A question on vehicle aerodynamics

[grey]

ok...i have a small question on aerodynamics for cars:

in fluid mechanics, we studied the flow of air past an airfoil, which is designed to reduce drag (and obviously lift has a lot to do with it, but that is irrelevant for me now, so i am ignoring it). My question is that why aren't cars designed that way? that is, 'fatter' at the front and 'sleek' at the back? in fact, i seem to think that they are designed in the opposite way...why is that?

 

[mgb_phys]

Because you have to fit people and an engine in them.
Ultra economy racing (where people go for miles on a teaspoon of fuel) are shaped like a teardrop , but for most cars it is a question of rounding off a square box a little and avoiding a trailing vortex.

 

[grey]

hi, thanks for the reply

actually the 'ultra economy racing' is what we intend to do and that made me ask
a further question...is the tear-drop shape effective as compared to the opposite-tear-drop shape at low speeds? by low, i mean something like 30 kph (around 8 m/s)?

 

[Cyrus]

ok...i have a small question on aerodynamics for cars:

in fluid mechanics, we studied the flow of air past an airfoil, which is designed to reduce drag (and obviously lift has a lot to do with it, but that is irrelevant for me now, so i am ignoring it).

Who taught\told you this? Airfoils are designed to optimize lift to drag ratios.

My question is that why aren't cars designed that way? that is, 'fatter' at the front and 'sleek' at the back? in fact, i seem to think that they are designed in the opposite way...why is that?

They're not. Car's are designed with streamlining in mind. Read [1]:

[1] http://en.wikipedia.org/wiki/Kammback

 

[minger]

The tear-drop shape is effective because it is optimum at keeping the boundary layer attached. As flow slows down and pressure increases, we refer to it as an adverse pressure gradient. This can lead to boundary layer separation, which hurts drag.

The idea is that if I'm going to sit in a car, then it has to be "X" high...somewhere. So, put it towards the front, keep the favorable pressure gradient are small, and then SLOWLY reduce the thickness as to ensure the BL stays attached.

If you check out:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940008699_1994008699.pdf
you can see the flow separating just past the turning point. That's because the adverse pressure gradient is too high. Reduce the change in thickness and reduce that gradient. That's the idea.

 

[xxChrisxx]

The reason why cars arent like that is because they look stupid, and the person driving it looks stupid. Customer tastes and marketing are the primary reasons for why cars arent streamlined bodies.

Opposite teardrop shaped arent effective at all, for cars the vast majority of the Cd is from pressure drag caused by separated flow. (This reminds of the Austin allegro that was acutally more aerodynamically efficient in reverse). Having a wide bit at the back, casues the gradient to be too large and you get separated flow.

If you want to go for ultra economy, a long car that is teardorp shaped where the drvier lies down will be the best.

 

[Cyrus]

The reason why cars arent like that is because they look stupid, and the person driving it looks stupid. Customer tastes and marketing are the primary reasons for why cars arent streamlined bodies.

Cars are like helicopters. The unaerodynamic shapes they assume are a result of needing to have utility. If you get a chance to drive a kammback car, you quickly realize the roofline on the back seats is very low, and the shallow slope of the roofline leads to large blind spots and horrible rear view visibility. Not only is inside cabin space reduced, the car becomes a safety issue when it's tear drop shaped.

 

[xxChrisxx]

Be that as it may. Bottom line is for whatever reasons you can come up with (and yours were good ones). Cars aren't teardrop shaped because people won't buy them.

 

[Cyrus]

Be that as it may. Bottom line is for whatever reasons you can come up with (and yours were good ones). Cars aren't teardrop shaped because people won't buy them.

It's not a matter of 'people wouldn't buy one', it's that the vehicle would not have any utility. If it got 100MPG, people would absolutely buy a teardrop shaped car. It would also go faster. I see no justification as to why you think people would not buy one. People buy american cars, don't they? QED.

 

[grey]

ok...thanks guys for the views...here is a design i made on google sketch-up
it s a bit crude, and no dimensions are given, but if you can critique i would be over-joyed
also, tips, ideas for further improvement are most welcome

and yeah, the prospected speed (max) is around 30 km/h (8.3 m/s)

 

[Ranger Mike]

that was a good one..( by American cars..).I had a good laugh on that one...the original question was " why aren't cars designed that way? that is, 'fatter' at the front and 'sleek' at the back?
Many versions of this have been prototyped and Automobile Marketing Departments have test marketed these designs in the public consumer marketplace for years..we do see the evolution trending toward more effective Areo designs..but the overwhelming marketing driver is ..Big Government, with EPA and CAFE Standards.. Big Brother dictates the MPG minimum and Big Auto moves mountains to get there..when this stuff hit the fan in the mid 1970's, the big Cube V8 engines got dumped..no more 426, 426, 440, 454, 455, 460 cubic inch displacement engines...then the CAFE regulations kicked in and the two tin big cars got on the Jenny Craig diet and weight was reduced.( so was crash safety but this was not hyped because it would make Big Gov. look bad)
thins settled out for a while until the green stuff became trendy and everyone was herded like sheep by the 'man made global warming" sham. Now General Motors ( owned d by the Government ) is making pregnant roller skates that will only be purchased by Federal, State and Local government offices and greenie weenie tree huggers who want to feel good abut them selves. The general public will not want these clown cars and the top down dictated product like will go the way of the Edsel,Gremlin, and Yugo.
So the bottom line is...given the confines of government regulations ...CAFE, EPA, DOT and others..how do we economically manufacture an automobile that will comply to all these regulations AND appeal to the purchasing public more than the off shore Competitor.
It is one big juggling act.

