I stumbled accross a computer generated wind tunnel simulation for the Fiero the other day. I've seen more than one discussion about our cars and aerodynamics, etc., and thought they might be of interest. The site doesn't really tell much about what the colors mean though, which was annoying...
This is the text from the site that went along with these illustrations:
Fiero Automobile
Gridgen was used by Aerosoft, Inc. to make this grid for a Pontiac Fiero sports car. Gridgen's direct interface to Aerosoft's GASP CFD code was then used to preprocess the grid and boundary condition files for a 60 miles per hour flowfield solution. Shown below are visualizations of pressure contours and velocity vectors colored by the air pressure.
I stumbled accross a computer generated wind tunnel simulation for the Fiero the other day. I've seen more than one discussion about our cars and aerodynamics, etc., and thought they might be of interest. The site doesn't really tell much about what the colors mean though, which was annoying...
This is the text from the site that went along with these illustrations:
Fiero Automobile
Gridgen was used by Aerosoft, Inc. to make this grid for a Pontiac Fiero sports car. Gridgen's direct interface to Aerosoft's GASP CFD code was then used to preprocess the grid and boundary condition files for a 60 miles per hour flowfield solution. Shown below are visualizations of pressure contours and velocity vectors colored by the air pressure.
i can verify this one being accurate as ive seen this sort of air flow with snow and leaves.
I'd really like to see that same information at speeds in excess on 110 mph. That information might settle a lot of other arguments. Thanks for posting that info.
------------------ Ron
It's the Soldier, not the reporter Who has given us the freedom of the press. It's the Soldier, not the poet, Who has given us the freedom of speech. It's the Soldier, not the politicians That ensures our right to Life, Liberty and the Pursuit of Happiness. It's the Soldier who salutes the flag, Who serves beneath the flag, And whose coffin is draped by the flag.
Thanks for the great pictures. Could you post a link to the site? I'd like to see if there are any other interesting info worth looking at. Thanks, Bob
i think the top and bottom ones are the pressure zones on the car and the middle one is the pressure waves generated by the car. and yes, blue is low/cold and red is high/hot. i was actually thinking of using those lil fins that the evo has on the back of its roof to smooth out the transition over the decklid and also move more air over it increase the performance of the 'fastback' spoiler. looks like it would actually work.
[This message has been edited by megafreakindeth (edited 01-21-2008).]
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07:00 PM
Dizzixx Member
Posts: 1470 From: Salt Lake, Utah, United States Registered: Oct 2005
Two of you on this thread (timgray and megafreakindeth) have both mentioned blue means low pressure and red means high.
Are you referring only to the fourth of the illustrations pictured above? I say that because that is the only one of those color illustrations that isn't almost all red, and from logic alone, I would think that everything can't be above average in terms of pressure.
Any further clarification of the meaning of these illustrations certainly would be appreciated by those of us unfamiliar with their format.
This "explanatory" comment of those illustrations' original developers certainly didn't seem very helpful to me: "Shown below are visualizations of pressure contours and velocity vectors colored by the air pressure."
Say what?
They could just as well have said, "Shown below are color illustrations which, if you aren't familiar with them, you shouldn't even attempt to understand, especially since we're certainly not going to help explain them to you!"
If it isnt modeled with the up front compartment then it probably wouldnt settle much.....
Oh contrair: If you notice in all the illistrations the center of the hood - where we put our hood vents - is a low pressure area (& that's true at any speed). The way you read these, would be the (as stated) the cooler colors (blue, etc.) are the low pressure areas, & the hot colors (red is the hottest) are the high-pressure areas. Yellow or yellow-green appears to be neutral (static atmospheric pressure). And yes ignore the color of the red bodies in 2 & 3. Also I think you'll find the closer the lines the greater (or lower) the pressure is. I think these are slightly flawed, though. I think you'll find the sides of the front bumper have a higher pressure as well as the base of the windshield (the cowl); also the sides of the doors will be higher as the air rolls off of the sides of the windshield. ~ Paul aka "Tha Driver"
Originally posted by Tha Driver: The way you read these, would be the (as stated) the cooler colors (blue, etc.) are the low pressure areas, & the hot colors (red is the hottest) are the high-pressure areas. Yellow or yellow-green appears to be neutral (static atmospheric pressure). And yes ignore the color of the red bodies in 2 & 3.
Thanks for that clarification. The above comments, augmenting those of timgray and megafreakindeth, do far more to explain the meaning of those color illustrations than did those of the illustrations' original developers.
