Let's talk aerodynamics
06-17-2003, 06:20 AM
#31
Drifting
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The rear diffuser in the so-called 'ground effects' is not to create downforce, but to reduce drag. The principle is that as the air is travelling faster under the car (it goes in a straight line.... smoothly rather than over a lumpy shape), then it out from the undertray at a faster speed that the air travelling from the top/sides, thus creating air pockets behind the car and thus creating drag. The idea of the diffuser is that the volume of air is expanded, and thus the air from under the car is slowed to the same speed as it arrives from the other parts of the vehicle, and thus reduces drag.
This is the reason that no two F1 cars have the same diffuser design. If that was the case, then they would slow down/not slow down the air enough!
I do also agree though that the rear valance on the 951 creates downforce. It's basically a wing under the car!
As for the Article Thom provided, It's a pretty good read. But I can't see where he states that the vented nose panels won't work!?? All I see is that he concludes about the null-speed air behind the headlights (pretty much expected!)
This is the reason that no two F1 cars have the same diffuser design. If that was the case, then they would slow down/not slow down the air enough!
I do also agree though that the rear valance on the 951 creates downforce. It's basically a wing under the car!
As for the Article Thom provided, It's a pretty good read. But I can't see where he states that the vented nose panels won't work!?? All I see is that he concludes about the null-speed air behind the headlights (pretty much expected!)
06-17-2003, 06:59 AM
#32
Nordschleife Master
The term "diffuser" would make you think that, wouldn't it? But the trick is that the air on TOP of an airfoil is actually travelling faster than the air below, even though it is travelling a greater distance!!! I'm not 100% sure if this holds true for F1 cars, because they are not shaped like a cross section of a wing, or like a 911...
Danno was right, faster moving air means less pressure. The airflow on TOP of a wing is a LOW Pressure area, and thus the air above is moving faster than the air below.
Same thing on a car, the air on TOP is moving faster, causing a low pressure zone, and causing lift. (As we agreed on earlier!)
So no, the diffuser doesn't slow the air down, that would be counter-intuitive to what you want.
Danno was right, faster moving air means less pressure. The airflow on TOP of a wing is a LOW Pressure area, and thus the air above is moving faster than the air below.
Same thing on a car, the air on TOP is moving faster, causing a low pressure zone, and causing lift. (As we agreed on earlier!)
So no, the diffuser doesn't slow the air down, that would be counter-intuitive to what you want.
06-17-2003, 07:45 AM
#33
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Rich Sandor:
<strong>If he needs a more accurate model, I can supply one myself. Just tell me what format to put it in.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Thanks in advance for posting it here.
<strong>If he needs a more accurate model, I can supply one myself. Just tell me what format to put it in.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Thanks in advance for posting it here.
06-17-2003, 07:47 AM
#34
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by pikey7:
<strong>But I can't see where he states that the vented nose panels won't work!??</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">He does not state it, that has only been my opinion so far. Thus I'm looking forward to seeing charts demonstrating I am wrong
<strong>But I can't see where he states that the vented nose panels won't work!??</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">He does not state it, that has only been my opinion so far. Thus I'm looking forward to seeing charts demonstrating I am wrong
06-17-2003, 08:37 AM
#35
Nordschleife Master
Thom, can he import the 3d model in any format? I have one that I can put into 3DS Max format pretty quick, but it needs some tweaking first on the nose section. Does it matter how many polygons the object is? I would guess that the software needs very high res models to model airflow accurately, but the more polygons in the model, the slower it will load in the software.
I'd love to see a side view of the whole car model with airflow lines!!! That software he's using must be $$$$ big bucks!
I'd love to see a side view of the whole car model with airflow lines!!! That software he's using must be $$$$ big bucks!
06-18-2003, 12:12 AM
#36
Drifting
Sorry I couldn't let this just die.....
[QUOTE]Originally posted by Rich Sandor:
[QB]Okay, we can agree that having the undertrays smooth out the underbody airflow, right?
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Right.