 

[xxChrisxx]

It's not a matter of 'people wouldn't buy one', it's that the vehicle would not have any utility. If it got 100MPG, people would absolutely buy a teardrop shaped car. It would also go faster. I see no justification as to why you think people would not buy one. People buy american cars, don't they? QED.

Because I along with many other drivers would rather look awesome in a flashy and impractical but good looking car.

So until someone designed a teardrop car that doesn't look...well dreadful frankly. Noone will want one. Even then, you have the practicality issues you mentioned.

As much as people may say they want a car that gets uber fuel economy, it could do a million miles to the gallon but if its shaped like a portable toilet no one will buy it. Nobody on this planet wants to be A: laughed at. B: known as 'the sensible one', its just not cool.

Also no one really cares what you think, no one really cares what I think. Its not on topic and will and up leading to us being stubborn and bickering. So as to not derail the thread shall we leave it there?

 

[xxChrisxx]

ok...thanks guys for the views...here is a design i made on google sketch-up
it s a bit crude, and no dimensions are given, but if you can critique i would be over-joyed
also, tips, ideas for further improvement are most welcome

and yeah, the prospected speed (max) is around 30 km/h (8.3 m/s)

Hmm... dimensions are really needed. It looks ok, but its a bit wide, you want the frontal area to be as small as possible.

You may also want to smooth it out a bit, edges generally = bad.

It's difficlt to make a proper judgement without running it through CFD. Also what about ride height?

 

[grey]

edges...point noted
wide...actually, its a very crude diagram...but max width is going to be around 1.2 m around the wheels and less than 1 m above
height...around 1.2 m max

and i read on some website, http://www.cardesignonline.com/design/aerodynamics/aerodynamics2.php
it says that 'drag due to rear slope angle would be at its peak at 30 degree, and min at 10 degrees'
is this correct?

theres no way i can do 10 degrees...max possible is around 25, which would, if the above statement is correct, increase drag

 

[xxChrisxx]

It's all to do with separation of air and form drag (so drag that comes from the shape, such as skin friction).

It's a trade off between, having separated flow (pressure drag) and skin friction (form/parasitic drag). A very short car with a high slope will have low form drag because there isn't much material, but will have high pressure drag due to separated flow. A very long car will have more form drag but little to no pressure drag.

This is the reason why very long lower speed trains can have blunt noses, the pressure build up is negligable compared to skin fricion. But a short high speed train will have a bullet shaped nose.What you want is the slope that does not let any flow seperate, but is not longer than necessary.

If the best you can get is near to 30 degrees for a full tear drop, you may consider using a lower angle but cutting off the rear, like the Kamm tail car Cyrus was talking about.

The other option is to simply make the car longer or lower, obviosly if you are building to a spec then this may not be possible.

I can't really comment on specifics of this car because there simply isn't enough data to make a reasoned guess.

 

[Cyrus]

ok...thanks guys for the views...here is a design i made on google sketch-up
it s a bit crude, and no dimensions are given, but if you can critique i would be over-joyed
also, tips, ideas for further improvement are most welcome

and yeah, the prospected speed (max) is around 30 km/h (8.3 m/s)

All you drew was a shape... I see no tires, no engine, no drive train, no cabin, no trunk. And that's the point. Until you do some serious thinking and analysis what you provided isn't nothing more than a drawing. You will find that your cars shape will have to change drastically due to the subsystems, stresses, and kinematics needed to make the car work. Right now you're toying around. Come up with something serious and you're going to learn a lot about automobiles in the process. Go on websites and find out the specs of actual components and fit them into your design. It will give you an appreciation for engineering, which is drastically different from what you're doing - design.

 

[Ranger Mike]

Grey, you keep doing what you are doing!..I am positve you will do just fine...never let negative feedback kill off the inventive side of your idea making...keep posting and never let doom and gloomers get you down...now get back on the drawing board and send us all another good post/ CAD pic! We old guys need to be reminded of the refreshing, new , energized way to do things ...some times...go get um, Tiger!

 

[grey]

If the best you can get is near to 30 degrees for a full tear drop, you may consider using a lower angle but cutting off the rear, like the Kamm tail car Cyrus was talking about.

thanks again

ok...i think we ll have to use this kamm back concept...but does it make sense to put things that make flow turbulent at the place the kamm back ends? something like the dimples on golf ball? i saw in some video on youtube that some new car they designed (i don't remember who, sorry) it had small fin-like objects to do the very same thing?

All you drew was a shape... I see no tires, no engine, no drive train, no cabin, no trunk. And that's the point. Until you do some serious thinking and analysis what you provided isn't nothing more than a drawing. You will find that your cars shape will have to change drastically due to the subsystems, stresses, and kinematics needed to make the car work. Right now you're toying around. Come up with something serious and you're going to learn a lot about automobiles in the process. Go on websites and find out the specs of actual components and fit them into your design. It will give you an appreciation for engineering, which is drastically different from what you're doing - design.

you have a perfectly valid point, but at the moment, i am doing an internship...i have no one else working on this project; at the moment, i am on ground zero...so i am really sorry i am doing this like i am and putting you guys in trouble

what i had in mind was to have an idea of the aerodynamics, and hence the shape...then design the chassis according to the shape...then calculate the weight, etc needed and according to that i ll need the power source, which can be a simple engine or a wholee hybrid system depending on what i have and what i need

i ll try to do what you have told me nevertheless, thanks

 

[Ranger Mike]

before you get into vortex generators ...check out drag coefficents and the like..the whizzy bits like vortex generators, dimples etc .. come after the basics...i recommend
get up on the internet and research the following

Bernoulli law of constant pressure
flow characteristics with in boundary layer
aero dynamic drag formula
parasite drag
Cd numbers
air velocity and pressure curves for airfoils
wing tip vortices
ground effects

will give you pretty good start on figuring out air / aero etc..
As Waddel Wilson ( famous NASCAR crew cheif ) once said.." anytime you are moving, yer pushing AIR"

 

[xxChrisxx]

All you drew was a shape... I see no tires, no engine, no drive train, no cabin, no trunk. And that's the point. Until you do some serious thinking and analysis what you provided isn't nothing more than a drawing. You will find that your cars shape will have to change drastically due to the subsystems, stresses, and kinematics needed to make the car work. Right now you're toying around. Come up with something serious and you're going to learn a lot about automobiles in the process. Go on websites and find out the specs of actual components and fit them into your design. It will give you an appreciation for engineering, which is drastically different from what you're doing - design.

