Aero mods and cooling

[the duke]

Post. I'll join in sometime later.

 

[Siestarider]

SS: Thinking about your splitter cooling question, if a splitter does produce front downforce, it has to create a higher pressure point above it than without it.

How big and shape of that high pressure "bubble" could be good for cooling. If the higher presssure area is "short" maybe it just affects IC.

Pressure could be "mapped" with manometer. But I am not confident of manometer readings in turbulent air such as outside car any distance at all from skin. I believe pitot tubes can do this, but someone with more manometer knowledge than I is needed to figure out how to map it correctly.

 

[15pbfist]

Wow a lot of info in this one glad someone is going off data not just I think [MENTION=1391]Siestarider[/MENTION] I had moved my battery to the trunk and it reset my computer every time I went to start the car back up it was kind of a bother and it also messed with the dyno it would cut out at diff rpms

 

[Siestarider]

Aero testing today on exterior of car, got some flexible vinyl hose that will attach to exterior paint with masking tape. Same conditions as before, all measurements at 80 mph.

Upper surface lower IC opening +3.0"
Center of bumper +4.0"
Top center bumper by Ford emblem -3.5"
Center hood +0.4"
Center hood 1" below wiper valence +0.4"
Center windshield above wipers +2.5"
Center roof 6" back from windshield -3.0"
Center roof -1.7"
Center roof 6" in front of antenna -1.0"
Upper center hatch "spoiler" -0.2"
Center trailing edge spoiler +0.3"

Also took three measurements on hood exterior where WRC rally vents are located, -1.7, -2.0 and -2.8"

Things that surprised me:
1. Our hood has positive pressure up center hump, and I assumed the hump was for style. Maybe not. Slight positive pressure negates lift at speed.
2. High negative pressures on hood surface and sides adjacent to rear half of headlights where WRC rally vents are, even though I found slight negative pressures beneath hood in this area (-0.2 to -0.3) in first report of results, the strong pressure differential between underside and top says those vent locations would pull air from engine bay.
3. Measurements down length of roof to hatch spoiler suggest the spoiler is not just for looks, it is helping reduce lift at speed.

Most surprising to me is the pressure differential between underside and exterior of hood where the WRC vents are. I am so stoked by these data, I want to cut holes and see what happens to the rest of the engine bay pressures. Theory says filling low pressure areas with ducted inflows improves aero.

But main thing remains finding more efficient ways to loose heat. These two low pressure areas look ideal. Delta 2" pressure gradient is tantalizing.

 

[LT Berzerker]

Awesome to hear about the venting

 

[Perfblue15]

Any idea if the rs style vents will do anything? I have a set I'm thinking about installing on my car. I just dont want to cut my hood if it won't help anything.

 

[Siestarider]

I started these tests with detailed mapping underside of hood, looking for locations to vent heated air out.

All 8 spots I tested on driver's side of underside of hood were similar and slightly negative pressure, so I looked deeper into engine bay and in front of radiator and down behind air dam. Learned some interesting stuff by leaving my original test plan.

Today was one set of tests down car centerline, just to get an overview. Already had a bunch of my assumptions trashed, good riddance.

For curiosity, three exterior tests where the WRC hood vents are. Mainly because vents on a race car are functional, and my measured engine bay pressures did not explain them. Now we know. Big negative pressures on the exterior of car in that location appear adequate to extract air from inside engine compartment.

I believe that the answer to where hood vents should go depends on what you want from them, and whats possible given the aero behavior. I want better heat extraction while tracking. But if you wanted to pull cool air into engine comparpment, these data suggest looking along hood centerline, where +0.4 exterior matches up with -0.3 underside hood.

I do not have enough information to start cutting holes in my car, much less advise anyone where holes should go.

I have a much greater appreciation of the car's aero behavior, especially with radiator sealed to cowl and air dam extension, from playing with manometer. Thinking ahead to Cyborg, even more heat to waste. Likely oil cooler is required. Maybe radiator too, but if I can find a way to use a smaller oil cooler and keep stock radiator by venting more heat, that is the way I would go. For that matter, if I could change only venting and gain enough cooling I would.

Things to keep in mind. SS mentioned a splitter, if you have one, it probably changes aero. My experimental air dam is 1" off pavement, that definitely changes aero. Soon as you cut holes in hood, aero inside engine bay will change. How? Probably be another surprise. So lets treat these data as clues, that is about all they are.

