Aerodynamics flow diagram?
12-05-07, 10:19 AM
#26
Rider of the Sky
I really wanted to chime in at the start of this thread, but Black91n/a has this one completely covered. I will say though that
= wrong
Flip a aircraft wing upside down and it'll still be a wing. Indy cars use anywhere from single to three-element wings, and I promise it's not to make them fly.
= wrong
Flip a aircraft wing upside down and it'll still be a wing. Indy cars use anywhere from single to three-element wings, and I promise it's not to make them fly.
12-05-07, 03:02 PM
#27
Original poster
That flow diagram shows the base model RX-7 without any aero kit.
The "GTU" additions you are asking about are the factory "aero kit" that included the 3 piece front lip, aluminum belly pan, pieces in front of rear wheels and the rear spoiler.
They were designed in Mazda's Miyoshi wind tunnel to compliment the 2nd gen RX-7 and were available as an option on all 2nd gens as well as being standard on the sport model in the US.
The "aero kit" dropped the base model Cd from .31 to .29 as well as halving the rear coefficient of lift from .14 to .07.
Front coefficient of lift stayed the same at .08. The Base model with its 185/70-14 tires has a Cd X A of .56.
From the Yamaguchi 2nd gen book.
I just needed a flow diagram for the 2nd underneath tray and other aero dynamic additions to the GTU.
The "GTU" additions you are asking about are the factory "aero kit" that included the 3 piece front lip, aluminum belly pan, pieces in front of rear wheels and the rear spoiler.
They were designed in Mazda's Miyoshi wind tunnel to compliment the 2nd gen RX-7 and were available as an option on all 2nd gens as well as being standard on the sport model in the US.
The "aero kit" dropped the base model Cd from .31 to .29 as well as halving the rear coefficient of lift from .14 to .07.
Front coefficient of lift stayed the same at .08. The Base model with its 185/70-14 tires has a Cd X A of .56.
From the Yamaguchi 2nd gen book.
12-05-07, 05:26 PM
#28
i didnt know that the sport had so many extra goodies. my car is a sport and it has the "duckbill" spoiler and the alum undertray extension. but i didnt see any extra parts on it. but anyway.. now it has all s5 aero except for the spoiler and alum undertray.
neat!
neat!
12-07-07, 08:03 AM
#29
Rotary Freak
Join Date: Dec 2006
Location: New Hampshire
Posts: 1,791
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Evil Aviator
You actually listened to somebody who thinks the Amero is real?
Damnit.....
Nz, I knew how well the reverse vented hoods work, I was just thinking of forward venting which wouldn't work I don't think I don't know why I was thinking that but I guess I was really screwed up since I was misinformed
Last edited by RotaMan99; 12-07-07 at 08:12 AM.
12-07-07, 08:19 AM
#30
meh
Join Date: Sep 2002
Location: Tallahassee
Posts: 801
Likes: 0
Received 0 Likes
on
0 Posts
Oh man, not again... 
How can one simple drawing be responsible for so much misinformation posted again and again?
The diagrams represent force vectors, i.e. the direction the air's static pressure is pressing against the car. Arrows pointing away from the car represent pressure lower than ambient; arrows pointing into from the car represent pressure higher than ambient.
No, airflow traveling up the windscreen separates from the body at the sudden change of direction at the top, the reattaches further along. The same thing happen at the top of the rear windscreen and again at the tail.
The bottom of the windscreen is obviously a high pressure area, again because the air is being forced to change direction quickly. This is why nearly every car in the world has the ventilation intake located there, and why air flows into the cabin even if the fan's off.
Sorry, but you really don't understand how aerodynamics work. That's completely wrong.
Here's an Autospeed article that tests just that. The pressure difference above and below the hood at various points was measured to find the ideal location of a vent. The results speak for themselves.
http://www.autospeed.com/cms/A_2162/article.html
If you aren't a member, this pic shows the pressure differences measured. Positive numbers show where the pressure above the hood is greater than the pressure below it. Negative numbers show where the pressure above the hood is less than the pressure below it. See where all the negative numbers are?
http://us1.webpublications.com.au/st...1/2162_7mg.jpg
Venting close to the front of the hood is the proper way to vent the engine bay, and there are hundreds of OEM vented hoods and properly developed race car hoods that do exactly that.