Ok, now this is on topic so I am free to fight about it.

Before you start being impertanent, read the original posts. This isn't a car to tool around town in, this is a car for an endurance racing project. It's perfectly acceptable to start by dsigning the outer cladding. Driver comfort and practicalites are secondary. It doesn't need a trunk and the rest of the stuff can be crammed in where ever. Sizes can even be increased if the rest of the stuff can't be packaged.

Also you've got to start somewhere with design, with an endurance racer, aero is as good as any. I didnt realize people needed a complete engineering project to post here.

The OP's ideas, questions and design starting point are perfectly valid. So unless you have something decent to contribute please stop filling the thread with drivel just for the sake of argument.

 

[xxChrisxx]

thanks again

ok...i think we ll have to use this kamm back concept...but does it make sense to put things that make flow turbulent at the place the kamm back ends? something like the dimples on golf ball? i saw in some video on youtube that some new car they designed (i don't remember who, sorry) it had small fin-like objects to do the very same thing?

Well obviously a full teardrop is ideal, but the advantage of a Kamm back is that it gives marginally more drag for a more practical design. You will get pressure drag as the flow will separate but you've got to decide if a Kamm tail of 10 degrees will be better or worse than a full teardrop at 25 degrees in terms of drag.

I'm not telling you what to do, or what will be best. Those design decisions will have to be yours in the end. I'm just giving a few options of how to achieve the goal.

 

[Cyrus]

thanks again

ok...i think we ll have to use this kamm back concept...but does it make sense to put things that make flow turbulent at the place the kamm back ends? something like the dimples on golf ball? i saw in some video on youtube that some new car they designed (i don't remember who, sorry) it had small fin-like objects to do the very same thing?


you have a perfectly valid point, but at the moment, i am doing an internship...i have no one else working on this project; at the moment, i am on ground zero...so i am really sorry i am doing this like i am and putting you guys in trouble

what i had in mind was to have an idea of the aerodynamics, and hence the shape...then design the chassis according to the shape...then calculate the weight, etc needed and according to that i ll need the power source, which can be a simple engine or a wholee hybrid system depending on what i have and what i need

i ll try to do what you have told me nevertheless, thanks

Hi Grey. As an engineer that's been on a design team before, I need to point some things out here that I don't see you doing (which is why I'm giving you grief). If you are serious about any form of design on a car, you really need to take a scientific approach. In any design process, you need to do some background literary research to see what's been done, and where there is room for potential innovation. Simply drawing a car and then saying such things as "ok...i think we ll have to use this kamm back concept...but does it make sense to put things that make flow turbulent at the place the kamm back ends" shows me you are making it up as you read posts. I would go to the library and find some SAE articles on automobile design. You don't need anything technical, but you do need to have some basic, realistic understandings of the systems needed to make the car work.

Also, any good design is made with to conform to constraints. As of yet, I see you have no such constraints. Pick an engine size, a maximum car length, number of passengers. You're drawing doesn't have anything as basic as a linear dimension in it.

Once you do some background research, you can make a serious effort at a drawing. I'm sorry to be blunt, but what you're doing right now is not engineering. I would hate to see you guys actually start building something on a whim, because that's how it comes off the more I read your posts. I don't see any engineering rationale.

 

[grey]

thanks for the kind words, and advice...

Ranger Mike...actually we have studied most of what you told me to look, but i looked up those things anyway...basically decided to go back to the basics...now a question that comes to my mind is that, why, as most resurces on the boundary layer say, do we need to keep it attached to the vehicle?
In the boundary layer, the viscous forces are dominant...so viscous drag increases?
i think that for cars, the pressure drag would have a dominant effect (at those speeds) so we would rather have viscous than pressure if and only if the overall drag is reduced...am i right here?

 

[Ranger Mike]

excellent question - the short answer is that we want to keep the boundary layer attached so we can manage it ( control it like we control sprung weight). We want the layer detaching past the end of the car. If portions of the air stream detach before this, we have created more drag than if is detaches after passing completely over the car. Same reason two race cars run faster while drafting ( slip stream).

 

[xxChrisxx]

You want the boundary layer attached for precisely the reason you said, pressure drag. Pressure drag is far more killing to overall Cd than skin friction is.

After the flow separates it creates a pocket of low pressure that wants to drag the car back, this combined with the high pressure pocket in front of the car is what casues the drag.

You may also think that nice smooth laminar boundary will be better, this isn't acutally true. A turbulent boundary layer is more energetic and will stay attached to the outer skin for longer.

It's important for you to know why the boundary layer seperates. Read up on adverse pressure gradient.

 

[grey]

excellent question - the short answer is that we want to keep the boundary layer attached so we can manage it ( control it like we control sprung weight). We want the layer detaching past the end of the car. If portions of the air stream detach before this, we have created more drag than if is detaches after passing completely over the car. Same reason two race cars run faster while drafting ( slip stream).

Noted!