"We all agree that running WITH the undertrays reduces oversteer. Therefor, it gives you UNDERSTEER, therefore, there is less weight on the front end of the car, or suspension. There fore, it is LIFTING the car. Therefore, having the undertray present, induces more lift."
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No we don't all agree. Read Danno's post at the top of this page.
"HOWEVER, the smooth airflow it creates, reduces Form Drag at lower speeds, but increases Induced Drag at higher speeds. So it's a trade off."
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This I would agree with if we were discussing wing tip vortices on a two dimensional wing but on a three dimensional object the math gets too complicated for me this late.
"Ground effect as you described it is a misnomer. In aerodynamics, ground effect reduces drag by eliminating downwash from the wing at low levels. It does not affect lift at all, it reduces drag only, and therefore increases speed and acceleration."
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Really? You need to read your book again. A wing on a road car is an inverted airplane wing right? The lift of a wing increases with ground proximity. So that would also mean the the same wing inverted would increase downforce with ground proximity right? I can referance more than a dozen books on aerodynamics to support this if you like. The whole concept of using the underside of a car to create downforce is not a new one. It goes back to the '70's. As Danno mentioned Ferrari has been doing this at least since the 355 and has been quite successful at creating downforce with little to no wing or splitter on their cars. McLaren did this as well and even introduced two fans to aid in air flow under the chassis.
"A car is already on the ground, and therfore always benefits from ground effect. Having air flow smoothly under the car is not ground effect. That is a term that was stolen from the aviation world and bastardised for Formula One, and then again stolen and used by the ricer body kit crowd."
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Not really. Aviation aerodynamics and automotive aerodynamics are based on the same laws of physics. The term "ground effects" has been misused by the auto industry in many ways. It takes more than smooth air flow under the car to create ground effect. Maybe I should have been more clear but I didn't really intend for all of this to get too deep as I am not really sure everyone else is really that interested.
"Ground effects in formula one, as it was originally used, constituted of a plastic skirt around the rocker panels of the car, which actually made full contact with the ground, preventing air from exiting out the sides, and funneled it all to the back, where a rear diffuser shot it upwards, giving more downforce. It was so effective, that cornering speeds increased to such a point that the FIA banned them right away."
--------------------------------------------------
Again no. It takes more than a plastic skirt and a diffuser to get you there. Formula One and Indy cars use a sculpted bottom and tunnels to create downforce but that's another subject. The whole idea is to shape and smooth the bottom of the car to mimic the shape of a wing, turn the rear up and add a longitudinal diffuser (which is just a form of wing or airfoil).
The point being that if you can properly create gound effect you can add considerable downforce with minimal drag. This is why some race car rules prohibit or limit the contouring of the underbody. I have done work with aerodynamics and have been successful doing so.
[QUOTE]Originally posted by Rich Sandor:
[QB]Okay, we can agree that having the undertrays smooth out the underbody airflow, right?
-------------------------------------------------
Right.
"We all agree that running WITH the undertrays reduces oversteer. Therefor, it gives you UNDERSTEER, therefore, there is less weight on the front end of the car, or suspension. There fore, it is LIFTING the car. Therefore, having the undertray present, induces more lift."
--------------------------------------------------
No we don't all agree. Read Danno's post at the top of this page.
"HOWEVER, the smooth airflow it creates, reduces Form Drag at lower speeds, but increases Induced Drag at higher speeds. So it's a trade off."
-------------------------------------------------
This I would agree with if we were discussing wing tip vortices on a two dimensional wing but on a three dimensional object the math gets too complicated for me this late.
"Ground effect as you described it is a misnomer. In aerodynamics, ground effect reduces drag by eliminating downwash from the wing at low levels. It does not affect lift at all, it reduces drag only, and therefore increases speed and acceleration."