How can one simple drawing be responsible for so much misinformation posted again and again?The diagrams represent force vectors, i.e. the direction the air's static pressure is pressing against the car. Arrows pointing away from the car represent pressure lower than ambient; arrows pointing into from the car represent pressure higher than ambient.
No, airflow traveling up the windscreen separates from the body at the sudden change of direction at the top, the reattaches further along. The same thing happen at the top of the rear windscreen and again at the tail.
The bottom of the windscreen is obviously a high pressure area, again because the air is being forced to change direction quickly. This is why nearly every car in the world has the ventilation intake located there, and why air flows into the cabin even if the fan's off.
Sorry, but you really don't understand how aerodynamics work. That's completely wrong.
Here's an Autospeed article that tests just that. The pressure difference above and below the hood at various points was measured to find the ideal location of a vent. The results speak for themselves.
http://www.autospeed.com/cms/A_2162/article.html
If you aren't a member, this pic shows the pressure differences measured. Positive numbers show where the pressure above the hood is greater than the pressure below it. Negative numbers show where the pressure above the hood is less than the pressure below it. See where all the negative numbers are?
http://us1.webpublications.com.au/st...1/2162_7mg.jpg
Venting close to the front of the hood is the proper way to vent the engine bay, and there are hundreds of OEM vented hoods and properly developed race car hoods that do exactly that.
12-07-07, 10:09 AM
#32
A cowl hood isn't meant to release hot air from the engine bay, instead it brings cool air to the intake air filter on a typical carbed v8 motor. On that car(LT1) it wouldn't work because the air filter is far away from the cowl.
12-07-07, 10:29 AM
#34
Lives on the Forum
The bottom of the windscreen is a high pressure area, that is a FACT. No matter how many of you are ignorant, it doesn't change the fact that it is at high pressure and that lines in show high pressure (air pushing on the surface) and lines out show low pressure (air "pulling" on the surface).
12-07-07, 10:38 AM
#35
Oh man, not again... How can one simple drawing be responsible for so much misinformation posted again and again?
The diagrams represent force vectors, i.e. the direction the air's static pressure is pressing against the car. Arrows pointing away from the car represent pressure lower than ambient; arrows pointing into from the car represent pressure higher than ambient.
No, airflow traveling up the windscreen separates from the body at the sudden change of direction at the top, the reattaches further along. The same thing happen at the top of the rear windscreen and again at the tail.
The bottom of the windscreen is obviously a high pressure area, again because the air is being forced to change direction quickly. This is why nearly every car in the world has the ventilation intake located there, and why air flows into the cabin even if the fan's off.
Sorry, but you really don't understand how aerodynamics work. That's completely wrong.
Here's an Autospeed article that tests just that. The pressure difference above and below the hood at various points was measured to find the ideal location of a vent. The results speak for themselves.
http://www.autospeed.com/cms/A_2162/article.html
If you aren't a member, this pic shows the pressure differences measured. Positive numbers show where the pressure above the hood is greater than the pressure below it. Negative numbers show where the pressure above the hood is less than the pressure below it. See where all the negative numbers are?
http://us1.webpublications.com.au/st...1/2162_7mg.jpg
Venting close to the front of the hood is the proper way to vent the engine bay, and there are hundreds of OEM vented hoods and properly developed race car hoods that do exactly that.
How can one simple drawing be responsible for so much misinformation posted again and again?The diagrams represent force vectors, i.e. the direction the air's static pressure is pressing against the car. Arrows pointing away from the car represent pressure lower than ambient; arrows pointing into from the car represent pressure higher than ambient.
No, airflow traveling up the windscreen separates from the body at the sudden change of direction at the top, the reattaches further along. The same thing happen at the top of the rear windscreen and again at the tail.
