"Tunnels" on Top of Car! (Cadillac)

[Tom and Vipers]

\"Tunnels\" on Top of Car! (Cadillac)

This view of the Cad is just plain ****. Talk about not letting any of the air on top of the body spill over the sides!

It's almost like there is an endwall along the entire length of the car.

This is a great picture for end walls. The front ramps, as then converge and the flow piles up, the endwalls get higher.

And then there is the "hidden" huge front wing that is just below the body and not readily seen in this photo.

No doubt, aero is what intrigues me about race car design. HP is a detail.

 

[jrkermode]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom you're not well. I know. I'm suffering from the same thing! Perhaps there's a support group? Of course, if you don't yet have Hoerner's books and a subscription to TV Motorsports( http://www.insideracingtechnology.com/ ), you may yet be saved.


All that Monkey business is just another example of how ground effects dictates everything in the upper levels of motorsport.

Those bulges, troughs and fences are there to feed that rear wing. The wing, in turn, is there to "extend" the rear diffuser.

Notice how low the wing is. At first glance that would appear to be a stupid place to put it, in all that dirty air, especially as the rules allow putting the wing up in clean air. However, the rules restrict how far the rear diffuser can extend beyond the rear axle line. The wing is allowed to extend back further. So, by placing the wing way down there in the back, its helping to extend the effect of the rear diffuser. If you're familiar with fixed wing aircraft, imagine the diffuser being like the main wing and the rear wing like an enormous flap.

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

You've got to be right on with that rear wing. It is LOW and it is AFT.

What I see is that there is a dorsal tunnel and "diffuser." The tunnels go around the driver then start ramping down while the end wall heights increase. Then the tunnels go inboard and join between the rear wheels. And then dump just below the rear wing. There should be a downward component in this dump at the end of the body where all of a sudden there is a wing surface which now reacts this downward component, i.e. suction.

Of course there is a ventral diffuser which is imparting an upward component to the belly flow, i.e. suction on the bottom aft of the rear axle.

I'm not sure how these 2 effects combine - independantly they make sence, however, I wonder if they compromise each other? Probably, there is enough room for both children to play here!

I did read in the article that the bottom of these cars has to be FLAT BETWEEN the axles - so there is a rear diffuser and a front wing. (The front wing is another story.)

I just can't get over those tunnels! They look like the Panama Canal. There should be a mill with a water wheel in there somewhere! ...or kids riding a log down the water trough.

Once I got the hang of what to do with the stagnation bow wave, end walls were the order of business. So how do you keep the air on top of the car on top of the car? How about an endwall that runs the entire length of the car? Sort of like make the top of the car a 3-sided box (no top.)

Well, that's what I see in this Cadillac. (and I thought I was crazy.)

I am a mechanical engineer and did 15 years at Pratt & Whitney and am fairly bored of stress and vibration analysis, however, I could get real motivated working on race car aerodynamics. Might as well dream about working on Ferrari F1 aero. (If you gonna' dream, DREAM BIG!)

 

[GTS Dean]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom,

That's a *very* interesting view of the car. What I glean from it is that they're not only trying to work the ground effects under the car, they are controlling pressure gradients on top of the bodywork. Most sportscars I've studied over the last several years have pretty much flat bottoms. All the sculpting occurs around the engine-transaxle diffuser.

Those Cadillac contours are reminiscent of the late '80s to early '90s IndyCar underwing tunnels. It's just that they're on top. Perhaps they are using FlowTran to model & play with the boundary layer effects on a scale that includes flow as much as 12-18 inches above the bodywork.

All the old "P-cars" moved the rear wings well clear of the bodywork so that they would be in cleaner air.

Do you subscribe to Racecar Engineering? That's a superb publication!

 

[CAS]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom and Dean,

I really, really want to understand what you two are talking about, because aero fascinates me as well. Could you put it in more laymens (sp?)-terms, but refrain from dumbing it down. Thanks

.

