Diffusers and belly pans

[blueovalz]

If indeed it does work well, then perhaps with some proper underbody work and panels, I may be able to make a diffuser incorporating the rear underbelly tray on the BlueOvalZ as such:

 

[Drax240z]

Geeze Terry, don't tell me you are going to rip it all apart again and start fiberglassing!

[blueovalz]

Naw, that won't be happening. But something like this can be made separately from the car and attached later. The only issue I would have is that I don't want to put something on the car that represents something it's not (such as, the appearance of a working diffuser when it doesn't really do anything)

[johnc]

Turbulent air is always low pressure. That's why wheel wells are low pressure and also why diverters are added in fron diffusers.

[tannji]

What I wonder is... why must the air travelling on both sides of the wing travel the span of the wing in the same amount of time?

 

Imagine the car in a wind tunnel instead of moving on a track. The car is staionary, and moving air is pushed past the wing by the tunnel's impellers. All the air is essentially moving at the same speed, with the same momentum, and the same enertia. The reason the air "must" travel around the wing in the same amount of time is that that the air has enertia, it is already moving that fast, and therefore will exert force to try to continue moving that fast. The forces apparently manifest as vacqum and pressure, depending on where you look on the wing.

[Drax240z]

I'm not sure I buy that... you are saying that 2 objects of similar mass & inertia (air molecules) are travelling different paths, with the same initial energy and starting in exactly the same position. Because KE=1/2*M*V^2 you are saying that the one that takes the longer path MUST be at a higher velocity (and therefore lower pressure due to the ideal gas law) in order to arrive at the end of the wing chord at the SAME time as the other particle.

 

Yes this all makes sense to me. However, I don't see what is making the definition that both particles MUST arrive at the end of the chord at the same time, why wouldn't the one travelling the longer path experience a greater loss of energy due to friction, and arrive later than the one that had the shorter path?

[johnc]

I don't see what is making the definition that both particles MUST arrive at the end of the chord at the same time, why wouldn't the one travelling the longer path experience a greater loss of energy due to friction, and arrive later than the one that had the shorter path?

 

I think looking at this assuming the air is moving is wrong and also thinking of the air as a bunch of separate molecules is wrong. In reality, air is a compressable fluid (for purposes of this discussion) and the wing is moving through it. It takes energy to separate the fluid, with the fluid above the wing being compressed, and that energy comes from the moving wing and is referred to as drag. The fluid is not loosing energy, it is gaining energy from the movement of the wing.

 

http://naca.larc.nasa.gov/reports/1932/naca-report-383/

http://aerodyn.org/Drag/speed-drag.html

[Drax240z]

Ah John, a good point in that this shouldn't necessarily be considered on a molecular level, as the interaction with the environment is not complete when just considering the path of 1 molecule, the interaction of that molecule with the surrounding ones has great effect as well.

 

Naca report 383 is very interesting in regards to optimal angles of attack but I'm not seeing it's relevance to the discussion... Unless somehow I've missed something in the first 8 pages.

 

Hrm... still thinking a bit on this one. Considering the wing & fluid as a system does seem to indicate that there is energy being transferred from the wing to the fluid. (in the form of a higher velocity) However, I would say the counterpoint is that the fluid is transferring energy to the wing as well in the form of heat. (due to friction) I suspect the actual magnitude of energy transferred is much greater in the wing->fluid transfer though.

 

Anyway, a properly designed diffuser will add downforce to the rear of a car. (isn't that the topic?)

[johnc]

383 was the earliest report I could find that had some numbers to calcualte drag and thus determine energy values. When you do a search on the NASA Tech Reports Server and enter "wing" and "drag" you get back thousands of results.

 

I think pretty much 100% of the "give back" to the wing from the air is drag. Heat becomes an issue at higher velocities but at the extremely low speeds (relative to flight) we're talking about I don't think thermal changes can be measured.

[Drax240z]

Yeah it's not like we are talking about Thrust SSC here. I'd be curious to know what kind of effects the engineers of an F1 team would expect due to frictional heating of their airfoils however. I would imagine that their local air velocities can be quite high compared with their actual speeds.

[Owen]

Hey Douglass,

I was wondering if you could elaborate more on your comments?