 

[Wimp Lo]

I think your notion of aerodynamics is a bit off.

I skimmed over the paper you linked in your first post and didn't see anything terribly exciting. One thing that I might have missed or the author thought was so obvious to omit it is a discussion of the ground effect. I don't recall anything about boundary layers either (but I just skimmed).

Let's start with a boundary layer. Air is a compressible fluid and although we don't think of it this way, it is a bit sticky. Imagine a place flat as Kansas but with a slight breeze. If you measure the wind velocity at various heights you will see that it approaches zero at ground level and past a certain height of many feet high, the velocity becomes fairly constant. The portion between the ground and constant velocity is a boundary layer where air is trying to stick to the ground but the wind above is dragging it along.

An example of using the boundary layer is a chimney. Bernoulli's principal states that fast moving gasses have lower pressure than slow moving gas so with a chimney that has openings in the low pressure boundary layer and up higher near the constant velocity region, the pressure differential pulls air up and out of the top of the chimney when it is windy.

Ground effect is what you get when you are flying in the boundary layer.

You have been looking at pressure differentials which is good but you need to think about pressure gradients and how you are accelerating the air.

If you look at a car without a boundary layer (flying), you have a mostly flat bottom with a sort of rounded top, kind of like a poorly designed airfoil. It will produce lift (not efficiently). The way we get down force in a car is to make sure the pressure under the car is less than on top. The main way cars try to reduce pressure under the body is to stop air from getting under there. The main way it gets in is through the engine compartment. The second way they make under the car a low pressure area is that they try to accelerate the air out through Venturi's or adding rake to the flat bottom. Your air dam reduced lift by reducing the amount of air that gets under the car but that doesn't explain why your coast down time increased.

Everything has a boundary layer including the bottom of your car so as you drive over the road you have between your car and the road a gradient of velocities approaching zero at the road surface and approaching the velocity of the car at its bottom surface. That creates a lot of shearing and acceleration in the air under the car and that makes drag. The same fix for lift work to fix drag too, you get the air out from under the car. This effect is what increased your coast down time.

Now when we thing about velocity gradients, we also need to think about pressure gradients. When a car is driving and hits an air molecule, it has to accelerate to get around it and the force that causes that acceleration is measured as pressure. When your air dam scrapes the boundary layer air off the ground, it creates a pressure gradient ahead of the dam by a number of inches. That means there is going to be high pressure air under the nose. That's not so good for lift.

What a splitter does is reach out hopefully beyond most of the pressure gradient to have that air pressure pushing down on the top of the splitter and also make the air travel up and over or around the car rather than under it.

Now, how much air do you want going through the engine compartment? Only as much as you need to cool the engine. Race cars often tape up their front end to reduce drag due to air flow through the radiator. I believe the improvement comes from less air under the car more than anything else.

I think WRC style vents should improve cooling and possibly drag. A suitably long splitter will do the same and increase front end grip without penalty.

With WRC style vents and a splitter that extends back in to a partial belly tray might be able to force more air out the WRC vents and allow less under the car making it even better.

With a splitter you are making a large high pressure zone near the grill opening, its far easier to duct that air wherever you need it than add a scoop to the hood and those RS style hood vents seem less than ideal when the car is in motion.

 

[Chuckable]

Wish I hadn't dropped out of the engineering program back in college

I don't believe that anyone is manufacturing an extended belly plan for our car, but I know there is at least 1 skid plate available. Not sure if that would be very effective, though, given that it's meant to protect the drivetrain rather than for an aerodynamic effect.

RAAMAudio was working on something custom if I remember correctly?

 

[Wimp Lo]

Luckily a splitter and tray are not terribly complicated.

I'm thinking about copying RAAMAudio's setup and maybe extending it a little but the WRC vents are what I really want.

 

[Siestarider]

Anyone who has tried to land a plane, or paid attention while riding in one as plane lands is familiar with ground effect. I really enjoy fluid dynamics and theory, but it is a branch of science that is based mostly on empirical findings, and even those tenets become less useful in turbulent fluids. I propose we can all agree that air inside engine bay, and air leaving engine bay to whatever lower pressure area is available, is turbulent.

The paper I linked to is the best I could find on aero effects on car engine and engine bay cooling. Nothing to do with ground effects in their test equipment or modeling. Really nothing to do with anything but cooling, ducting and aero efficiency, which is the topic of interest this thread is directed toward.