--------------------------------------------------
Really? You need to read your book again. A wing on a road car is an inverted airplane wing right? The lift of a wing increases with ground proximity. So that would also mean the the same wing inverted would increase downforce with ground proximity right? I can referance more than a dozen books on aerodynamics to support this if you like. The whole concept of using the underside of a car to create downforce is not a new one. It goes back to the '70's. As Danno mentioned Ferrari has been doing this at least since the 355 and has been quite successful at creating downforce with little to no wing or splitter on their cars. McLaren did this as well and even introduced two fans to aid in air flow under the chassis.
"A car is already on the ground, and therfore always benefits from ground effect. Having air flow smoothly under the car is not ground effect. That is a term that was stolen from the aviation world and bastardised for Formula One, and then again stolen and used by the ricer body kit crowd."
--------------------------------------------------
Not really. Aviation aerodynamics and automotive aerodynamics are based on the same laws of physics. The term "ground effects" has been misused by the auto industry in many ways. It takes more than smooth air flow under the car to create ground effect. Maybe I should have been more clear but I didn't really intend for all of this to get too deep as I am not really sure everyone else is really that interested.
"Ground effects in formula one, as it was originally used, constituted of a plastic skirt around the rocker panels of the car, which actually made full contact with the ground, preventing air from exiting out the sides, and funneled it all to the back, where a rear diffuser shot it upwards, giving more downforce. It was so effective, that cornering speeds increased to such a point that the FIA banned them right away."
--------------------------------------------------
Again no. It takes more than a plastic skirt and a diffuser to get you there. Formula One and Indy cars use a sculpted bottom and tunnels to create downforce but that's another subject. The whole idea is to shape and smooth the bottom of the car to mimic the shape of a wing, turn the rear up and add a longitudinal diffuser (which is just a form of wing or airfoil).
The point being that if you can properly create gound effect you can add considerable downforce with minimal drag. This is why some race car rules prohibit or limit the contouring of the underbody. I have done work with aerodynamics and have been successful doing so.
06-18-2003, 12:45 AM
#37
Race Director
"The point being that if you can properly create gound effect you can add considerable downforce with minimal drag. "
Ahhhh, that's the holy-grail of aerodynamics, eh? To get a net lift of zero, or even downforce without using an opposing downforce component to combat the natural lift (wings).
We even see this with airplanes as well. The B2-bomber being one with a flying-wing design, using the entire plane for lift. Commercial airliners are going to the lifting-body design as well. Just turn it upside down for a car.
Ahhhh, that's the holy-grail of aerodynamics, eh? To get a net lift of zero, or even downforce without using an opposing downforce component to combat the natural lift (wings).
We even see this with airplanes as well. The B2-bomber being one with a flying-wing design, using the entire plane for lift. Commercial airliners are going to the lifting-body design as well. Just turn it upside down for a car.
06-18-2003, 01:26 AM
#39
Gawd, I love these discussions... reminds me of all the lectures I slept through in college!
Seriously, tho, keep it coming, its fascinating stuff.
Can I ask a question here, as a side topic? The factory venting of the intercooler on the 951 includes ducting to force the air through it. When people install a vented nose piece on their cars, it looks like they usually omit the ducting. Wouldn't this create a vacuum in front of the intercooler, since the air would diffuse around it rather than being forced through it? Even if you installed the duct and cut out access in it for the vent in the nose, I would think this would still create turbulance in front of the intercooler that would be counterproductive. A lot of guys swear that the vented nose is effective tho. Any guesses as to who is right?
Regards,
Seriously, tho, keep it coming, its fascinating stuff.
Can I ask a question here, as a side topic? The factory venting of the intercooler on the 951 includes ducting to force the air through it. When people install a vented nose piece on their cars, it looks like they usually omit the ducting. Wouldn't this create a vacuum in front of the intercooler, since the air would diffuse around it rather than being forced through it? Even if you installed the duct and cut out access in it for the vent in the nose, I would think this would still create turbulance in front of the intercooler that would be counterproductive. A lot of guys swear that the vented nose is effective tho. Any guesses as to who is right?
Regards,
06-18-2003, 02:37 AM
#40
Nordschleife Master
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Chris Prack:
<strong>Sorry I couldn't let this just die.....