Clint

 

[GTS Dean]

Re: \"Tunnels\" on Top of Car! (Cadillac)

In simplest terms, the airflow speeds up and slows down relative to the car's velocity at different points all around, over and under the bodywork. Like the venturi on a carburetor, when the air velocity speeds up, the pressure drops suction (or downforce) is created. Careful design of the bodywork contours can dramatically influence the pressure distributions acting on the car.

 

[WCKDVPR]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom, Clint, et al,

Get this book!
"Race Car Aerodynamics: Designing for Speed", by Joseph Katz
$25 from Amazon

It covers all aspects of aerodynamics at both the technical and laymans level and uses real cars (open wheel, GTU, GTP, production, etc) for examples.

Trust me, one read of this book and you will never again look the same way at some of those "high downforce" wings that people claim to have/sell.

FYI, jrkermode did a lot of this aerodynamic research for my - as he likes to call it - "street car". He used to work for the race car division of Mazda North American and is one smart engineer. Listen to what he has to say.

Jim - It IS a street car! Now, about that support group, I hope it is free because I spent all my money on aero stuff.....

Regards,

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

You can learn a lot about aero and wing flutter just by playing with your hand out the window of a moving car - we all have lots of windtunnel testing already under our belts - but this was when we were kids and we have forgotten most of it.

Regarding flutter:

Flutter occurs when the phase between the translation and rotation of an airfoil is such that work is done on the airfoil.

Do the hand out the window trick and do the up and down snake thing. If you don't hold your arm, your hand will have your arm flapping up and down like crazy.

This is flutter. You solve it by making your arm (i.e., wing) stiffer.

You can "flip the phase" of your arm and now you are "pushing" the air with your hand - this is how your kick works in swimming.

Regarding aero loads:

Air is a fluid that is compressible and viscous.

The primary concept in all of aero, the way I see it is stagnation pressure. This is the pressure that builds up on a surface normal to a flow. The resulting force is the highest that can be obtained from a flow. In fact, it is (rho)(vel)(vel)/2 where rho is the density of the flow and vel is it's velocity.

This is why the orientation of a pressure probe is important. A pitot tube is oriented to measure the stagnation pressure. A probe oriented 90 deg from that or perpendicular to the flow measures the static pressure, that is, the pressure w/o flow velocity effects.

How here is the BIG DEAL. Where ever you build up stagnation pressure, like the vertical front end of a car, that high pressure spills in all directions which are at a lower pressure.

Other Visualizations:

If you ever diverted a small stream in the woods, you know how you have to dam it up and turn it. Same principals hold for air flow.

And don't forget, if you have flow in a pipe thru an elbow, there is a constant reaction at that elbow when there is flow velocity. If you draw a control volumn around the elbow, you have to introduce a resultant force to balance the momentum or the 2 different flow directions.

Also, regarding this elbow reaction. When a valve is closed in a large pipeline which is flowing water or oil. The closing of the valve is done very slow (of the order of hours) because as the flow is reduced, the entire mass of the fluid in the pipe is being decelerated and as we know F=MA.

There have been cases where the valve control failed and the valve snapped shut and miles of pipe whipped out of the ground do the inertia of the fluid.

Anyhow, I hope I've given some examples that may help to visualize this stuff.

I never paid any attention to race car aero until about maybe 6 mos ago and frankly, it makes airplanes look like kites. Air plane aero is very simple whereas race car aero is very complicated with lots more features and effects.

 

[jrkermode]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom,

Hate to confuse you more, but, for cars, they almost never care about stagnation pressure as you have described it. Yes, pressure acts on the front of the car, but by rounding the corners you create low pressure areas acting on the corners of the nose. These low pressure areas pull the car forward in an equal amount to what the stagnation pressure pushes it back. Therefore the drag that acts on a car is just the effort of pushing the air aside and that big bubble of low pressure being dragged along by the car (base pressure). The take home; if you want low drag, disturb the air as little as possible as you pass (streamline everything) and have the tail end as tapered as you can to minimize the base pressure.

Buy the book, you'll really enjoy it.

 

[CAS]

Re: \"Tunnels\" on Top of Car! (Cadillac)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jrkermode:

These low pressure areas pull the car forward in an equal amount to what the stagnation pressure pushes it back.<HR></BLOCKQUOTE>

Using this logic, wouldn't the cars Cd be .00 since the two forces are equal?

