Internal Body Aerodynamics

[mfain]

Originally Posted by vette427sbc

Pappy- So the silver car showed no/minimal airflow out of the fender and the same car with no gills/grilles showed a notable difference?
Interesting considering you would think the gills/grilles would create a low pressure area being that they are under the (assuming) attached airflow over the fender

Yes, that's true. The silver car is mine, and I track it. I have a slight push with the hood vents removed, but the fender grills installed. The push goes away with the fender grilles removed due to increased downforce. The problem I see with the ACR (and the Salt Flat Daytona) is that the rotating tire creates an airflow pattern that pushes air FORWARD at the top of the tire. That air can't make the sharp 135 degree turn to exit the fender louvers. With the louvers removed, the air escapes vertically as verified by yarn tuft tests I have done. The openings in the fender would probably be more effective with a slight raised lip on their leading edge to create a low pressure area. I did that with the fabricated side vents on my 56 to create a low pressure area to help pull air through the oil cooler that is located just behind the vent. BTW, the duct work for the oil cooler starts with air taken from above the radiator (hard to do on a C-3) and runs through a diffuser of sorts to expand the volume prior to the cooler, and then "nozzle" duct work to the fender vent exit. This is the P-51 "Meredith Effect" that Phantomjock describes. You take the inlet air and expand the duct cross-section in a smooth, controlled manner (generally less than 15 degrees divergence angle of the walls) to keep the airflow laminar (non-turbulent) and to increase pressure/decrease velocity at the face of the radiator, then "nozzle" the exit air in the same controlled manner to increase its velocity before discharge into the air stream. At our track speeds an inlet area of roughly 30-40% of the radiator cross-sectional area seems to work well. The air inlet on the 250 mph+ Daytona Charger was less than 20% of the radiator surface, and it had no cooling issues whatsoever, even with 2200 HP. Minimizing obstructions in the incoming airflow - such as horns and braces in the C-3 - greatly reduces air turbulence which both hurts cooling and aero drag. I have posted photos of my 56's oil cooler duct work and some carbon fiber panels I built to smooth the radiator entry airflow on my 62 in other threads, but the system won't let me repost them in this thread. I will try to find a way to provide links to those photos.

Pappy

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Edit: I think this link will get to the 62's radiator duct work: [Only registered and activated users can see links. Click Here To Register...]
This link will get to the 56's oil cooler duct work: [Only registered and activated users can see links. Click Here To Register...]

[vette427sbc]

Jim- Like you noted, my plan is to isolate the wheel wells from everything else and get as much air as possible to exit through the gills. The gills will be dedicated for this and only this airflow. One of my reasonings for this is because assuming I can get the airflow needed, this will make the coolers much more efficient. The fenderwell has "free" high pressure, ambient(ish) temperature air, and if I can manage to control it, I can use smaller coolers, no fans AND rid the car of some drag. FYI the small cooler fans from Setrab are 5lbs each!!

Your Baldwin Motion style gills are probably much better for evacuating air, as are the Ecklers vents, but Im trying to maintain a subtle appearance. I do love the way both of those mods look though... Just not the look Im going for on this car.

Another thing worth noting is to somehow employ wheel spats... I believe these help with drawing air out through the spokes (assuming you dont have a tire to fender gap )

[phantomjock]

Chris -

Carry those "spats" forward on the wheel arch -- right to the leading edge then to the top of the arch - a set of wheel gurney. They would effectively create a vortex running down each side of the car. but not do much for under hood/wheel well (as I recall) but keeps air from migrating under the car.


Sounds like you've a plan coming together. I didn't realize the fans were that heavy - I'd better check mine now.

Like Pappy said, wheel rotation really throws the air forward. Somewhere I have a pic - I thought I posted on here once - from a smoke test in a wind tunnel with rolling road. Really eye-opening.

Cheers - Jim

[vette427sbc]

Pappy- VERY impressive work with the ducting and CF pieces!

[rtj]

Vette427sbc

The louvers I cut were 73 and up style. Probably 80 to 90% area increase.

The 68 and 69 have a dummy louver you might be able to open. At least that’s what my 68 had.

Find photos of the 4takt silver car. Probably the most aero I’ve on a c3.

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You could also consider the grand sport fix with scoops and/or ext. coolers.


All:

Good info. Keep it coming.

[69427]

Originally Posted by vette427sbc

All excellent info!

69427- I like your idea of closing off as much of the front as possible. In fact, I did the same thing with the grilles when you first posted this mod years ago. If you have any pictures of how much you have closed off the underside vents too that would be great! I think with a more efficient airflow design/ducting after those inlets, a smaller (ie: lighter) radiator could be used also. The airflow in the nose pre-radiator has got to be a turbulent mess.
.............................