 

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Owen,

Strictly speeking the diffuser should work on a totally sealed under pan so that no air can get above it....

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This is what I was thinking too, hence all my questions, but I think it was John who said that the air above the diffuser (in between the gas tank, etc. and the diffuser) was necessary to create downforce.

I'm no engineer, but wouldn't downforce at the rear be created by upper airflow (above car, over rear spoiler) and lower airflow (below pan and diffuser)? And not or less by "middle airflow" (air right above the diffuser) and lower airflow?

 

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If you dont have end plate the the low pressure developed by the upturn will suck in air from the sides making the diffuser redundant. I have used a similar aproach on my Z

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What end plates are you referring to? The vertical fins of the diffuser?

 

thanks,

Owen

[Dead Roman]

This is what I was thinking too' date=' hence all my questions, but I think it was John who said that the air above the diffuser (in between the gas tank, etc. and the diffuser) was necessary to create downforce.

I'm no engineer, but wouldn't downforce at the rear be created by upper airflow (above car, over rear spoiler) and lower airflow (below pan and diffuser)? And not or less by "middle airflow" (air right above the diffuser) and lower airflow?

 

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If you dont have end plate the the low pressure developed by the upturn will suck in air from the sides making the diffuser redundant. I have used a similar aproach on my Z

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What end plates are you referring to? The vertical fins of the diffuser?

 

thanks,

Owen[/quote']

 

 

To the best of my limited knowldge. the air flowing in erratic paths under the car is the problem. your going to have pockets of high and low pressure with an open undercarriage, Due to the exhaust,suspension..etc. The reason you would have a belly pan under the car would be to give the flowing air a direct path into, and out from under the car. This is going to creat a constistant pressure uner the car at any given speed. If you look under an f1 or indy car they have varios ducts and paths for the air to follow. for the underbody diffuser to work you would have to have a constant and smoothe airflow ofe both sufaces of your diffuser(im just gonna say airfoil from now on). If you have turbulent air on one side and free flowing air on the opposing side of an airfoil, you get very erratic performance. I cant see a bellypan such as Z8Red's(no offense man i think its sweet) actually creating any noticeable downforce unless somehow what we see in the pic is actually the exit of a duct, In which case the undercarraige of the car would be closed in from all airflow.

 

if i said anythign that sounds stupid tell me it wont hurt my feelings, and i hope im not repeating stuff that has already been said.

[Z-TARD]

When I look at the math aspect of all this, I just here popping noises from deep inside my brain cavity, but it seems to me that the caveman/rock-paper-scissors explanation of how this might work is this:

 

1) Belly pan smoothes undercar airflow (I think the important part here is to smooth the aiflow without doind anything to increase the air pressure under the car such as sudden reductions in ground clearance. Venting all radiator cooling air over the top of the hood would help here as well)

 

2)As the belly pan nears the rear of the vehicle, it slopes upwards at a slight angle (7 to 9 degrees IIRC). This causes the air under the car to increase volume to fill the extra space created by the extra ground clearance, resulting in low pressure under the car.

 

3) Side plates keep the highr pressure air on the outside of the car from rushing in to fill the low pressure area created by the belly pan/ diffuser.

 

On a Z without a rear spoiler, the rear diffuser might even help reduce the amount of lift created by the downward slope of the rear hatch. Imagine the airflow from the rear of the vehicle being pulled down slightly from the low pressure air exiting from under the rear of the vehicle, in effect reducing the amount of "pull" exerted by the air over the hatch. Not sure waht kind of effect it would have on a car with a spoiler, due to the greater degree of separation between over the car/under the car airflow.

 

So who's gonna be the first to tie hundreds of 4" lengths of yarn to their car and video tape airflow at freeway speeds?

 

Mike

[ZROSSA]

Owen,

If you can get air from underneath the car on top of the diffuser then its no longer a diffuser, it becomes a wing working in a very turbulant position. Im not really sure what John C. is sugesting myself. Everything I have read and studied suggests otherwise. By end plates I do mean the vertical section of the diffuser. To make these things work we really need to get the cars much lower then possable for the street.

 

Douglas

[johnc]

Im not really sure what John C. is sugesting myself.