Really? You need to read your book again. A wing on a road car is an inverted airplane wing right? The lift of a wing increases with ground proximity. So that would also mean the the same wing inverted would increase downforce with ground proximity right? I can referance more than a dozen books on aerodynamics to support this if you like.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">No. An airfoil's coefficient of lift increases with ground proximity due to AIR DENSITY, not ground effect. You find me a sentence in a book that specifcally says that you can reduce downwash AND increase lift at the same time, and I'll concede. Have you ever seen flaps on an aircraft? When I bring down the flaps on my aircraft, I'm changing the camber of my wing, and adding more downwash, which is basically induced drag. That's why I say when I'm flying along the runway at 5 feet above ground, I dont' have more lift, I simply have less drag, (and probably less lift) but that significant decrease in drag allows me to accelerate faster...
The B2-bomber being one with a flying-wing design, using the entire plane for lift. Commercial airliners are going to the lifting-body design as well. Just turn it upside down for a car.
Yeah, I've drawn lots of concept cars with that idea in mind. It's hard to pull something off that is low enough, works, and looks good at the same time! Plus there is so much stuff under the car that gets in the way of the airflow you want to achieve..
Wouldn't this create a vacuum in front of the intercooler, since the air would diffuse around it rather than being forced through it? Even if you installed the duct and cut out access in it for the vent in the nose, I would think this would still create turbulance in front of the intercooler that would be counterproductive. A lot of guys swear that the vented nose is effective tho. Any guesses as to who is right?
The ideal is having air circulate correctly, but having a vacuum suck out the warm air and replace it with cooler turbulent air, I would think is better than leaving stale hot air in front of the top part of the intercooler, as it is when stock.
Anyways, I know that argueing over the internet is retarded, but if any of us learns anything through it, than it's worth it. So bring it on..
<strong>Sorry I couldn't let this just die.....
Really? You need to read your book again. A wing on a road car is an inverted airplane wing right? The lift of a wing increases with ground proximity. So that would also mean the the same wing inverted would increase downforce with ground proximity right? I can referance more than a dozen books on aerodynamics to support this if you like.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">No. An airfoil's coefficient of lift increases with ground proximity due to AIR DENSITY, not ground effect. You find me a sentence in a book that specifcally says that you can reduce downwash AND increase lift at the same time, and I'll concede. Have you ever seen flaps on an aircraft? When I bring down the flaps on my aircraft, I'm changing the camber of my wing, and adding more downwash, which is basically induced drag. That's why I say when I'm flying along the runway at 5 feet above ground, I dont' have more lift, I simply have less drag, (and probably less lift) but that significant decrease in drag allows me to accelerate faster...
The B2-bomber being one with a flying-wing design, using the entire plane for lift. Commercial airliners are going to the lifting-body design as well. Just turn it upside down for a car.
Yeah, I've drawn lots of concept cars with that idea in mind. It's hard to pull something off that is low enough, works, and looks good at the same time! Plus there is so much stuff under the car that gets in the way of the airflow you want to achieve..
Wouldn't this create a vacuum in front of the intercooler, since the air would diffuse around it rather than being forced through it? Even if you installed the duct and cut out access in it for the vent in the nose, I would think this would still create turbulance in front of the intercooler that would be counterproductive. A lot of guys swear that the vented nose is effective tho. Any guesses as to who is right?
The ideal is having air circulate correctly, but having a vacuum suck out the warm air and replace it with cooler turbulent air, I would think is better than leaving stale hot air in front of the top part of the intercooler, as it is when stock.
Anyways, I know that argueing over the internet is retarded, but if any of us learns anything through it, than it's worth it. So bring it on..
06-19-2003, 12:13 PM
#41
Drifting
My home computer is having a crisis but once it's back up I will see if I can scan a page and post it Rich.
This is somewhat retarded but we do it anyway...... at least it's still civil!
This is somewhat retarded but we do it anyway...... at least it's still civil!