The bottom of the windscreen is obviously a high pressure area, again because the air is being forced to change direction quickly. This is why nearly every car in the world has the ventilation intake located there, and why air flows into the cabin even if the fan's off.
Sorry, but you really don't understand how aerodynamics work. That's completely wrong.
Here's an Autospeed article that tests just that. The pressure difference above and below the hood at various points was measured to find the ideal location of a vent. The results speak for themselves.
http://www.autospeed.com/cms/A_2162/article.html
If you aren't a member, this pic shows the pressure differences measured. Positive numbers show where the pressure above the hood is greater than the pressure below it. Negative numbers show where the pressure above the hood is less than the pressure below it. See where all the negative numbers are?
http://us1.webpublications.com.au/st...1/2162_7mg.jpg
Venting close to the front of the hood is the proper way to vent the engine bay, and there are hundreds of OEM vented hoods and properly developed race car hoods that do exactly that.
http://www.autospeed.com/cms/A_2162/article.html
They are showing boxy cars, not streamlined cars like the rx-7. The flat front is creating a high press. area, and when the air flows up (because the front is flat and some of the air has to go up over the top) over the front of the hood it is still moving upward creating a low pressure spot just beind the front bumper on the hood.
Picture it as if you were to put your had out a car window at 50mph.. which way do you have to push your hand? against the force of the wind resistance. Its that simple. Thats the reaction force.
12-07-07, 11:08 AM
#36
This is interesting info. In addition to Mazda's dedication to improvements in the aero department, we can see by this illustration why our cars have such a solid "feel" and are relatively rattle-free compared to other car manufacturers. I'm impressed with the techniques used at obtaining structural rigidity. I had a Mazda B2000 that I drove for 14 years that felt as solid when I sold it as it did when new. Good stuff.
12-07-07, 01:36 PM
#37
Lives on the Forum
Air is essentially incompressible at speeds below about 0.3 Mach, which is where cars operate. That is a very important point to remember and has s heavy influence on how the flow behaves. It doesn't compress as the car moves through the flow. Air accelerates as it's deflected out of the way by a moving object like a car, as it speeds up the pressure drops, as is shown by Bernoulli.
It DOES show the higher pressure on the nose, look at the inward facing lines on the very front, up to where the nose goes to about 45 degrees. Over top of the hood the air is being deflected and therefore it is being accelerated and the pressure drops, causing the low pressure on the front of the hood. As it reaches the base of the windshiled it deccelerates due to friction and stagnates (stops as it hits the windshield) and the pressure is higher there. As it goes across the back of the car it decelerates as the disturbance lessens (sloping hatch) so the air decelerates and the pressure returns to normal. There's a turbulent wake behind the car, causing a low pressure area causing pressure drag.
Besides, in the page from Mazda themselves it states a positive coefficient of lift, meaning the car creates lift, and if the arrows pointing up were a higher pressure then the car would make downforce, and that's not the case, so arrows out mean lower pressure.
If you want to know more about car aero, read the package of articles from Racecar Engineering here: http://www.fatcatmotorsports.com/iga...o_articles.zip
If you don't have any fluid dynamics or aero knowledge then please don't post your doubts about this, since they are completely baseless and totally wrong.
OUT MEANS LOWER PRESSURE, IN MEANS HIGHER PRESSURE, THIS IS A FACT!!!!!
It DOES show the higher pressure on the nose, look at the inward facing lines on the very front, up to where the nose goes to about 45 degrees. Over top of the hood the air is being deflected and therefore it is being accelerated and the pressure drops, causing the low pressure on the front of the hood. As it reaches the base of the windshiled it deccelerates due to friction and stagnates (stops as it hits the windshield) and the pressure is higher there. As it goes across the back of the car it decelerates as the disturbance lessens (sloping hatch) so the air decelerates and the pressure returns to normal. There's a turbulent wake behind the car, causing a low pressure area causing pressure drag.