It's important for you to know why the boundary layer seperates. Read up on adverse pressure gradient.

will do

Ok, while i have been studying a bit on one side, i tried to make the thing in AutoCAD...posting the gif here (dont know how to post dwg)...hope both of you guys can review it there
its still not there (things like the horizontal edges have to be rounded, and a round front has to be pu etc)
also, put most of the dimensions in order (theres a limit by the rules, for example the length has to be between 220-350 cm, so i put 350 max...stayed within the limits :-) ) but for some curved areas, i just relied on trial-and-error..still much to be done
and made it a solid coz i thought it d be easier to analyse

 

[xxChrisxx]

I realize this may be preliminary design, but ultimately do you have haccess to CFD or a windtunnel? A picture of the shape gives a general idea but conveys no useful information as to performance.

Cyrus' last post is dead right, you do need to have a clear idea of where you are going before even considering putting pen to paper for a proper drawing. Your initial drawing was fair enough to give us a general idea, but you've changed it now with seemingly no real reason.

Try and answer the following questions regarding the last picture.

Why have you changed the design?
Will that be better?
How do you know?

Unless you can give a comprehensive answer to all of those questions, don't change anything and don't draw anything because its meaningless if you don't know where you are heading.

 

[grey]

ok...well to be honest i did try to put in changes for reasons, tho i am not sure if they are good enough
1. this one is in AutoCAD, with (mostly) proper dimensions. It shows that such a design is possible while conforming to the limitations in dimensions set by the rules
(the angle at the 'trailing edge' is 20 degrees, but i think, given the low speeds, it should be ok; but to be sure, we need access to some CFD software (we don't have it at the moment, but i am sure some professor at the college would), like you say. Wind Tunnel i am afraid is not a possibility here.
2. Some changes i have made:
a) sweep-up at the underside at the rear

http://www.cardesignonline.com/design/aerodynamics/aerodynamics2.php][/PLAIN]
The underbody should be as smooth and continuous as possible, and should sweep up slightly at the rear

b) tried to smooth off the edges, not all of them yet though

http://www.cardesignonline.com/design/aerodynamics/aerodynamics2.php][/PLAIN]
There should be no sharp angles (except where it is necessary to avoid cross-wind instability).

How do i know? Thats where i am stuck...is there a way of telling without using softwares or wind-tunnels?

 

[PFMike]

Hi Grey
Good luck on your car building and racing efforts. I think you'll get much more focused answers on this forum if we have a better idea of what your goal is. I've competed and served as a judge in numerous car building and racing events and it always comes down to the goals and rules. You've mentioned "rules" a few times, what is the event that you want to compete in? What's the website for the rules? Depending on the event requirements I may be able to point you to some very useful resources that can help you develop your car.

 

[grey]

my apologies for this impertinence all along...
so i think i was missing this trick...really sorry...
number of passengers= 1 (min weight 70 kg)
weight of vehicle = 160 kg (max)
length = 220-350 cm
width = 120-130 cm
height = 100-130 cm
wheel base= 120 cm (min)
axle length = 100 cm (front, min), 80 cm (rear, min)
wheel rim dia (16-17 inches)

i think these are the main things
power source = there's a choice, but we have decided on IC engine with gasoline most prolly our fuel
hybrid is an option we are considering but that depends on the weight-power ratio, iff we can squeeze into the limits
obviously, i can't give you the power of the engine coz that i think should be worked out after we know what we are up against (in terms of how much to move, and drag)

the rest you can see in the rules (for UrbanConcept) in the site

the event is shell eco marathon asia
http://www.shell.com/home/content/eco-marathon-en/asia/welcome_sem_asia.html

obviously, i reread Cyrus' last post again...so much data regarding the constraints i have posted. regarding looking up research papers, i am not sure as to what to look for...design? our key aim is to reduce drag (my aim is to keep the co-efficient of drag x frontal area <0.1) and minimise weight (that obviously depends on materials, surface area, power system etc, which we can't get a figure until the design of the chassis is done)

if anything, i have missed, do tell so i post it

 

[Cyrus]

my apologies for this impertinence all along...
so i think i was missing this trick...really sorry...
number of passengers= 1 (min weight 70 kg)
weight of vehicle = 160 kg (max)
length = 220-350 cm
width = 120-130 cm
height = 100-130 cm
wheel base= 120 cm (min)
axle length = 100 cm (front, min), 80 cm (rear, min)
wheel rim dia (16-17 inches)

i think these are the main things
power source = there's a choice, but we have decided on IC engine with gasoline most prolly our fuel
hybrid is an option we are considering but that depends on the weight-power ratio, iff we can squeeze into the limits
obviously, i can't give you the power of the engine coz that i think should be worked out after we know what we are up against (in terms of how much to move, and drag)

the rest you can see in the rules (for UrbanConcept) in the site

the event is shell eco marathon asia
http://www.shell.com/home/content/eco-marathon-en/asia/welcome_sem_asia.html

obviously, i reread Cyrus' last post again...so much data regarding the constraints i have posted. regarding looking up research papers, i am not sure as to what to look for...design? our key aim is to reduce drag (my aim is to keep the co-efficient of drag x frontal area <0.1) and minimise weight (that obviously depends on materials, surface area, power system etc, which we can't get a figure until the design of the chassis is done)

if anything, i have missed, do tell so i post it

I'm glad to see you listened to my advice. Now you are on a path for success. Find yourself a copy of Fundamentals of Vehicle Dynamics by T.D. Gillespie. I will scan the refrences in the chapter on aerodynamics for you and post it later so you have a jumping off point in terms of your literary search, which is what you need to do right away. And, be INTENSIVE in this search for information. If you don't do this, you will waste your time reinventing the wheel. At best, you can find an existing design you like and simply use it. You can justify your choice based on the research you did, and I think that is just as impressive as coming up with a unique idea because it shows careful consideration and respect for research already done in this field. As for your statement:

"my aim is to keep the co-efficient of drag x frontal area <0.1"​

The value of C_D \times S is called the wetted area (or also the wet plate area). Where C_D is the coefficient of drag and S is the reference area.

