL to bring in some high energy freestream air.
Mitsubishi first went under the assumption that 30mm was the optimum height, but in their conclusion(and final production on the EVO) they opted for 20mm fins in the interest of lowering the drag of the VGs themselves.
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Old 06-09-2008, 10:10 AM   #54
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Mitsubishi first went under the assumption that 30mm was the optimum height, but in their conclusion(and final production on the EVO) they opted for 20mm fins in the interest of lowering the drag of the VGs themselves.
was that before or after installing the spoiler?
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Old 06-09-2008, 10:33 AM   #55
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Mitsubishi first went under the assumption that 30mm was the optimum height, but in their conclusion(and final production on the EVO) they opted for 20mm fins in the interest of lowering the drag of the VGs themselves.

It would be interesting to read that trade study. I wonder what other factors may have guided the design of their VGs.
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Old 06-09-2008, 10:52 AM   #56
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was that before or after installing the spoiler?
I think that the EVO MR already had the pedestal wing before the VGs were developed. But in any case, the wing is almost irrelevant because the VGs effects begin to take place long before the air influences the spoiler.

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It would be interesting to read that trade study. I wonder what other factors may have guided the design of their VGs.
Here it is: http://www.mitsubishi-motors.com/cor...004/16E_03.pdf
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Old 06-09-2008, 11:06 AM   #57
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I've lately been noticing that quite a few GMs have factory "vortex generators" or maybe I should call them slipstream energisers. They don't look like what you think though. A few other makes have similar devices....

What the GM ones are, are grooves let into rear curved surfaces, new GM fullsize vans have them in the rear pillar trim piece above the lights, their minivans have got them moulded into the lights. Several pontiac models have them moulded into the rear bumper... it looks like a styling thing, but I don't think it is... what they do I think is similar to how a NACA duct works, they allow a stream of air to bleed off from the main flow with minimal disruption to it, and this high speed stream is diverted and injected into the slipstream at a greater angle than the main body of air can turn round... This helps in the shedding of large parasitic vortexes and energises the rear airflow. There are similar types of devices incorporated into some of the rear diffusers on exotics.

I'm planning to take advantage of this type of device when I get my "quarter pool noodle" rear fairings project on. Firstly I'm quartering a pool noodle lengthwise, these will be stuck around marvins tailgate to allow airflow to deflect slightly behind him for a virtual boattail effect. However, I'm going to also carve notches in them, probably aimed something like this in top view //|\\ with the angles arranged to focus at a point about 2m behind the tailgate and half it's height. The side pieces will also attempt to aim at that point. I was going to go with separate turbulation devices, I may put a couple in front of the light clusters still, but I think this will work just as good and is cleaner looking.
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Old 06-09-2008, 11:15 AM   #58
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yup, the civic is a sedan, LX.
I have always wondered if stock Civic Sedans and Civic coupes have better aerodynamics than stock Civic Hatchbacks.

I know the sedans and coupes weigh a little more than the hatchbacks, but they definitely have more of a tear drop shape which ought to have a smaller low pressure area behind when compared to the hatchbacks- which just drop off abruptly.

Any thoughts on this?

If this was true, a 92-95 lightened civic coupe with D15Z1/VX trans might get better highway mpg than a VX hatchback.
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Old 06-09-2008, 11:52 AM   #59
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Coupes can have a problem where they form too efficient an airfoil shape with a positive angle of attack. This makes them create lift and the lift vector is tilted back a little from the vertical resulting in lift induced drag...

This is why coupes and kammbacks should not curve smoothly off to as near the floor as possible and should try to have clean edge seperation at about the half height mark, with a reasonable "trunk" type shape to make sure the air is turned to the horizontal before separating... New Beetles for example have horrible drag... until fitted with a spoiler...

So, pick your coupe carefully and be prepared to correct it with an appropriate "whale tail" or extra lip on the trunkette if it has less than about a couple of inches of surface to turn the airstream.

Supercars and exotics have a lot of upkick from their rear diffusers to try to help this problem and to try to alter the effective angle of attack of the body shape. Some saturn coupes have body lines that indicate the designer was thinking right, aiming to create more upwash at the back, I'd think those ones do better than the others.

That pretty pic above, the before and after pic, you'll notice that it changes the effective angle of rear airflow by shortening the effective airfoil shape by a couple of feet. This kinda tips the lift vector forward, stopping it making drag.

Modern vehicles seem to be bearing this principle in mind more. The mods I described for Marvin may have detrimental effects if he's not kicking up enough air from underneath. Lower down the sides the aero will be designed to move air upwards....

Now there is also an effect that creates downwash that will mess up the best intentions here. This is similar to tip vortices on an aircraft wing and because the "span" of a car is very narrow it can have significant effects. What happens is that the air will try to roll round the sides off the top of the vehicle. This is why lipped side skirts can be effective, they stop that air getting back under the car. Other measures might be effective also. For example roof fences are seen as "lift killers" for safety on race cars that go sideways, but I think they would also have an effect on reducing the "tip vortex" type down wash from the roof, improving the rear airflow by keeping it from being sucked down too much behind the vehicle, increasing the effective angle of attack and making induced drag. I keep wondering actually if there's actually something to be said for 1957s fins in that regard. Not how they were implemented then, but how something looking somewhat similar but actually functional might be used these days, it would look I think a lot like "winglets" on a learjet. Come to think of it, those Superbird fins probably had as much effect on downwash prevention as they did in "keeping the spoiler in clean air"
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Old 06-09-2008, 12:13 PM   #60
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Coupes can have a problem where they form too efficient an airfoil shape with a positive angle of attack. This makes them create lift and the lift vector is tilted back a little from the vertical resulting in lift induced drag...

This is why coupes and kammbacks should not curve smoothly off to as near the floor as possible and should try to have clean edge seperation at about the half height mark, with a reasonable "trunk" type shape to make sure the air is turned to the horizontal before separating... New Beetles for example have horrible drag... until fitted with a spoiler...
"
So, is there a rule of thumb on coupe/sedan shape to prevent this lift?

Should the back glass angle always be more shallow (less slope) than the windshield angle? This seems to be the case on the CRX as well as Prius and even the Nissan 240sx

As you said, there should be a separation point about halfway down the back.

So models with a steeper back glass (Fiero, early 1990's Dodge shadows etc.), will likely have more drag, right?
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