Those colour graphs IMHO are not accurate enough. You need numbers. If you had maybe 10 colours you'd be closer to seeing what is actually going on. You also need clearer flow lines especially trailing edge. That tells you how to correct any deficiencies or improve flow.
Nice graphics though. And generally informative. Thanks,
Arn
[This message has been edited by Arns85GT (edited 01-22-2008).]
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08:38 AM
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
The 3rd picture down is a representation of air velocity (the arrows) highlighted by pressure...Here red indicates a faster velocity and blue indicates slowest, though it's hard to see, the pic is pretty crappy. This also suggests that the Fiero does indeed conform to Bernoulli's principle and generates lift instead of downforce.
There aren't actually any flow lines in those pictures, what is represented is pressure regions.
[This message has been edited by FastIndyFiero (edited 01-22-2008).]
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09:09 PM
Daviero Member
Posts: 382 From: Thunder Bay, ON Canada Registered: Jan 2006
Also I think you'll find the closer the lines the greater (or lower) the pressure is. -----------------------------------------------------------------------------------------------------------------------
I think you will find that the closer the lines are together, the higher the rate of change of pressure - just like the isobars on a weather map......
------------------ Daviero - 88 N* GT
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09:30 PM
Jan 23rd, 2008
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
Have you seen the video "Fiero: A Car is Born?" It has at least one clip of the Fiero undergoing some wind tunnel testing (supposedly one of the first cars GM did this with according to the narrator) and even shows a "smoke stream" or whatever flowing over the car, and then it is placed right behind the rear window where you can see it basically just stagnating. I just bought a copy off a guy on e-bay. I think boomtastic has it up on his web site possibly.
Click on the "click here to launch the boomtastic racing media player" and then select "promotional pontiac videos," where you'll see "Fiero: A Car is Born." I don't recollect where it is at in the video, but it's in there.
-Gary
[This message has been edited by FieroFanatic13 (edited 01-23-2008).]
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11:41 AM
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
Also, here is a tidbit (that I know others must have on hand but I've never really seen this addressed with this actual reported info) I just posted up in a separate new topic thread and in response to another "high speed instability" thread, but it seems applicable here I think. It's from an Technical Design Analysis of the Fiero that appeared in Road & Track in 1983...
*note the ALL CAPS SECOND PARAGRAPH:
"Aerodynamics may not have been a high priority consideration in the original Fiero design. With a relatively low weight and small frontal area (by American standards), good EPA mileage figures were possible without going for the ultimate in low drag coefficient. The reported Cx is 0.377, which is not bad for such a short car, but not too strong an advertising point either. The obvious problem is the notchback rear window, which is almost a necessary evil in a mid-engine car. Although it allows easy engine access and ventilation, it really disturbs the upper air flow and increases drag while reducing potential down force from any rear spoiler.
THE FIERO'S OTHER PROBLEM IS THE NOSE-UP LEADING EDGE OF THE FRONT BUMPER. THIS DESIGN ALLOWS A GOOD OPEN ANGLE AND RADIATOR INLET, BUT IT ALSO RAMS A LOT OF AIR DOWN UNDER THE NOSE. NOT ONLY DOES THIS USUALLY INCREASE DRAG, BUT IT ALSO GENERATES A LOT OF LIFT, IN SPITE OF THE BOTTOM-BREATHER RADIATOR INLET. REPORTED FRONT LIFT FIGURES WERE ABOUT 120 LB AT 100 MPH, WHICH CAN BE SIGNIFICANT WHEN THE STATIC FRONT WEIGHT IS JUST OVER 1100 LB.
It also appears that the opened headlight buckets were not as well researched in the wind tunnel as the Corvette's, as they raise the Cx to 0.417. Be that as it may, the pop-off plastic body panel concept means that better aerodynamics can be incorporated easily in the future.
Louvers would be a first step. Some models of the Lotus Elise and I think all Exiges have some very nice louvers. The back end of the Fiero won't be as clean aerodynamically without significant work, but anything to hold off flow separation would help.
Edit: Louvers won't be as aesthetically clean on a Fiero, either, without careful consideration.
[This message has been edited by Fiero Brick (edited 01-23-2008).]
The 3rd picture down is a representation of air velocity (the arrows) highlighted by pressure...Here red indicates a faster velocity and blue indicates slowest, though it's hard to see, the pic is pretty crappy. This also suggests that the Fiero does indeed conform to Bernoulli's principle and generates lift instead of downforce.
There aren't actually any flow lines in those pictures, what is represented is pressure regions.
the LIFT on the backend though is assuming a airtight rear, the model does not take into consideration venting from the high pressure zone under the car through the engine bay and into the low pressure zone.
