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Old 03-21-2008, 07:04 AM   #11
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Hi clench..,

Well, heading on down the highway, the cross wind vector sums with the movement of the car. So the resulting air flow direction relative to the car is what, less than 20 degrees at 60 mph, for most situation. So, what is going on is these fences actually act as fences to straighten out the flow over the roof, rather than as side-ways spoilers.

But, even if they did act as side-ways spoilers, turbulence can be good or bad. If you did not have them, your going to get detachement of flow on the downwind rear corner of the car. The more side wind, the longer the zone of detachment from the rear forward. The fence could limit this detachment from the rear forward to the C pillar, or cause a rapid turbulence that causes the cross section of turbulence to be small, and having a minimul impact on the axial flow along the down wind side of the car.

So, that is the jist of the guestimation. From here out takes engineering.
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Old 03-21-2008, 07:52 AM   #12
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But, even if they did act as side-ways spoilers, turbulence can be good or bad. If you did not have them, your going to get detachement of flow on the downwind rear corner of the car. The more side wind, the longer the zone of detachment from the rear forward. The fence could limit this detachment from the rear forward to the C pillar, or cause a rapid turbulence that causes the cross section of turbulence to be small, and having a minimul impact on the axial flow along the down wind side of the car.

So, that is the jist of the guestimation. From here out takes engineering.
Yep, that's what I was figuring.
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Old 03-21-2008, 02:06 PM   #13
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I'm not sure how the NASCAR fence affects cD, but the VW 1 liter prototype had some sort of fin on the tail end. I assumed it was there to maintain attached flow.
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Old 03-21-2008, 02:47 PM   #14
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Originally Posted by theclencher View Post
If I'm going less than 60 (almost always) and there's a 90 degree xwind at 25-30 (frequently happens here) that makes a 45 degree vector. We know that flow separates above about 30 degrees; 15 against a ground plane.
that's a 30 degree vector at 60 mph w/ a 30 mph crosswind (or is it 22.5? been a while since physics). wind speed is half your speed so you don't split the difference equally
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Old 03-21-2008, 03:27 PM   #15
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Hi All,

Lets see. 60 mph, 30 mph 90 degree cross wind would be for every foot you move forward, the wind moves 1/2 a foot cross wise. A 45 degree angle would require a 60 mph cross wind. Back to the original problem, Arc Tan of 1/2 is 26.6 degrees relative air flow direction across the car. Kames..., I think you were confusing the Sin of 30 degrees (opposite over hypotenues) which is .5.
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Old 03-26-2008, 07:18 AM   #16
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Just realised something else related to these.

When an aircraft wing is in ground effect the tip vortices that create downwash are blocked by the ground within a span width of the ground....

So any vehicle shorter than it is wide will not be affected much by lift vortices...

But any vehicle taller than it is wide, i.e. it's lifting surface is a span or more off the ground, is likely to make "tip vortices" off the side of it's roof. These will create a downwash effect that alters the lift vector such that it is tilted backwards from the vertical, inducing drag...

So for taller vehicles, i.e. my minivan, some "tip vortex" control strategem may be beneficial, the equivalent of tip plates or winglets on an aircraft. Therefore a form of fence at the sides of the roof on a taller vehicle may lessen the effects of vortices forming downwash, and may thus improve the induced drag by tilting the lift vector forward of the vertical.
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Old 03-26-2008, 04:25 PM   #17
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i have inverse railing on my chevette on the roof. i think it may be more of a rain gutter than anything.
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Old 03-26-2008, 05:04 PM   #18
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Hi All,

The vortex forms when the air from a high pressured area flows sideways to a place where air is lower pressure, in an on-axis general flow.

So, at C pillar area of a car is one of those places. The air on top of the car is high pressure, and the air on the side of the car is stalling, sucking the air in from the roof. As the car slopes away going behind the C pillar, the vortex has a zone to complete the first loop.

Which is why Ernie Rodgers put thos big plates on the Beetle. They interupt the first loop of the vortex.
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