Aerodynamics

[nova]

<blockquote>quote:</font><hr>Originally posted by Backfire:

I only went through a few fluids courses before I graduated, but it seemed like streamlined objects saw better results from laminar flow, while blunt objects, like the golf ball, fared better with turbulent conditions. Is there any basis for this that you are aware of? Seems like cars would fall more towards blunt objects( have you looked at the rear of a corvette from the side ;) ).<hr></blockquote>

So far I've only had the basics of fluid dynamics so you probably know more than I do.

As for where cars fall I have no clue. They a really are a cross between the two of sorts. They have some blunt surfaces but are also strealined over other surfaces. What I'm thinking is that a car would perform best with laminar flow over the foreward sections which then transitions to turbulent flow towards the rear.

[CARdiac]

Sorry, I may have used incorrect terminology....but basically the theory was correct....considering I was a Sports Medicine Major and it has been 15 years since I even looked at a physicis book...I think I did ok.

Anyway...isn't fun how someone asks a question about saving money on gas and we went with it....and boy did we go!

[nova]

<blockquote>quote:</font><hr>Originally posted by CARdiac:
Anyway...isn't fun how someone asks a question about saving money on gas and we went with it....and boy did we go!<hr></blockquote>

That is pretty cool.

I remembered why a golfball does better in a turbulent flow where the flow seperates farther back on the ball. The flow seperation creates a dead zone at the rear of the ball. This air isn't moving as fast so the pressure is higher than that of the freestream. This pressure differential causes the air at the dead zone to attempt to move out creating a partial vacuum sucking the ball backwards somewhat.

I would assume the blunt rear sections of cars act the same way and thats the reason many new cars are rounded all the way around.

[Arctc Wolf]

drag and downforce don't relate as easy as you make it sound, magic1872. You can cause downforce w/o increasing drag. The new audi's in the LeMan's series racing use body extensions to add downforce but don't increase drag like a increased wing angle would. this allows the wing to have less angle to it to make the same downforce, but reduce drag as well, allowing the car to accelerate faster as well as have a higher top speed while still being able to stay on the ground.

also, you don't see any of those cars sticking up in the air really high now do you. the less air that goes beneath your car, the lower the air pressure beneath the car, therefore making the difference in air velocity over the car and under the car closer in relationship to each other, and therefore causing less lift and not needing as much downforce to keep the car on the ground.

because of this fact, I also have reason to believe that cold air intakes such as the slp cai, ftra, and the ssra all help reduce lift at higher speeds. why? because it's sucking up air from below the car that it would normally suck from the upper engince compartment/hood area. less air means less air pressure, which causes that air below the car to move faster, and since the air pressure beneath the car wants to equalize with the higher speed air above the car, the closer the relationship between the 2 air speeds, the less lift you will have at higher speeds.

However, by putting on a body kit, you increase the frontal area of the car, and since drag is related to frontal area, the mroe frontal area you have, the more drag is produced.

[magic1872]

Artic, I know about all kinds of drag and lift, I'm a Commercial Pilot. But I see what your saying.

[Backfire]

<blockquote>quote:</font><hr>also, you don't see any of those cars sticking up in the air really high now do you. the less air that goes beneath your car, the lower the air pressure beneath the car, therefore making the difference in air velocity over the car and under the car closer in relationship to each other, and therefore causing less lift and not needing as much downforce to keep the car on the ground.

because of this fact, I also have reason to believe that cold air intakes such as the slp cai, ftra, and the ssra all help reduce lift at higher speeds. why? because it's sucking up air from below the car that it would normally suck from the upper engince compartment/hood area. less air means less air pressure, which causes that air below the car to move faster, and since the air pressure beneath the car wants to equalize with the higher speed air above the car, the closer the relationship between the 2 air speeds, the less lift you will have at higher speeds.[/QB]<hr></blockquote>

Why does less air under the car result in lower pressure?? The velocity of the air dictates the pressure, not the volume of air. Unless you are suggesting that the air is forced through a smaller area thus resulting in higher velocities?? But again that isn't less air, but higher velocities. I would think that would depend upon the shape of the front bumper as well.