I measured lower pressures back there but not a lot, there is an incredible wind coming up out of the vents (even more if you remove those silly water plates underneath) at highway speeds. Adding another vent in the back deck would further enhance engine bay cooling, reduce under car pressures, and reduce the fastback notch low pressure zone.
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07:52 PM
Jan 24th, 2008
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
Are you talking about adding an additional vent above the engine in the decklid?
And you have noted a lot of flow through the existing vents at speed? It seems I've heard talk on here that they don't flow a lot of air? I have no idea myself, other than that they DO flow UP from the bottom.
Very interesting piece. I especially enjoyed a couple of excerpts here:
"It wasn't until the 1960s that automakers noticed that if they reduce the slope of the back of the car to 20 degrees or less, the airflow will follow the roofline and drop off the back of the car, greatly reducing drag. The term for this design was called Fastback, and an excellent example is the Porsche 935/78, better known as the "Moby Dick." The Fastback design isn't without its flaws, especially in the area of lift. Because it has a very large surface area (the roof, in effect, has been extended to the bumper) in contact with airflow, it suffers from a low pressure on top of the car all the time. Usually, wings are used to combat this problem. It seems that good drag and good lift are mutually exclusive-you can't have both of them in equal amounts at the same time."
AND:
"To Change or Not to Change
When you modify your car in any way, you are changing the original design and the vehicle's overall characteristics. Sometimes that is a good thing, while other times it can lead to negative results. Below is a list of common modifications and their estimated change in percentage of the overall drag coefficient (Cd) of an average car. For example, the outside rearview mirrors add a two to four percent increase in the Cd. However, it should be noted that your results may vary.
Modification % Change
Lowering the vehicleby 30mm approx. -5 Smooth wheel covers -1 to -3 Sealing body gaps -2 to -5 Underbody panels -1 to -7 Concealed headlamps +3 to +10 Outside rearview mirrors +2 to +5 Airflow into the enginecompartment +4 to +14 Brake cooling devices +2 to +5 Interior ventilation approx. +1 Open windows approx. +5 Open sunroof approx. +2 Wide tires +2 to +4
Windows flush with exterior approx. -1 Roof-mounted surfboard rack approx. +40
Conversations and theories about aerodynamics abound in the automotive industry, and if you wanted to throw puns around, you could say it is a lot of hot air passing over stiff resistance. Does a car go fast enough to warrant a full body kit? Is a spoiler or airdam necessary on American highways? Will I affect my car's performance enough to offset the cost of the variety of packages on the market today? These questions can be argued until your car becomes an antique, but the only solid answer with any kernel of truth that everyone can agree on is that most aerodynamic-themed accessories make your car look cool. Whether they improve handling, fuel economy, or 60-foot times are issues best left to physicists, engineers, and bench racers (unless, of course, you have the time slips to prove it)."
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10:32 AM
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
On side note about aerodynamics and the Fiero, I find it interesting that the BEST aero numbers for a stock Fiero are the original Pace Car, '85 GT and '86-'87 SE Aero Package and possibly the 87-88 base body (with wing/spoiler) styles. I think the common expectation for most would be that the "fastback" GT would have been the best, but it's not a true fastback, and either those buttresses or those vertical tail lights apparently reduced aero efficiency a little compared to the notchback aero cars. I found that interesting...
Fiero Drag Coefficients I've found in various road tests:
1984 - 1986 Base Coupe: .377
1984 - 1986 Base Coupe (with wider 215/60/14 tires) : .41
1987 - 1988 Base Coupe: .357
1987 - 1988 Base Coupe (with wing/spoiler): .35
1984 Indy, 1985 GT, 1986-87 SE (WITH wing/spoiler): .35
1986-88 GT "fastback": .36
[This message has been edited by FieroFanatic13 (edited 01-25-2008).]
Yours is an interesting compilation of stats, FieroFanatic13.
If they're not "apples to oranges" comparisons, it appears that based solely upon the six items you've posted, the biggest "single-variable" difference --- where only one thing at a time is being changed --- is simply the presence or absence of slightly larger tires on the `84-`86 Base Coupe (.41 with them, versus .377 without them).
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08:23 PM
Jan 27th, 2008
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
Yours is an interesting compilation of stats, FieroFanatic13.
If they're not "apples to oranges" comparisons, it appears that based solely upon the six items you've posted, the biggest "single-variable" difference --- where only one thing at a time is being changed --- is simply the presence or absence of slightly larger tires on the `84-`86 Base Coupe (.41 with them, versus .377 without them).