 

[PFMike]

Grey
Thanks for the great answer. No need to apologize. Most of us start a conversation with something in mind and often leave that out until later.

The Shell event (and urban concept category) has been run for years. After a while the top winning teams converge on one or two optimal solutions to the problem (rules). Concentrate on the top six teams over SEVERAL years and learn as much as possible from each of their experiences and especially their changes in design. Make charts tracking changes and competition results.

Use these prior experiences and rules to establish minimums and maximums. This will give you the basic boundaries within which you will be creating your design.

Also note Rules Article 53 Visibility. The driver seating and viewing position will establish one of the basic outer parameters for your vehicle shape and drag coefficient. You will want the driver to lay down as far as possible while wearing the helmet and safely driving the vehicle to minimize frontal area.

Note the mirrors rule. Oversized and poorly designed mirrors and attachments can cause a surprisingly large amount of drag.

Explore wheel fans to manage turbulence and vortices created by the wheels and tires. Use NACA vent openings for any required ducts (example: radiator cooling).

Consider strategy. I don't see any minimum speed requirements, no stop-and-go braking, or slolom (driving a weaving pattern through obstacles) either. Write to race authorities and get this clear before going too far on your design efforts. Design your vehicle for its optimum efficient operating range and the actual running in this event. This is a fuel efficiency event. Design to win.

As a basic consideration, complete your vehicle at least three months prior to the event. Build in several very ACTIVE months work before the event with the basic car fully completed. Driving (practice, practice, practice), live testing (roll down, ink testing, streamers), and time to make adjustments/changes to the design (spoilers, foils, air dams, streamlining undercarriage) can make as much difference to drag and final race results as an equal amount of time adjusting designs in software or drawing programs.

You should get a copy of the book "The Leading Edge: Aerodynamic Design of Ultra-streamlined Land Vehicles" by Goro Tamai, Robert Bentley Publishers, ISBN 0-8376-0860-0, copyright 1999. Corrections available on the publishers web site.

Goro Tamai was an Massachusetts Institute of Technology (MIT) engineering student that was the aerodynamicist for MIT's solar racing, and Tour de Sol commuter cars. He went on to win the Young SAE Engineer of the year award and work for General Motors designing among others the "Precept" concept car. The book's main focus is solar continental racers but it covers aerodynamic design for highly efficient competition cars as well. He walks through all the basic questions an aero designer considers in automobile efficiency, shows his calculations, shows real world testing procedures, and explains how to apply it to your project. It is an outstanding resource that is technical, solidly grounded, understandable, and immediately applicable. https://www.amazon.com/dp/0837608600/?tag=pfamazon01-20

This book is a great source for your project. Please note that most aero books and engineering rules of thumb are NOT for wheeled ground vehicles. And the few that are are usually written for high speed (lots of fuel) racing.

Ask any further questions, and keep us up to date as you go forward.

 

[grey]

Cyrus,
I have got hold of the book...studying has commenced

PF Mike,
thanks again...just to clear it up a little
1.I don't think drivers are allowed to lay down

2. minimum speed. Minimun average speed for 6 laps of 2.706 km track (16.236 km) is given to be (in the facebook group of this event) 30 km/h...this includes some stops (3 stops of 10 secs each last time)

One particular feature of this group is that vehicles competing in this group will require "stop and go" driving

there are no obstacles though

will try to get data from last years' races
will try to get those books too...thanks!:-)

 

[Cyrus]

Attaboy, now you're doing engineering work.

 

[famousken]

koenigsegg ccx isn't teardrop shaped and it has a drag coefficent of 0.3

 

[xxChrisxx]

Its never going to win a fuel endurance race though is it...

0.3 may be fairly good for a road going car but doesn't cut the mustard for ultra low drag racing.

 

[famousken]

Its never going to win a fuel endurance race though is it...

0.3 may be fairly good for a road going car but doesn't cut the mustard for ultra low drag racing.

But it makes up for it in cool points ;)

 

[xxChrisxx]

Very true.

 

[Cyrus]

koenigsegg ccx isn't teardrop shaped and it has a drag coefficent of 0.3

That doesn't mean anything...it's the wetted area that matters if you want to compare two cars. I could define the reference area a mile wide for a mini van and have a drag coefficient of 0.000000000000000000000000007.

 

[famousken]

That doesn't mean anything...it's the wetted area that matters if you want to compare two cars. I could define the reference area a mile wide for a mini van and have a drag coefficient of 0.000000000000000000000000007.

The wetted area is just the frontal area of the car isn't it? So to figure drag you would use
Cd X A^2?

Here is a link to a car that VW is working on, most definatly teardrop shaped
http://www.ultimatecarpage.com/car/1316/Volkswagen-1-Litre.html

 

[xxChrisxx]

That doesn't mean anything...it's the wetted area that matters if you want to compare two cars. I could define the reference area a mile wide for a mini van and have a drag coefficient of 0.000000000000000000000000007.

Why the frig would you do that? You COULD define the area as that but you'd be utterly stupid to as it tells you nothing useful. Thats like saying I'm going to work out the Reynods number for a flow over a wing and define the characterisitc length as the diameter of the earth. Also say you've calculated your Cd empirically fby finding the drag force and then calcualting Cd.

And of course it means something Cd is how you compare drag between the shape of cars to take out the annoying variables like area and speed, its nondimensional for a bloody reason.