Clint

 

[jrkermode]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Clint,

That's it exactly. The net drag (pressure) due to the air on the nose of the car is negligible. The net pressure at the nose may even be pulling the car forward, a "nose Cd" &lt; 0!

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Lift and Drag Formulas are all of the form

(SP)(A)(C)

SP = Stagnation Pressure = Rho*V*V/2
A = Area
C = Life or drag coefficient

So I would argue that every aspect of aerodynamics hinges on stagnation pressure.

I look at it like this: Aero load is some fraction of stagnation load where stagnation load is the maximum possible aero load.

The phenomenon which generates stagnation load is the same as that which produces an aero load.

Any time I see endwalls on a 1-sided flow surface, I "see" stagnation pressure.

I am dying to get the book of course!

 

[jrkermode]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom,

You're making the mistake of trying to bring "aeronautics" to bear on vehicles. The two cases are very different. If I recall correctly, the most aerodynamically advanced automobile, the Honda Insight, has a Cd similar to a 1920's aircraft, the DC3. Another way of thinking about it; a 450 HP Viper and 160 HP Cessna have similar top speeds. That's why I say drag, not frontal pressure is the "thing" with cars.

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

I don't know, I would think that downforce is the issue with cars with drag a consequence of downforce.

And of course, downforce, is aero load or pressure. While leads me back to aero pressure of which the special 90 deg case is stagnation pressure.

Again, anytime you need endwalls, you are trying to "pool" a "stagnation" area.

...I gotta' get that book......

Tom

PS I hope you see the Porsche, Victory by Design on Speedvision at 2:30 AM tomorrow morning

 

[WCKDVPR]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom,

There is no excuse for not having purchased and read that book by now. Order online, next day air, it is a 1 - 2 day read and you are back on this site in 3 - 4 days with enlightened words of wisdom. How can you pass this opportunity up?

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Ah WCKDVPR

You are my aero conscience!

However, I am busier than a one-armed paper hanger!

I'm in the midst of a modified supercharged engine build for my Lark AND "project managing" the rebuild of Bad Vipers suspension and drive train in NV while I'm in PA.

Not to mention I haven't done any house work in years...

Tom and Dust Bunnies...

 

[CAP]

Re: \"Tunnels\" on Top of Car! (Cadillac)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Tom and Vipers:
There have been cases where the valve control failed and the valve snapped shut and miles of pipe whipped out of the ground do the inertia of the fluid.
<HR></BLOCKQUOTE>

A tiny little point, but inertia is the resistance to acceleration (and or deceleration) - which is somewhat relevant but....

Momentum, a vector consisting of a direction and the product of mass and velocity, describes more accurately the forces at work causing the pipe to pile up when a control valve is closed rapidly under flow conditions. Same as "water hammer".

Hope this isn't too fine a point.

 

[WCKDVPR]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Tom,

In the time it took you to post your excuses, you could have ordered the book!

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Thank you sir(s), may I have another?

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Craig,

Maybe you can explain this.

I have heard physicists say that when you get a finer understanding of energy, you think of kinetic energy, (mv**2)/2 as the "the time rate of change of linear momentum"

d(mv)/dt = (mv**2)/2

What perplexes me about this is that the concepts of potential and kinetic energy seem "clear" to me, however, I have no concept of what linear momentum actually is.

Springs, harmonic oscillators, and escape velocities really lend themselves to these energy concepts, however, conservation of linear momentum for flow thru a control volume, collisions, and such, while mathematically simple, escapes me.

I'll throw Tensors in with that too.

Tom

 

[CAP]

Re: \"Tunnels\" on Top of Car! (Cadillac)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Tom and Vipers:
Craig,

Maybe you can explain this.

I have heard physicists say that when you get a finer understanding of energy, you think of kinetic energy, (mv**2)/2 as the "the time rate of change of linear momentum"

d(mv)/dt = (mv**2)/2

What perplexes me about this is that the concepts of potential and kinetic energy seem "clear" to me, however, I have no concept of what linear momentum actually is.