The stats for the early body style WERE only the Tires for the most part...those wider tires on the standard body from 84-86 certainly impacted the drag quite a bit...
BUT, keep in mind that the Aero bodied cars came WITH THE WIDER TIRES and STILL HAD A LOWER drag coefficient than the base model body with skinnier tires.
-Gary
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07:24 PM
Jan 28th, 2008
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
Are you talking about adding an additional vent above the engine in the decklid?
And you have noted a lot of flow through the existing vents at speed? It seems I've heard talk on here that they don't flow a lot of air? I have no idea myself, other than that they DO flow UP from the bottom.
-Gary
Yes add more vents to help relieve the low pressure zone in that location. a vent at the rear of the engine compartment and front along with removign the rain plates on the existing vents will dramatically increase the flow. Just removing the rain guards makes a huge difference on the Fastback air movement in back there. (from my readings, it made a big difference in the area of each vent.)
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08:21 PM
Jan 29th, 2008
edfiero Member
Posts: 965 From: Coatesville, PA Registered: Nov 2004
So lets get to some usable information. With 2 otherwise identical cars, if you decrease the drag coeffient from .40 to .35, what does that translate to in gas mileage improvement??
[This message has been edited by edfiero (edited 01-29-2008).]
So lets get to some usable information. With 2 otherwise identical cars, if you decrease the drag coeffient from .40 to .35, what does that translate to in gas mileage improvement??
Edmunds.com says:
How Drag Affects Mileage
When you consider aerodynamics from a fuel economy standpoint, you're primarily looking at coefficient of drag (known in the business as "Cd"). Essentially, this is how easily a vehicle moves through the air, though drag isn't the only factor that is considered. "There's more to aerodynamics than just drag," says Doug Frasher, strategic design chief at the Volvo Monitoring & Concept Center. "There's downforce and lift. And there is yawing moment, which is basically when you're in a crosswind, how much does the vehicle get steered by the wind that is pushing on it? And then there's noise. So we try to look for all of those factors.
"For a full-size truck, a change in drag coefficient of 0.01 is approximately equal to an improvement in fuel economy of 0.1 mpg on the combined city/highway driving cycle," says GM's Schenkel. "The same drag coefficient reduction can improve a car's fuel economy by approximately 0.2 mpg."
Volvo's Frasher says the force acting against a car by the air it moves is a function of:
Cd x Frontal Area x Density of Air x Speed Squared
Speed clearly is an important part of the equation. At stop-and-go speeds, drag isn't a big deal, but the faster you go, the more it matters. At 70 mph, you've got four times the force working against your vehicle that you have at 35 mph.
To put Cd changes in perspective, Frasher put some numbers to a hypothetical sedan. Our imaginary car has a curb weight of 3,527 pounds, a Cd of 0.30, a frontal area of 23.7 square feet and 9 pounds of rolling resistance for every 1,000 pounds of weight.
(BUT HERE IS THE KICKER IF YOU ASK ME
According to Frasher, "If we put a gas-burning engine in this car, expect reasonable performance and drive it on a combined driving cycle, we can expect to get 23.8 mpg…. Add 10 percent to the drag coefficient, we'll now get 23.3 mpg…. Take 10 percent from the drag coefficient, we'll now get 24.3 mpg."
[This message has been edited by FieroFanatic13 (edited 01-30-2008).]
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09:27 AM
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
SO, by the math according to that article on Edmunds at least, reducing your drag coefficient from .41 to .35 would mean NOT VERY MUCH in the MPG department.
Notice that said "on a combined driving cycle". That means both stop & go city driving & highway driving. On the highway it would be a lot more mileage increase. Going from .40 to .35 is an 8% decrease in drag, which I would think would result in at least a 5% increase in gas mileage on the highway - probably more in a lightweight car with narrow tires & a 5-speed. 5% is substantial. ~ Paul aka "Tha Driver"
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01:17 PM
FieroFanatic13 Member
Posts: 3521 From: Big Rapids, MI, USA Registered: Jul 2006
Notice that said "on a combined driving cycle". That means both stop & go city driving & highway driving. On the highway it would be a lot more mileage increase. Going from .40 to .35 is an 8% decrease in drag, which I would think would result in at least a 5% increase in gas mileage on the highway - probably more in a lightweight car with narrow tires & a 5-speed. 5% is substantial. ~ Paul aka "Tha Driver"
But in the end, don't we measure by what we get in a driving cycle? I see what you mean that it would really impact someone who drives primarily on the highway though. I can't speak to your estimated %5, but it would help if you weren't doing "the cycle" as much...
Could you post a link to the site? 