He was making a statement that the drag coeffient for a CCX is 0.3... which it is. Just like the Cd for a Lotus 7 is 0.7. And just like the Cd for an F1 car varied according to how much downforce its set to. They are all calcualted in a standard way. So there is bugger all wrong with his statement.

Can you just take a step back and stop yourself from making stupid posts simply for something to do. I wouldn't mind you being argumentative if you contibuted something remotely useful to furthering the discussion.
More to the point, what on Earth was the point of your post, how was it relevent?

EDIT: And don't bother waving your hands and trying to justify yourself by saying CdA is more important than Cd, because it isnt. If you are going for that mentality you may as well just throw everything in and talk about Fd. He was referring to the shape only.

 

[Cyrus]

Why the frig would you do that? You COULD define the area as that but you'd be utterly stupid to as it tells you nothing useful. Thats like saying I'm going to work out the Reynods number for a flow over a wing and define the characterisitc length as the diameter of the earth. Also say you've calculated your Cd empirically fby finding the drag force and then calcualting Cd.

And of course it means something Cd is how you compare drag between the shape of cars to take out the annoying variables like area and speed, its nondimensional for a bloody reason.

Please go back and re-read my post. The point is that if the cars do not have the same frontal area it is meaningless to directly compare the drag coefficient.

He was making a statement that the drag coeffient for a CCX is 0.3... which it is. Just like the Cd for a Lotus 7 is 0.7. And just like the Cd for an F1 car varied according to how much downforce its set to. They are all calcualted in a standard way. So there is bugger all wrong with his statement.

Provided they have the same reference area, then yes. Is there a standard refernce area used in automotive aerodynamics?

Can you just take a step back and stop yourself from making stupid posts simply for something to do. I wouldn't mind you being argumentative if you contibuted something remotely useful to furthering the discussion.
More to the point, what on Earth was the point of your post, how was it relevent?

EDIT: And don't bother waving your hands and trying to justify yourself by saying CdA is more important than Cd, because it isnt. If you are going for that mentality you may as well just throw everything in and talk about Fd. He was referring to the shape only.

I have reported this portion of your post.

 

[Cyrus]

PS, do you notice how the tabulated values here are wetted parameters?

http://en.wikipedia.org/wiki/Automobile_drag_coefficient

I.e., C_D A.

While designers pay attention to the overall shape of the automobile, they also bear in mind that reducing the frontal area of the shape helps reduce the drag. The combination of drag coefficient and area is CdA (or CxA), a multiplication of the Cd value by the area.

In aerodynamics, the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient is called drag area. In 2003, Car and Driver adapted this metric and adopted it as a more intuitive way to compare the aerodynamic efficiency of various automobiles. Average full-size passenger cars have a drag area of roughly 8.5 ft² (.79 m²). Reported drag area ranges from the 1999 Honda Insight at 5.1 ft² (.47 m²) to the 2003 Hummer H2 at 26.3 ft² (2.44 m²). The drag area of a bicycle is also in the range of 6.5-7.5 ft².[16]

Also, don't put words in my mouth:

EDIT: And don't bother waving your hands and trying to justify yourself by saying CdA is more important than Cd, because it isnt. If you are going for that mentality you may as well just throw everything in and talk about Fd. He was referring to the shape only.

 

[Cyrus]

Please read page 307, notce anything special about the y-axis?

http://books.google.com/books?id=nM...sult&resnum=4#v=onepage&q=wetted area&f=false

This is why you can't add the contributions of drag in coefficient form from each part of the car unless they all have the same reference area. Each car is going to have a different reference area, which is typically the frontal area of the car. Hence, why you can't simply compare one Cd value to another and have it be meaningful. This is why saying things like 'car X has a Cd of such and such" is as meaningless as boasting about your car having 900HP, and failing to mention it weighs as much as a big rig.

 

[Cyrus]

And of course it means something Cd is how you compare drag between the shape of cars to take out the annoying variables like area and speed, its nondimensional for a bloody reason.

No, this is where your problem lies. Cd is not defined the way it is to "take out the annoying variables like area and speed". Cd is a nondimensional parameter for a bloody reason you failed to mention, similitude. People don't put full size cars inside wind tunnels because its very, very expensive. Instead, they put scale models. In order to put a scale model, you must have dynamic similitude, which occurs through the buckingham-Pi theorem of nondimensionalization. You can then SCALE UP the value of Cd from the model to the full size production car by scaling the REFERENCE AREA.

This does not mean Cd is a valid metric for comparison of cars. The wetted area is a better metric.

EDIT: And don't bother waving your hands and trying to justify yourself by saying CdA is more important than Cd, because it isnt. If you are going for that mentality you may as well just throw everything in and talk about Fd. He was referring to the shape only.

My paragraph above explains this in detail.

 

[xxChrisxx]

Hmm ok I've decided I am going to have to be careful trying to explain the reasoning on with this one as you do have me worried I am wrong. It's been a while since I've had to do any aero work.

As we arent comparing cars directly only the shapes. And although you need the same reference area to directly compare shapes. Cd gives a better indication of how good the shape is when you know nothing about the car than CdA. CdA is a better metric if you want to compare cars themselves and not just the geometry.

You come from an aerospace background don't you?. As cars are bluff bodies comparison methods differ slightly, wetted area doesn't really mean much if the flow has separated and the drag is coming from the pressure. We always use frontal area for a car. (unless you are using wetted area to mean frontal area)

The reason for this is that CdA can give misleading ideas when talking about bluff bodies rather than spteamlined. A low Cd car with a high CSA can give a higher CdA when compared to a high Cd car with a low CSA.

For example:

Ok we have a small family hatchback shaped car with a drag coefficient of 0.3 with a frontal area of 0.5m^2
We have much larger modern GT car with a drag coefficent of 0.25. With a frontal area of 0.7m^2.