Wimp's conclusion that extending the air dam effectively reduces the air flowing beneath the car seems plausible. My spot pressure measurements so far suggest that one obvious effect of "getting the air out from under" is reduced air pressure, ie negative pressures beneath the engine bay, highest behind air dam, but negative everywhere I tested.

I assume that "preventing air flow" beneath the car means better cooling from heat exchangers in the nose, as we all know, nature abhors a vacuum. So more cool air should enter nose and exit as warmed air beneath car. The technical paper referenced seems to me to conclude that in general the further back warmed air can be exhausted beneath car, the better.

Finding negative pressures on upper side of hood behind headlights suggests to me the WRC vents are in the right place to vent engine bay heat. However, just because there is negative pressure vs ambient at those locations does not also mean they will flow lots of air out of engine bay. That is where pressure testing on skin of car fails to inform us adequately. The size and shape of that low pressure area will determine cooling capacity available from vents, and I have no idea how to measure that part. Probably what wind tunnels are for.

My interpretation of the coast down effects with air dam experiment is that the effective Cd of the car was decreased, at least under the 90-45 mph test, in spite of added frontal area. Simple, that is the only variable that could have changed.

Lets cut to the chase. We have substantial evidence that properly located hood vents can improve cooling. Whoever cuts their hood up first to prove or disprove it wins.

Something. Whatever pioneers get, gold or arrows in the back.

 

[Wimp Lo]

Just think about this: From the side view, your air dam could be simplified to a short vertical line that neglecting the wheels lowers the bottom of the cross section by an inch or two. What would happen if you added a similar short vertical line at the car's roof to raise the top of the cross section by an inch or two?

Without the ground effect, that increase in the frontal area is going to hurt your cD and then multiply it by the area increase, instead of very good, it is very bad for your aerodynamics.

Back to that paper, it has a simplified representation of an engine bay but it doesn't look like they simulated the ground effect for air exiting the bottom of the engine bay. That is probably valid for a tall SUV or truck but with a low car that is exploiting the ground effect, you need to limit the amount of engine bay air that exits underneath the car.

There is a good book to read that covers most of this: Race Car Vehicle Dynamics by Milliken and published by the SAE press. It is kind of expensive but worth it.

 

[Siestarider]

Back to the airplane landing, ground effect is evident before the lower side of a wing reaches the height of an SUV. Yes, a wing is not a car, but even slow airplanes land 80 mph or more.

One could argue that without ground effect, the low pressure zone beneath any car or truck at speed would not exist, therefore dumping cooling air down and back from engine bay would not work.

The extended air dam enhances this effect, and results in a "slipperier" car. I suspect closing the front engine bay bottom opening will slightly enhance this effect. RAAM has slready done it, plus experimented with some other aero refinements, but without data its hard to say how well it works.

Venting some engine bay air at rear sides of hood should, as I understand it, add some engine bay cooling and provide some additional reduction in effective Cd. In my opinion, aero theory can not predict how much these mods will change the overall behavior of the car and cooling.

It would be fun to use sensors to measure lift and downforce at wheels with speed at the same time, not all that tough to do, but my contribution was buying and using a manometer to get interested parties this far.

Someone has to do it and measure it.

 

[Wimp Lo]

Yes ground effect happens at quite a height with an airplane but the wingspan is huge and the distance that you can see obvious ground effects varies relatively to the size of the aerodynamic surface.

Also, with an SUV, it is still operating in the ground effect region, it just has no way to take advantage of it because it is either too high or too narrow to create a low pressure area underneath.

Simply put, the more air you can duct to somewhere other than under the car becomes free down force and a drag reduction if the car is low enough.

 

[RAAMaudio]

Fun stuff here, keep it up

Though I had not data from them I had a C6 World Challenge race body made for a C5 race car I bought to build into a NASA ST1 car back in 07. It was stock looking with just minor tweaks to flow air better and they still spent 200+ hours in a wind tunnel perfecting it. Approx 50 hours on the wing, 50 on the roof line, 50 on the undertray which was the coolest part and most expensive. It had two curved massive inverted NACA ducts that directed air out towards the front wheel wells. The rest of the body had the remaining time in the tunnel, all rounded off numbers per area.

