Air Curtain

[Vman455]

Originally Posted by mcrews

THIS!
"Back in the day", front ducts were for cooling the brakes.

Not just back in the day. The current Viper has inlets at the outer edge of the bumper for air curtains:



...along with black ducts at the lower corners of the grill for brake cooling:

Originally Posted by hat_man

Not that I would even know how to implement something like this, but I do like to learn as much as I can. After reading the last 10 pages it seems like these air curtains work by moving air from the higher pressure area in from of the vehicle (bumper area) through slots, and increasing it's velocity by narrowing the slots into slits before expelling the air across the face of the front wheels. This in turn either helps fill the cavity of the front wheel well causing a "boundary" so that the air moving around the vehicle doesn't get "scrambled" causing drag? Or the air coming through the slits is of a greater velocity (pressure?) than the air moving around the corner (bumper) creating this "curtain", again keeping the attached air from getting "scrambled"?

I am assuming these slits have their opening in the wheel well somewhere near the vertical mid-line of the tire or are they closer to the outer edge of the tire?

I know the "particulars" of this are way above my head, but man I do love to learn.

I looked at the various BMW air curtain ducts up close at the Chicago Auto Show last week, especially on the i8. They're several inches long, roughly in the middle of the wheel opening, and very close to the outside edge of the wheel well. Also, the angle of the ducting looks to be critical--if the jet shoots too far to the inside, it will simply hit the tire, and if too far to the outside, it will disrupt airflow coming off the bumper. I doubt anyone here could get these to work without a wind tunnel.

[freebeard]

hat_man -- That's pretty much it. I've seen examples that blow onto the tread and others that blow across the face of the wheel.

Originally Posted by Vman455

Also, the angle of the ducting looks to be critical...

How so?

[CigaR007]

Originally Posted by Vman455

Also, the angle of the ducting looks to be critical--if the jet shoots too far to the inside, it will simply hit the tire, and if too far to the outside, it will disrupt airflow coming off the bumper. I doubt anyone here could get these to work without a wind tunnel.

I am currently studying and analyzing various OEM air curtains and stumbled upon this CFD depiction of the air curtain in action.



I will try to implement a front wheel air curtain on my Honda CR-Z, based on existing and proven examples. The illustration above helps to visualize a viable ducting setup. I am currently in the process of getting and building all the necessary parts to assemble the prototype.

[Vman455]

I don't think I was correct in making that statement--I've since noticed that the size and placement of air curtain ducts in production cars vary considerably. The Ford Ranger, for example, has a small duct well inboard of the wheel arch edge:



The Hyundai Veloster N uses a duct with an outside edge truncated well ahead of the wheel opening:



I'll be curious what you find. I've tried two designs of external duct now and found that one hurt stability and the other improved it.

[JulianEdgar]

My external 'Edgarwit' curtains gave a measurable reduction in drag.

[CigaR007]

My plan is to install an internal duct. The incoming high pressure air will go through a naca duct on the bumper lower fascia, routed to the wheel well by a 2.5" pipe. The air curtain exit flow duct will be rectangular and installed vertically (6-7inches) and the opening will be slim in nature, maybe half an inch, no more.

[CigaR007]

Hey Julian, can we assume that the frontal area was slightly increased with your Edgarwit curtains ?

[JulianEdgar]

Originally Posted by CigaR007

My plan is to install an internal duct. The incoming high pressure air will go through a naca duct on the bumper lower fascia, routed to the wheel well by a 2.5" pipe. The air curtain exit flow will be rectangular and slim, maybe half an inch, no more.

Ensure you are picking up air from the stagnation zone or I think you won't have enough pressure difference to get good flow.

[JulianEdgar]

Originally Posted by CigaR007

Hey Julian, can we assume that the frontal area was slightly increased with your Edgarwit curtains ?

Yes, fractionally. But the testing was of total drag (not just Cd) and it showed a reduction.

[freebeard]

Based on what I see in the rain on I-5, what pictures don't show is a forward rolling bale of air that pushes out normal to the wheel face. The air curtain's energy sorta slices that off, so I don't think the lateral placing is critical, but a converging duct is.

One thing that's changed since Permalink #93 is I've pranged the new front bumper, so now I can drill holes in it.