Springs, harmonic oscillators, and escape velocities really lend themselves to these energy concepts, however, conservation of linear momentum for flow thru a control volume, collisions, and such, while mathematically simple, escapes me.

I'll throw Tensors in with that too.

Tom

<HR></BLOCKQUOTE>

1) Never heard that time rate of change explanation before and it doesn't make much sense to me based on what I know about momentum and KE, and wouldn't the expression d(mv)/dt = m dv/dt = ma = F ?

2) Below is an exercise to mentally capture the 1/2mv^2 vs. mv difference. (I c and p'd from the internet.)

Suppose that you were captured by an evil physicist who gave you the following choice:

You must either:

Stand in front of a 1000 kg. truck moving at 1 m/s, or
Stand in front of a 1 kg. meatball moving at 1000 m/s.
What's your choice?

Hopefully, you picked the truck! It's a big truck, but it is moving rather slowly (about walking speed), so assuming you don't fall down when it hits you (That would be bad...) the truck is just going to bump into you and move you out of the way.

On the other hand, you probably suspect intuitively that the meatball is a very dangerous object. It isn't that massive, but it is moving very fast (about 10 football fields per second) - and when it hit you it would do considerable damage to you, and keep going!

Consider the momentum and kinetic energy of the truck and the meatball:

Truck:

Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s
Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules

Meatball:

Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s
Meatball's kinetic energy = 0.5 mv2 = (0.5)(1 kg)(1000 m/s)2 = 500 000 Joules

We know intuitively that the meatball is more dangerous than the truck, yet the momenta of the truck and the meatball are the same. On the other hand, the meatball has 1 000 times the kinetic energy of the truck!

Clearly, momentum and kinetic energy tell different things about an object!

3) KE is a scalar and can be added in the manner of regular numbers. Momentum is a vector and is additive in the same manner as forces are.

4) The fact that both KE and momentum are conserved can enable calculations of system properties from physical measurements. (Remember the ballistic pendulum from physics? You use the conservative relationship of KE and momentum to determine the velocity of the bullet shot into wooden block which swings to a measurable height.)

5) The two physical descriptors, momentum and KE go hand in hand when assessing the total physical characteristics of a system.

6) Your basic question: What is linear momentum. It's a vector with a magnitude of mv. That's all. It's significant because it is a conserved property and is additive/subtractive using vector mathematics.

I'm not sure any of this clears anything up, but I tried.

 

[GTS Dean]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Now that Caddy is officially a "has been" from LeMans competition - I recommend this month's Racecar Engineering mag (which I just received today). It has a multi-page article on CFD development of the Audi R8. Cool stuff.

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

Yes, Force is the Rate of Change of Momentum.

I got my wires crossed,

d (1/2 mv**2)/dt = mv

so Momentum is the Time Rate of Change of KE.

I don't know if the "understanding" is the deriviative of KE or the integral of MV.

KE (1/2 mv**2), 1st deriviative is MV, 2nd is F (= MA)

So here we have 3 Conservations Laws:

Force,
Momentum,
Energy

Could one conclude that all integrals of F = MA are conserved?

While it is a standard operation to take the deriviative or integral of both sides of an equation,

Doesn't a single equation (F=MA) spawn 2 other equations that can be used as a system to solve a problem?

Perhaps it has to do with the Constant of Integration?

Time for bed......

 

[Tom and Vipers]

Re: \"Tunnels\" on Top of Car! (Cadillac)

...also need to look into the relationship between Force, Work, and Power...

Expressions for work involve change in energies.

Power is time rate of change of work.

Momentum is not involved.

No clues about the nature of momentum here...

 

[CAP]

Re: \"Tunnels\" on Top of Car! (Cadillac)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Tom and Vipers:
So here we have 3 Conservations Laws:

Force,
Momentum,
Energy

Could one conclude that all integrals of F = MA are conserved?

Doesn't a single equation (F=MA) spawn 2 other equations that can be used as a system to solve a problem?
<HR></BLOCKQUOTE>

Sounds good to me.