It cost me $15k and looked like a cheap pile of sheat for that kind of money, luckily I did not buy the $25k CF version to save 20 more lbs! I called a race shop manager buddy to ask him if they were that bad and he said always, 100 hours shop time minimum to clean them up, fit better, paint....

-------------

Best part was I figured out how to make such an undertray for other cars and it did help but did not do it on this car as not much room to do it well though I might try again once I have a house and shop sometime in the future.

 

[Wimp Lo]

I'm thinking about buying a Triple R splitter to copy the bolt pattern on and the cut up and graft the side pieces on to a piece of Alumilite to make a splitter in any shape I choose and easy to repair if I crunch it. If I add an air dam, it will be at the front of the splitter.

I'm hoping that there might be a WRC or GRC Fiesta at the local auto show coming up, if there is I will beg to get a tracing of the hood vents and the profile of the edge.

I don't have a wind tunnel but I bet whoever designed the vents did.

I am also kind of interested in the aerodynamics of the Boomba wing risers because I have heard it keeps the hatch cleaner. If I can do that without screwing up the cD, I like a clean hatch.

 

[RAAMaudio]

A splitter is far more effective if it includes an under tray, even half depth one instead of full depth like I built.

The TR splitter has a down turned lip all around the bottom side, I sanded off the back and the Alumalite fit right in well behind the front edge.

If you want a longer one you could just sand off the front as well and make the Alumalite stick out further.

I understand your thoughts about a vertical damn on the front of the splitter yet not really sure how well it would work out which is partially due to constant bottoming out and if soft enough to take some abuse it would also fold back at higher speeds. I have built a few of those but once real splitters and undertrays became well known that is what I have chosen to do and with great results.

Very good idea on the hood vents, let us know what you come up with

As for the spoiler risers(there is no wing on the FiST) nobody has done any testing, even yarn and a video camera, that I know of, keeping the window cleaner does not mean a good aero design. If you want to improve aero you would want to cover the top of the stock unit to smooth it out and seal the holes and then add a bit of a Guerney flap on the trailing edge and still this would not gain much but better than losing some or guessing at it. To really get decent aero you will need something far more aggressive, why I have a none ST spoiler to look into making some sort of quick release mounts to use a real wing on track or even use the stock FiST spoiler.



I did not do so as it bottoms out far too easily just stopping at the end of the TR unit but I have been considering ways to extend it for the track with quick release mounts and aluminum sheet as stiff and low weight. I would just attach it to the bottom of what I now have when on track and take it off for the street.

 

[Wimp Lo]

I like the idea of modifying the Triple R to hold an extended splitter but I think the top surface becomes redundant if a stiff material is used to extend it.

What I don't have is a template for the holes which the Triple R could be before I cut it up, then I'll bolt the pieces together and give it a vinyl wrap to make it pretty...

The air dam would be track only and the whole splitter and dam would be maybe $50 in materials to start over if I crunch one.

 

[RAAMaudio]

I love Alumalite for making many parts, cheap, stiff, very low weight, easy to work with.....

What I like about using the TR unit as I have or if extended is that it makes for a very solid mount to the car if done right and very little extra weight. I do understand being able to make cheaper replacements, I was going to buy a second unit and mod it to have a spare with me. Even though they cost a bit I have paid many many times more for CF units in the past and never will again.

I have extra screws into the lower part of the stock bumper cover and made aluminum mounts to the lower corners of the radiator support with two more screws per side and a rear mount/2 point brace/inverted lip(Guerney flap in effect) I made for free from scrap 1" sq tubing so my setup is very solid.

A cool thing about doing it this way the gap in the middle of the stock cover is open in front, sealed on the bottom and back that has a curved towards the tire tunnel to tap into for cooling the brakes.

If you build a full length undertray you will want to add two small ducts in just before the engine and transaxle or as I did one larger one to draw air up into the bay to create more flow out the rear to help keep the engine, etc, cooler.

My reasoning for the ducts on the undertray, how to mod the stock spoiler or far better to use a real wing as well as only needing a two point subframe brace are from inputs by the Ford Race Shop Manager at MMP(if still open, Ford may of closed it up with the closing of the track) a professional with a very high level race car experience.

 

[RAAMaudio]

I am going to use Alumalite to build a new rear floor and cargo wall, tire well cover, etc...

I have seen it used for quite a bit of rear lower aero work and I have made hundreds of filler panels for access holes in doors and speaker baffles, mostly for C5 and C7 Vettes.