Besides, in the page from Mazda themselves it states a positive coefficient of lift, meaning the car creates lift, and if the arrows pointing up were a higher pressure then the car would make downforce, and that's not the case, so arrows out mean lower pressure.
If you want to know more about car aero, read the package of articles from Racecar Engineering here: http://www.fatcatmotorsports.com/iga...o_articles.zip
If you don't have any fluid dynamics or aero knowledge then please don't post your doubts about this, since they are completely baseless and totally wrong.
OUT MEANS LOWER PRESSURE, IN MEANS HIGHER PRESSURE, THIS IS A FACT!!!!!
12-07-07, 02:41 PM
#38
(Long - read to the end or I won't make sense)
Yes - as odd as it looks (to those less familiar), the diagram above shows the force of the air on the car. These are *not* reaction forces by the car itself to the air. Think of it this way...picture the air hitting the nose of the car. The air just doesn't flow around the nose and over the hood undisturbed. It bounces off, creating a low pressure area temporarily. But because of this low pressure area, air will want to move back to this area eventually (depending on speed). As the negative pressure diminishes farther away from the nose, you get to the point near the base of the windshield where the air starts to come back, which becomes positive pressure. Near the back, the negative pressure left by the profile of the car goes down, and increases to positive pressure near the back, if a spoiler was present (automotive type) (not present in the airflow diagram).
Also, it will help if we remember that wings and spoilers are often thrown about verbally and this leads to confusion when we look at their uses in the automotive and aerospace fields. A "wing" or "spoiler" can refer to the same thing in the automotive industry. A wing as usually found in our cars in this sense is more like a spoiler, because it spoils the air, creates downforce (and drag). On race cars, the rear wing is truly a wing (I'm referring to cars like F1, CART and IRL, but others are included), mounted essentially upside-down. In the aerospace industry, a spoiler spoils the airflow (and lift) created by the wing as mentioned above.
The thing that helps me to visualize what happens is to focus on the front of the car.
Yes - as odd as it looks (to those less familiar), the diagram above shows the force of the air on the car. These are *not* reaction forces by the car itself to the air. Think of it this way...picture the air hitting the nose of the car. The air just doesn't flow around the nose and over the hood undisturbed. It bounces off, creating a low pressure area temporarily. But because of this low pressure area, air will want to move back to this area eventually (depending on speed). As the negative pressure diminishes farther away from the nose, you get to the point near the base of the windshield where the air starts to come back, which becomes positive pressure. Near the back, the negative pressure left by the profile of the car goes down, and increases to positive pressure near the back, if a spoiler was present (automotive type) (not present in the airflow diagram).
Also, it will help if we remember that wings and spoilers are often thrown about verbally and this leads to confusion when we look at their uses in the automotive and aerospace fields. A "wing" or "spoiler" can refer to the same thing in the automotive industry. A wing as usually found in our cars in this sense is more like a spoiler, because it spoils the air, creates downforce (and drag). On race cars, the rear wing is truly a wing (I'm referring to cars like F1, CART and IRL, but others are included), mounted essentially upside-down. In the aerospace industry, a spoiler spoils the airflow (and lift) created by the wing as mentioned above.
The thing that helps me to visualize what happens is to focus on the front of the car.
12-07-07, 03:02 PM
#39
Ok... sorry I'm not a scientist and don't understand a lot of the lingo used in your link.
But, all complex terminology aside.
If I put my hand out the window of a car at 50mph I have to push it against the wind.
In the pictures you can see the air being compressed... or what ever you want to call it.
Refer back to the aerodynamic forces pic and notice the arrows...
see where the flow lines are closer together in the wind tunnel pic and similarly the aerodynamic forces picture the arrows are pointed outward.
But, all complex terminology aside.
If I put my hand out the window of a car at 50mph I have to push it against the wind.
In the pictures you can see the air being compressed... or what ever you want to call it.
Refer back to the aerodynamic forces pic and notice the arrows...
see where the flow lines are closer together in the wind tunnel pic and similarly the aerodynamic forces picture the arrows are pointed outward.
