Rain Gutter Side Skirts

[mazdamx640]

Looks like a neat and cheap project. Did you notice a gain in fuel economy and or more stability going down the road?

[aerohead]

Originally Posted by NeilBlanchard

Hi folks,

Can I ask what is the intended aerodynamic function of these side skirts?

Personally, it feels a bit counterintuitive -- I would think that you want to have air "escape" out from under the car; and not to trap it under the car. And none of the ultra-low Cd cars that I am aware of have anything like this.

They're borrowed from racing where they segregate the flow between sides and bottom (as pickups are using step rails today),making a 'flush' pathway for the air across the tires.Sometimes,air fence flow straighteners are added to keep the air moving longitudinally,not transversly).
The John Shinella' Trans Am Firebird units were shown in the Lockheed tunnel to cut drag.
Lower skirts have been used in ground-effects cars,but I don't think there's a drag advantage.
IMHO I would not have them lower than the belly.I used them on the CRX

[mwilliamshs]

Originally Posted by aerohead

...Sometimes,air fence flow straighteners are added to keep the air moving longitudinally,not transversly)....

I'm listening

[aerohead]

Originally Posted by mwilliamshs

I'm listening

The 1987 Olds AEROTECH would be an example
http://deansgarage.com/wp-content/ga...n_at_speed.jpg
A bunch of Pontiacs had these as styling features.

[mazdamx640]

I want one

[mwilliamshs]

are the straighteners the horizontal surfaces (and the transition to such) at the rear of the skirts?

[aerohead]

Originally Posted by mwilliamshs

are the straighteners the horizontal surfaces (and the transition to such) at the rear of the skirts?

Yes.They intrude into the airstream and create a physical boundary which directs the flow and helps isolate it from hostile pressure gradient across the 'border' which might otherwise induce a span-wise flow,spinning up into power-robbing vortices.
Once the flow is aligned with the fence,it builds momentum which tends to keep it moving in that direction.

[mwilliamshs]

the horizontal fins on the side skirts...

Originally Posted by aerohead

...They intrude into the airstream

They stick out from the car

and create a physical boundary

and air can't go through them

which directs the flow

so the air gets pushed by the fin

and helps isolate it

and this keeps it separated


from hostile pressure gradient across the 'border' which might otherwise induce a span-wise flow,spinning up into power-robbing vortices.

from...????

Once the flow is aligned with the fence,

once the air gets pushed by the fence

it builds momentum which tends to keep it moving in that direction.

it keeps going that way

is that about right?

[aerohead]

Originally Posted by mwilliamshs

the horizontal fins on the side skirts...

Yep,you've got a good handle on it.
For the spanwise flow,here's a better explanation:

If the pressure below the car was higher than the pressure besides the car,the high pressure air would bleed up into the lower pressure air alongside the car,moving transversly,or,sideways (spanwise if on a wing).
This transverse flow spins up into vortices,as a fast-moving tributary meeting a slow-moving river,or as the jet stream spins warm,rising,moist gulf air over Colorado into supercell tornadoes.
The vortices lockup kinetic energy which can never be converted to pressure,ultimately raising the pressure drag.
If the fence can keep the two pressure regimes apart,we can skip the vorticity.
Winglets on commercial airliner wingtips are a form of exaggerated fence which helps spanwise vorticity from forming.