The CdA for both is:
Hatch: 0.15 m^2
Sports: 0.175 m^2

Now if we knew nothing about the Cd of these cars, but wanted to pick one of the shapes we could have to conclude the hatchback is better on data availalbe when the GT car produces more drag simply becuase it is bigger doesn't lead to a good comparison as if the Gt car were the same size it would give a lower CdA. Basically the shape of the GT car goes through the air better than the hatchback.

As you stated Cd is there for simlitude, which allows scale models to be used. Imagine two cars of different CSA are just scales models of the car that you want.

We take the CdA, divide by the CSA of the acutal car to get the Cd then multiply it by the new CSA of the car we are going to build. Why not just cut the step to comparing a similar 'wetted area' and just compare the Cd values. Obviously this only holds for similar conditions (same Re same Mach etc etc).

Now when you are dealing with streamlined bodies such as planes/wings. Wetted area is more useful becuase the flow stays attached and the majority of the drag comes from skin friction which is area dependent. Which is why I like the fact you used a helecopter aerodynamics book to prove a point about car aero. I know in general aero is aero, but the method about comparing cars always tends to use Cd becuase the flow seperates.

One textbook example I can think of that demonstrates this is you have a small diameter circe and a large aerofoil i think the characteristic length of the aerofiol was 10x that of the circe (i'll try to find this example from my books and fill in the details)
They both have the same drag force (so the same CdA as they were tested under the same conditions) but the streamlined aerofoil has the lower Cd.

Now its obvious that when you scale the aerofoil down to the same charactersic length as the circle it will have a much lower drag.I'd like to apologive for the tone of my last post, it was hastily typed. Dont get me wrong Cyrus I like you and your direct approach, but sometimes you piss me off with needlessly condecending or combative posts. I wouldn't mind if they were wrong and talking bollocks but the guys post was clearly anecdotal. I pointed out that it didnt matter and he used the phrase 'cool points', that more than anything else shows he wasnt tryin to make a serious point.

 

[Astronuc]

In aerodynamics, the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient is called drag area. In 2003, Car and Driver adapted this metric and adopted it as a more intuitive way to compare the aerodynamic efficiency of various automobiles.

Hmmm - back when I studied the fundamentals of fluid dynamics, we looked at both frontal area and surface area and we had to calculate the normal dynamic force and friction (shear) force. Then we'd have to consider flow separation, which put a static force on an object due to the differential pressure between front and back.

Of course, a car is designed for utility and operation/environment. The utilities of a race car, sports car and family passenger car are all different, and so are their environments and operation. The utility (function) is just one of the constraints with which a designer must deal.

And there is the coolness factor for sports car and family car. A race car just needs to be fast, reliable and safe at the higher speeds at which it performs, and the designer isn't constrained by the comfort or appeal of the driver.

When I looked at the ultimatecarpage site, the banner at the top had a picture of couple of the Gulf Porsche 917's from back in the early 70's. Fantastic car, but it was quite uncomfortable for the driver. The space for the driver was minimized to reduce frontal area. Visibility to the rear was a problem too, because external rear-view mirrors would project into the airflow and the rear window was small. The only time one needed it was when someone was trying to pass (not too often) or the driver was passing another car (most of the time).

http://www.khulsey.com/stockphotography/porsche_917k_le-mans.jpeg

 

[Cyrus]

Cyrus we arent comparing cars directly only the shapes. And although you need the same reference area to directly compare shapes. Cd gives a better indication of how good the shape is when you know nothing about the car than CdA. CdA is a better metric if you want to compare cars themselves and not just the geometry.

If you are going to compare cars on nothing but shape, then you need to make sure both cars have the same frontal area for a direct comparison of Cd. Otherwise, CdA is the only thing that make sense as a comparison.

You have also got to be really careful Cyrus, because you blatantly come from an aerospace background and are used to dealign with streamlined bodies. Car's as you probably konw are bluff bodies. This is why comparison methods differe slightly, wetted area doesn't really mean much if the flow has separated and the drag is coming from the pressure.

We always use frontal area for a car.

The problem is that the frontal area is not a constant for every car.

The reason for this is that CdA can give misleading ideas when talking about bluff bodies rather than spteamlined. A low Cd car with a high CSA can give a higher CdA when compared to a high Cd car with a low CSA.

I don't know what "CSA" stands for: I think you mean frontal area for CSA. We call this S in aerodynamic standards. If that's the case, then what you wrote in the second sentence is exactly my point. The influence of area cannot be ignored for exactly that reason. When comparing two cars a given dynamic pressure, q, I care about which one has a higher drag force because it will require more power. CdA tells me this, Cd does not. Cd only helps for similitude of scale models.

For example:

Ok we have a small family hatchback shaped car with a drag coefficient of 0.3 with a frontal area of 0.5m^2
We have much larger modern GT car with a drag coefficent of 0.25. With a frontal area of 0.7m^2.

The CdA for both is:
Hatch: 0.15 m^2
Sports: 0.175 m^2


Now if we knew nothing about the Cd of these cars, but wanted to pick one of the shapes we could have to conclude the hatchback is better when the GT car produces more drag simply becuase it is bigger doesn't lead to a good comparison as if the Gt car were the same size it would give a lower CdA. Basically the shape of the GT car goes through the air better than the hatchback.

No, it says that the Hatch does go through the air better at a given speed than the GT. The hatchback will absolutely require less power to stay at a fixed speed. If you want to compare the two cars at a purely geometric standpoint in terms of the aerodynamics, give them both the same refernence area. Then you can compare the Cd. In fact, this is what you are doing implicitly in your argument of "just compare Cd"!

As you stated Cd is there for simlitude, which allows scale models to be used. Imagine two cars of different CSA are just scales models of the car that you want.