12-07-07, 03:36 PM
#41
Lives on the Forum
I've studied fluid dynamics and aerodynamics, Nick_d_TII, you obviously haven't, so how about we listen to me?
Having the smoke lines become closer together doesn't show that the air is being compressed, it doesn't compress or expand in any significant way in flows of these speeds. All that is showing is where the flow is going, and in fact when the lines become closer the flow is moving faster and the pressure is lower. The hand out the window shows stagnation at the front causing high pressure and the low pressure turbulent wake behind. A car isn't a hand and the flow over the hood acts differently because it's not just hitting a blunt object, it's flowing over a streamlined object. You can see in the diagram that at the very front there is stagnation and there is higher pressure (arrows in) and at the rear there is the low pressure wake (arrows out).
When looking at the diagram remember that those are forces normal(perpendicular, at 90 degrees to) to the surface, and those lines on the hood are basically acting up, so that's not having much impact on drag at all.
Having the smoke lines become closer together doesn't show that the air is being compressed, it doesn't compress or expand in any significant way in flows of these speeds. All that is showing is where the flow is going, and in fact when the lines become closer the flow is moving faster and the pressure is lower. The hand out the window shows stagnation at the front causing high pressure and the low pressure turbulent wake behind. A car isn't a hand and the flow over the hood acts differently because it's not just hitting a blunt object, it's flowing over a streamlined object. You can see in the diagram that at the very front there is stagnation and there is higher pressure (arrows in) and at the rear there is the low pressure wake (arrows out).
When looking at the diagram remember that those are forces normal(perpendicular, at 90 degrees to) to the surface, and those lines on the hood are basically acting up, so that's not having much impact on drag at all.
12-07-07, 04:54 PM
#42
I'm a boost creep...
Join Date: Jan 2002
Location: Auckland, New Zealand
Posts: 15,608
Likes: 0
Received 8 Likes
on
8 Posts
It works exactly the same way as the ventilation intake in my post above. It's a high pressure area, so air is naturally forced in.
No, no, no! You've just said basically the same as what Rotaman said, I just explained that this is completely wrong. Go back and read it again.
Think that all you like, I have hundreds of years of aerodynamic research to back me up.
They are not reaction forces, they are the the forces created by the air acting on the body. You can argue this 'til you're blue in the face, but you are WRONG.
Think that all you like, I have hundreds of years of aerodynamic research to back me up.
The arrows show the force reactions against the wind/air. The large arrows pointing away from the car show that the wind/air at a given speed is equally pushing against that much. They are reaction forces! Just think about it... the nose of the car is breaking the wind which has higher pressure/greater forces on it.
12-07-07, 08:23 PM
#43
ok... ok... I was just trying to make a simple assumption for some of us that aren't aerodynamic engineers as you may be. I retract my comment that your 100% incorrect, as you are 100% correct. My understanding is a little or a lot off base here.
I'm not a aerodynamic or fluid dynamic engineer.
When people do reverse hood scoops(cowls) doesn't that create a, mind my ignorant terminology, low pressure area that sucks the air out?
I'm trying to be constructive here, thanks for the references but I have a hard time understanding those complex formulas and terminology.
This stuff is obviously too complicated for me...
I'm not a aerodynamic or fluid dynamic engineer.
When people do reverse hood scoops(cowls) doesn't that create a, mind my ignorant terminology, low pressure area that sucks the air out?
I'm trying to be constructive here, thanks for the references but I have a hard time understanding those complex formulas and terminology.
This stuff is obviously too complicated for me...
Last edited by Nick_d_TII; 12-07-07 at 08:25 PM. Reason: pic
12-07-07, 08:27 PM
#44
Rotary Enthusiast
iTrader: (3)
Join Date: Jan 2007
Location: st. louis
Posts: 1,000
Likes: 0
Received 0 Likes
on
0 Posts
they call the reverse scoop hoods cowl INDUCTION hoods for a reason... to most people it would seem *** backwards but there is positive pressure at teh base of the windscreen. ill try to think up a good explanation as to why it is positive pressure at the base
12-07-07, 08:38 PM
#45
I'm a boost creep...