06-19-2003, 02:13 PM
#43
Burning Brakes
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quote:Originally posted by Chris Prack:
Sorry I couldn't let this just die.....
Really? You need to read your book again. A wing on a road car is an inverted airplane wing right? The lift of a wing increases with ground proximity. So that would also mean the the same wing inverted would increase downforce with ground proximity right? I can reference more than a dozen books on aerodynamics to support this if you like.No. An airfoil's coefficient of lift increases with ground proximity due to AIR DENSITY, not ground effect. You find me a sentence in a book that specifically says that you can reduce downwash AND increase lift at the same time, and I'll concede. Have you ever seen flaps on an aircraft? When I bring down the flaps on my aircraft, I'm changing the camber of my wing, and adding more downwash, which is basically induced drag. That's why I say when I'm flying along the runway at 5 feet above ground, I don't' have more lift, I simply have less drag, (and probably less lift) but that significant decrease in drag allows me to accelerate faster...
Rich, when you are in an airplane and truly into ground effect a lot of factors come into play which causes the wing to support the weight of the aircraft with less power than would normally be required when out of "ground effect". Wing aspect ratio, height above ground, wing airfoil etc. all modify the effect of "flying in ground effect". Density of the air actually changes very little between say 40 feet above ground and 5 feet above ground, but what does happen is you get a bit of extra lift due to the cushioning effect of the airflow now being forced under the aircraft due to the positive angle of attack.
In most cases a car is already way below the threshold height of ground effect and for the same reasons above many fctors influence the total lift of a car. One of the big factors is the airflow under the car but as you and others have said it becomes very difficult to pinpoint where the airflow goes. If you can keep the airflow under the car moving quickly or divert some of it around the car via the aerodynamics of the nose then you lower the pressure under the car thus negating lift. I do have a seat of the pants check on my car that it feels much better at speed with all the undertrays on. I do feel that they help negate lift and I wish I had the instrumentation to go check it out.
I agree with you and Chris we can all learn a little bit or maybe make us think a bit more about what makes our cars handle the entire performance envelope as well as they do.
quote:Originally posted by Chris Prack:
Sorry I couldn't let this just die.....
Really? You need to read your book again. A wing on a road car is an inverted airplane wing right? The lift of a wing increases with ground proximity. So that would also mean the the same wing inverted would increase downforce with ground proximity right? I can reference more than a dozen books on aerodynamics to support this if you like.No. An airfoil's coefficient of lift increases with ground proximity due to AIR DENSITY, not ground effect. You find me a sentence in a book that specifically says that you can reduce downwash AND increase lift at the same time, and I'll concede. Have you ever seen flaps on an aircraft? When I bring down the flaps on my aircraft, I'm changing the camber of my wing, and adding more downwash, which is basically induced drag. That's why I say when I'm flying along the runway at 5 feet above ground, I don't' have more lift, I simply have less drag, (and probably less lift) but that significant decrease in drag allows me to accelerate faster...
Rich, when you are in an airplane and truly into ground effect a lot of factors come into play which causes the wing to support the weight of the aircraft with less power than would normally be required when out of "ground effect". Wing aspect ratio, height above ground, wing airfoil etc. all modify the effect of "flying in ground effect". Density of the air actually changes very little between say 40 feet above ground and 5 feet above ground, but what does happen is you get a bit of extra lift due to the cushioning effect of the airflow now being forced under the aircraft due to the positive angle of attack.
In most cases a car is already way below the threshold height of ground effect and for the same reasons above many fctors influence the total lift of a car. One of the big factors is the airflow under the car but as you and others have said it becomes very difficult to pinpoint where the airflow goes. If you can keep the airflow under the car moving quickly or divert some of it around the car via the aerodynamics of the nose then you lower the pressure under the car thus negating lift. I do have a seat of the pants check on my car that it feels much better at speed with all the undertrays on. I do feel that they help negate lift and I wish I had the instrumentation to go check it out.
I agree with you and Chris we can all learn a little bit or maybe make us think a bit more about what makes our cars handle the entire performance envelope as well as they do.