We take the CdA, divide by the CSA of the acutal car to get the Cd then multiply it by the new CSA of the car we are going to build.

Eg above. We know that out frontal area of the car is going to be 0.3m^2 for example.

CAR 1:CdA 0.15
CAR 2:CdA 0.175

Ok so we divide by the current CSA and times by new CSA.

CAR 1: 0.15*(0.3/0.5) = 0.09
CAR 2: 0.175*(0.3/0.7) = 0.075

As the above is simply the Cd multiplied by the area we want. Why not just cut the step to comparing a similar 'wetted area' and just compare the Cd values. Obviously this only holds for similar conditions (same Re same Mach etc etc).

Because when you "cut the step" that is an assumption of having the same reference area value for both design shapes.

Cd is a funcion of Re and Ma and flow direction. Under the same conditions the Cd will be the same. So i'll repeat what I said above so you don't miss it. "Obviously this only holds for similar conditions (same Re same Mach etc etc)".

Yes, that needs to be held constant, but that isn't the issue here.

Of course when you scale both shaped down to the area you want the one with the lowe rvalue od Cd is the one that gives the lower drag. This is why at the start of designs, when you know the rough size of the car you want but want to focus totally on the shape its more useful to compare Cd values of other cars NOT CdA.

Exactly, becuase they both have the same area!

Now when you are dealing with streamlined bodies such as planes/wings. Wetted area is more useful becuase the flow stays attached and the majority of the drag comes from skin friction which is area dependent. Which is why I like the fact you used a helecopter aerodynamics book to prove a point about car aero. I know in general aero is aero, but the method about comparing cars always tends to use Cd becuase the flow seperates.

Attached or unattached flow doesn't matter, that is accounted for in the value of Cd. Helicopters are by no means streamlined, they are a highly unsteady aerodynamic environment due to the rotors. Any unsteadiness is captured by time varying aperiodic terms inside of Cd, i.e Cd = Cd(Re,Ma, time, and other explanitory variables).

I'll leave with this. For cars in the wind tunnel, you can cause them to have side slip angle and get a side force. What is interesting is that you can return the car back to centerline of the wind tunnel, but you will still measure a strong side force despite no yaw angle. A flow condition occurs where the car starts sheading a vortex that doesn't go away when the car gets back to center. Very counter-intuitive.

I will also note: I am glad the OP stated his car has a targed wetted area < 0.1 for the competition.

 

[famousken]

Quote:[People don't put full size cars inside wind tunnels because its very, very expensive. Instead, they put scale models. In order to put a scale model, you must have dynamic similitude, which occurs through the buckingham-Pi theorem of nondimensionalization. You can then SCALE UP the value of Cd from the model to the full size production car by scaling the REFERENCE AREA.]

People don't put full size cars in wind tunnels? They most certainly do! There are some things that just don's scale up well, what do you think the apparent air density is for a 1/16th scale model is compared to a full size car. Air is not going to behave the same way on a model as it will on a full size vehicle. vortexes may not appear on a smaller model, It is damn near impossible to account for air that may be flowing THROUGH the car in areas like the radiator, under bumpers, undercarriage, etc. Another reason they do it is to take measurements of road noise and drafts in the vehicle. Please do a little research before you make such a false statement, people are on this forum are trying to understand new concepts and they don't need to be misled. Here are some links I turned up in about 60s of googling.

http://www.gm.com/experience/autoshows-events/racing/technology/aerodynamics/index.jsp
http://exoticcars.about.com/od/guidedtours/ig/Mercedes-Benz-SLR-McLaren/SLR-McLaren-wind-tunnel.htm
http://trucks.about.com/od/2007fordtrucks/ig/2009-Ford-F-150-Truck-Pictures/09-Ford-F-150-Wind-Tunnel-Test.htm
http://www.carbodydesign.com/gallery/2009/05/14-volkswagen-polo/22/

 

[Cyrus]

Quote:[People don't put full size cars inside wind tunnels because its very, very expensive. Instead, they put scale models. In order to put a scale model, you must have dynamic similitude, which occurs through the buckingham-Pi theorem of nondimensionalization. You can then SCALE UP the value of Cd from the model to the full size production car by scaling the REFERENCE AREA.]

People don't put full size cars in wind tunnels? They most certainly do! There are some things that just don's scale up well, what do you think the apparent air density is for a 1/16th scale model is compared to a full size car. Air is not going to behave the same way on a model as it will on a full size vehicle. vortexes may not appear on a smaller model, It is damn near impossible to account for air that may be flowing THROUGH the car in areas like the radiator, under bumpers, undercarriage, etc. Another reason they do it is to take measurements of road noise and drafts in the vehicle. Please do a little research before you make such a false statement, people are on this forum are trying to understand new concepts and they don't need to be misled. Here are some links I turned up in about 60s of googling.

http://www.gm.com/experience/autoshows-events/racing/technology/aerodynamics/index.jsp
http://exoticcars.about.com/od/guidedtours/ig/Mercedes-Benz-SLR-McLaren/SLR-McLaren-wind-tunnel.htm
http://trucks.about.com/od/2007fordtrucks/ig/2009-Ford-F-150-Truck-Pictures/09-Ford-F-150-Wind-Tunnel-Test.htm
http://www.carbodydesign.com/gallery/2009/05/14-volkswagen-polo/22/

I should have said, don't always put a full size car in a wind tunnel, that was a bit careless on my part. Don't lecture me on air density about scale cars when I was in a wind tunnel doing testing and saw a scale car sitting there from previous testing. It was a ford tarus. In fact, back in the 90s GM did so much testing they had their own private section of the tunnel. Road noise is from the RMS signal of the vibrations on the road. That's not something a wind tunnel tells you.