And my guess would be that the air intake doesn't pull in anywhere near enough air to make a significant difference in air pressure below the car. Think about the volume of air that passes in the area down there. Say maybe 7" by 60" for the area of the space below the car. If you are travelling at 60mph at 6500RPM in a 3.8L, you are going to pull 7.5X10^5 in^3/min into the intake, while passing 2.2*10^7in^3/min under the car. Those numbers use a lot of "ideal" estimates including 100% volumetric efficiency. That is a little more than 3% of the airflow under the car. the faster you go, the lower the percentage. I really don't think that would be noticable considering that the stock systems draw from the similar areas.

[ August 23, 2002: Message edited by: Backfire ]</p>

[snotty]

Dont pay attention to artic, he is a 17 year old kid who reads to many magazines and listens to way to man backyard half a**ed mechanics, im sure the dude who is a pilot knows what he is talking about.

[magic1872]

Hey guys, here is the equation for caculating drag, if anyone cares. Now, you can figure out how much parasite drag your car produces.

Parasite Drag = F * V2 (Squared)/295
F= Frontal Area
V= Speed
This will give you parasite drag in pounds!!

Or you can calculate your car's induced Drag:
Induced Drag = ki * W2 (squared)/ V2 (Squared)
ki = Iduced Drag Constant (what ever it may be)
W = Weight
V = Speed

Or you can calculate your car's total Drag:

Total Drag = ( f * V2 (squared)/295) + (ki * W2(squared)/V2 (squard))

[magic1872]

Here is another way you could calculate it:

D = cd * r * (V2/2) a

D = Drag
cd= Coeficient of Drag
r = Density of the air
v = Speed
a = frontal area

[Arctc Wolf]

and you came from where snotty? don't talk about stuff you don't know anything about. you have no clue what i do, so don't try to act like you do. we don't need another person on this board acting like a kid when people are trying to help others out. unless you have something intelligent to say, stick to the lounge.

[12secondv6]

<blockquote>quote:</font><hr>Originally posted by CARdiac:
Everything you do for performance will more than likely decrease you fuel economy<hr></blockquote>

I see 31 mpg with my bird cruising at 75 mph.
My front end is dropped about an inch due to light weight k member and a arms
Skinnies sometimes or 225 50 16's in front
3 inch by 8 inch large scoop hanging under car for nice cold air/ ram air.
323 gears
pro yank 3200 stall converter
drag radials in back

[V6Maniac]

Well if you want something to help with aerodynamics why don't you try blackouts? :D They cover up those little pockets our lights sit in and I would assume that hased to help out with the aerodynamics some. Probably wouldn't notice any difference but they just look so cool. :cool:

[thebigwaldo]

Maybe clear covers would work better. The cops around here are jerks about that kind of stuff. Basically all I got here (besides a physics and fluid mechanics lesson) is get some headlight covers or maybe a air dam.

It's all good though, I love learning all this complicated stuff outside of school.

Now I need to go figure out how to measure the frontal area of my car... ;)

Edit: !sig mod

[ August 23, 2002: Message edited by: thebigwaldo ]</p>

[nova]

<blockquote>quote:</font><hr>Originally posted by thebigwaldo:
Now I need to go figure out how to measure the frontal area of my car... ;)

Edit: !sig mod

[ August 23, 2002: Message edited by: thebigwaldo ]<hr></blockquote>

Its not really frontal area you have to measure. Frontal area is what you measure when determining the aerodynamic drag of simple geometric solids.

For complex shapes you have to determine the function that describes the average cross sectional area of the object and then work some Calculus voodoo on it to work that into your aerodynamic drag equations.

Isn't fluid mechanics fun? :D :D :D :D

[John_D.]

As if this thing hasn't gone on long enough... [img]smile.gif[/img] Here's one more thought for you all.

Lift/downforce does not change the mass of the vehicle. Whether the car is being pushed up or down or neither, it still has the same inertial resistance to forward acceleration.

It may have more or less traction due to downforce on the tires, or more or less turbulence/drag due to cleaner airflow, but the car doesn't "weigh" any less in terms of mass and inertial forces.

Air resistance is the major contributing factor. Otherwise your car would just keep accelerating, at any throttle setting, on level ground. (until you redlined anyway...) :D As it is, acceleration ceases when air resistance builds to a point that equals the applied rwhp.