I don't have any pictures, but what I did was a pretty simple temporary block-off while I see how much more I can reduce the air going to the radiator. I had some scrap pieces of Lexan laying around, so I cut some (4) square pieces (about 5" x 5" in size), and then drilled a quarter inch hole in the centers of each. I just slide one of the squares up into the hole and let it rest on the hole perimeter, and then place another square directly underneath it on the bottom side of the hole (sandwiching the hole perimeter fiberglass), and then put the bolt/nut in the center hole and tighten things up to keep the lexan from moving or falling out. I then do the same for the other side. Nothing really new or clever, just a quick and convenient method while I figure out how much airflow I can block off.

We're on the same page regarding radiator size. I'd like to eventually end up with a smaller/lighter radiator. The reduction in coolant volume/weight in a smaller radiator ought to be measurable.

My objective in all this is more front end grip via less weight and less aero lift. I have no realistic expectation of actually getting downforce from any of the mods on my car, and drag reduction is way down on my list. My experience with the car currently is that I've got enough motor and a light enough weight to end up with a vehicle that accelerates well, and has a pretty decent speed coming into the braking zones. I actually appreciate a fair amount of drag to help slow this car down. I've got enough rotor/caliper/pad capacity to reach lockup if intended. What I lack is tire width in the stock fenders.

[phantomjock]

Fender Louvers (Continued)

Found the missing pic(s). This from Katz Race Car Aerodynamics



Katz adds in the explanation of the figure:

The flow field near wheels enclosed by various types of body work can be very complicated. NO5h1T However one aspect of this interaction shown in Fig (see above), is fairly typical to most enclosed-wheel vehicles. Usually the tire drives the airflow between the wheel well in a manner indicated in the figure. This flow pattern can be used to channel cooling flow, or to move flow from the bottom of the car in order to generate additional downforce.

my emphasis


For 69427: Seems like cutting louvers in the fenders would give you less weight AND downforce for the front end grip you want!

And this from a wind tunnel study. Note the smoke stream (and fender louvers)!



Somewhere I have a smoke study pic like the Katz figure - but can't find it just now. It would be nice to see the smoke trail lower in the wheel well and show the stream exit through the vertical gap. Oh well, we'll have to trust it works...

Cheers - Jim

[phantomjock]

Sorry if I have latched on to this topic like a junkyard dog that just found a fresh bone, but this topic really interests me!

Here is a really integrated approach. Wheel house, cooling, under hood, and under fender pressure reduction. Let me introduce the Maserati MC12:


Sure it looks just like a C3... well, maybe not,



but now, look under the hood:



What a layout! I'm guessing dual radiators in the 2 carbon fiber airboxes, and what a wheel house! I'd really like to see some stream flow or CFD of this setup, and I'll bet they've reduced that internal cooling drag component to just the through flow of the radiator core itself. And, those fans will provide a downforce component.

Awesome.

I'll go build some carburetors now and be quiet for a bit.

Cheers - Jim

[69427]

Originally Posted by phantomjock

Fender Louvers (Continued)

Found the missing pic(s). This from Katz Race Car Aerodynamics



Katz adds in the explanation of the figure:

my emphasis


For 69427: Seems like cutting louvers in the fenders would give you less weight AND downforce for the front end grip you want!

................................

Cheers - Jim

I can't argue with the facts/physics of your statement. But as I mentioned earlier, I like the stock/sleeper look of my antique. My personal experience on track days is that my car gets little to no attention before the sessions (as it's just a stock looking old car), but I frequently get guys walking over to my parking area after a session to comment on how surprised they were that my car was able to "run with the pack" of later, much more expensive sportscars. I get a bit of personal satisfaction from that experience. If I start doing obvious body modifications that make my car look like a street legal Greenwood race car then it had better run like it looks. I just prefer that my car runs better than it looks. JMHO

Regarding the drawing above, I've been playing around with a somewhat similar phenomenon (tire rotation causing air rotation in the wheel well). If you use your imagination on the drawing, picture the windshield gone, and that this is the left rear tire (going forward obviously). I riveted in a splitter/plane from the frame to as near the tire as possible, in an effort to catch the air that gets thrown downward from the tire rotation (and hopefully causing a slight downward pressure on the splitter/plane). When time permits I intend to hook up my differential pressure gauge to see if/how-much of a pressure delta is created. I don't expect much, but it is essentially a free/unnoticeable modification.

[phantomjock]

Would really like to see the pressure results. On my list too.

Cheers - Jim