Join Date: Jan 2002
Location: Auckland, New Zealand
Posts: 15,608
Likes: 0
Received 8 Likes
on
8 Posts
12-07-07, 08:50 PM
#47
Rotary Enthusiast
iTrader: (3)
Join Date: Jan 2007
Location: st. louis
Posts: 1,000
Likes: 0
Received 0 Likes
on
0 Posts
not entirely sure this is correct but if i were to make a quick guess as to why it is that the air stagnates there and thus has a higher pressure. its friday and im done thinking for the day so thats all you are going to get
12-07-07, 10:32 PM
#49
Lives on the Forum
The surface colour is Cp, it tells you about the relative pressures of the air. Ignore the lines (the colour of the lines tells you nothing about the pressure, only the speed) and concentrate on the surface. Basically red, orange, yellow means higher than ambient pressure, blues mean lower than ambient, green is ambient.
Stagnation is what it's called when the air hits a solid object, it is forced to stop (or at least slow down and get re-directed) and this creates a higher pressure, this is what happens at the base of the windshield and is why that's a high pressure area and this is what a cowel induction hood utilises to get air to the engine.
A cowel induction hood could possibly let air out the back if there's enough effect from the air being rammed into the front causing pressure in the engine bay, but if done properly it'll be sealed off from everything but the engine intake. It'll be dependant on the hood and the car as to what the exact effect would be, it'll be more pronounced on a car with a steeper windshield.
The actual differences in pressure are fairly small, but it acts over a large area, making the effects large.
Stagnation is what it's called when the air hits a solid object, it is forced to stop (or at least slow down and get re-directed) and this creates a higher pressure, this is what happens at the base of the windshield and is why that's a high pressure area and this is what a cowel induction hood utilises to get air to the engine.
A cowel induction hood could possibly let air out the back if there's enough effect from the air being rammed into the front causing pressure in the engine bay, but if done properly it'll be sealed off from everything but the engine intake. It'll be dependant on the hood and the car as to what the exact effect would be, it'll be more pronounced on a car with a steeper windshield.
The actual differences in pressure are fairly small, but it acts over a large area, making the effects large.
12-07-07, 11:45 PM
#50
I'm a boost creep...
Join Date: Jan 2002
Location: Auckland, New Zealand
Posts: 15,608
Likes: 0
Received 8 Likes
on
8 Posts
Cortesy of the Autospeed article I linked to above about siting bonnet vents (outlets, not inlets):
"The pressure on the surface of the bodywork depends on the aerodynamic flows over it. This DaimlerChrysler graphic shows the typical frontal pressure distribution of a car. Looking just at the bonnet you can see that there is low pressure (blue) where the air wraps around the leading edge of the bonnet, grading to high pressure (green) as the air reaches the obstruction which is the windscreen.
So you wouldn’t site a bonnet outlet vent close to the windscreen – in fact that’s usually where the cabin ventilation inlet ducts are... they’re taking advantage of that high pressure! Looking at just the exterior pressures, what we want at the vent location is the very lowest external pressure."
Looks all not forget that the image being argued about is from a Mazda sales brochure. It is not an engineering diagram, it's one being used to sell a car. Don't get too hung up on it.
"The pressure on the surface of the bodywork depends on the aerodynamic flows over it. This DaimlerChrysler graphic shows the typical frontal pressure distribution of a car. Looking just at the bonnet you can see that there is low pressure (blue) where the air wraps around the leading edge of the bonnet, grading to high pressure (green) as the air reaches the obstruction which is the windscreen.
So you wouldn’t site a bonnet outlet vent close to the windscreen – in fact that’s usually where the cabin ventilation inlet ducts are... they’re taking advantage of that high pressure! Looking at just the exterior pressures, what we want at the vent location is the very lowest external pressure."
Looks all not forget that the image being argued about is from a Mazda sales brochure. It is not an engineering diagram, it's one being used to sell a car. Don't get too hung up on it.