 

If you look at a lot of diffuser installs on full bodied cars, they are not completely integrated with the floor of the vehicle and they stick out behind the vehicle some amount. The high pressure air that's creating the downforce is the air just above the diffuser itself. I know its a subtle distinction but its important to keep it in mind to ensure you're not creating another low pressure area just above the diffuser.

[Guest bastaad525]

Ah... slightly OT here maybe, but after reading thru here and some other threads on bellypans and such, one thing that doesn't really get hit on... what is the most effective way to increase airflow thru the radiator on a 240z? I have the odd problem that my car can run all day in stop and go traffic and the temp sits right in the middle, even on the hottest days, but get on the freeway and get over 60mph, and temps climb 20-30 degrees or more. I figure it HAS to be an airflow problem, as I've addressed everything else. I'm running one of MSA's front air dams, and I do have an FMIC in front of the radiator, though the problem existed before I installed it.

[johnc]

My 810 does the same thing temp wise and a 1972 240Z I had also showed the same behavior. Your engine is operating under a load so its generating more BTUs. Unless its a cool day you will most likely see a temp increase in the gauge while driving with the engine at 2,500 rpm.

 

But, just in case... Make sure ALL the air entering the grill is going through the radiator. Seal any gaps between the radiator and the core support. Even gaps as small as 1/8" will allow a lot of air to bypass the radiator - especially gaps toward the bottom. Also, try adding some kind of seal on top of the core support that will seal it to the hood.

[ZR8ED]

Ok OK

 

I'll be the guinea pig here. I'm doing some exhaust work still, but I'll get out the yarn and the video camera as well... it'll be fun!

 

I'm surprised how technical this discussion has gotten. There are some good ideas.

 

Anyone else built one? Anyone tested one? Anyone have one on their Z? I should drop Tony D an email and get his thoughts.. Tony? are you out there lurking?

 

I'll let you know how the yard thing works... it is something I've always wanted to test.. Too bad I never did a before? Any chance someone here would like to video their car for comparisons? Stock?, Stock with wing?, wild?, wild with wing?

 

It wouldn't take to long..

 

Scott.

[Guest bastaad525]

My 810 does the same thing temp wise and a 1972 240Z I had also showed the same behavior. Your engine is operating under a load so its generating more BTUs. Unless its a cool day you will most likely see a temp increase in the gauge while driving with the engine at 2' date='500 rpm.

 

But, just in case... Make sure ALL the air entering the grill is going through the radiator. Seal any gaps between the radiator and the core support. Even gaps as small as 1/8" will allow a lot of air to bypass the radiator - especially gaps toward the bottom. Also, try adding some kind of seal on top of the core support that will seal it to the hood.[/quote']

 

Funny thing is it's definately not RPM dependant, but I don't fully understand how the load differs at different road speeds vs. different RPMS. For instance, on the street I often cruise at around 50-55mph , in 4th gear, at around 3200rpm, and the temps will stay right in the middle. On the freeway, I'm usually at about 75mph, but at the same rpm, but running 20-30+ degrees hotter. I had also seen some of the suggestions you mention before, but figured with the openings for the I/C piping and air filter out front, this might be a futile gesture. Here's an observation though... before I had the I/C, the two holes in the radiator support were wide open. Now with the silcone elbows going thru them, they are blocked off, not 100%, but still there is much less airflow able to pass thru there. For what it's worth, temps did not decrease at all due to this. My I/C is not very tall, and only blocks about half of the radiator, the bottom half is still free of obstruction. I'll try to seal up what holes I can in the rad support... I may just stuff them with plastic bags or something just to test if blocking the airflow there helps with temps at all. I do like the idea of laying some kind of strip over the top of the rad support to seal it against the hood, and I"ll take a better look along the bottom as well and see what can be sealed up.

 

So I dont need to think about creating any kind of belly pan, splitter or diffuser to help with this?

[johnc]

but I don't fully understand how the load differs at different road speeds vs. different RPMS.

 

Really? We're in this huge thread about aerodynamics and you don't understand how engine load differs based on road speed? I know you're smarter then that!

 

So I dont need to think about creating any kind of belly pan, splitter or diffuser to help with this?

 

Start with the basics.