Haters gonna hate
09-16-2010, 03:18 AM
#51
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Shin, I did track the car, but this summer I was off with my school's Formula SAE team helping get the car, that I spent all year helping construct, to become one of the world's best. That left me penniless for racing my own car. Next season is looking slightly better if I don't go to Europe with them.
I KNOW that the changes I've made will minimally affect any downforce or drag made by it, but it will still change the airflow. I've seen what huge changes made to the diffuser on our formula car only make minute tenths of a percent changes in downforce and drag. Whereas not having the diffuser altogether cause sthe drag and downforce to both reduced by 15-20% and 60-70%. And this is on a 330 pound car that has a 0-60 of less than 4 seconds.
Our RX-8's, like most cars, are kind of wedge shaped with the pointy end first. At lower speeds, this causes a bit of downforce. However at high speeds the car will act like a wing, create some lift and give you that floaty feeling. The air traveling over the top is at a lower pressure than the bottom because it must travel faster. (Try it yourself with a small strip of paper. Hold one end at your bottom lip so you can blow over it. Blow and you'll see the strip of paper rise because the faster moving air is at a lower pressure.)
I can see how what I've done, coupled with the pipey exhaust, could cause some more turbulence since there are not any venturis or even a flat undertray. But, giving the same amount of air more area to occupy and escape cannot create lift if its on the high pressure side. Air cannot curl under, this is because both sides curl towards the dead area right behind the trunk and that the air coming off the top is at a lower pressure than the bottom.
Lol. And no, I didn't get hit with an egg.
"Pop tha hood"?
I KNOW that the changes I've made will minimally affect any downforce or drag made by it, but it will still change the airflow. I've seen what huge changes made to the diffuser on our formula car only make minute tenths of a percent changes in downforce and drag. Whereas not having the diffuser altogether cause sthe drag and downforce to both reduced by 15-20% and 60-70%. And this is on a 330 pound car that has a 0-60 of less than 4 seconds.
Our RX-8's, like most cars, are kind of wedge shaped with the pointy end first. At lower speeds, this causes a bit of downforce. However at high speeds the car will act like a wing, create some lift and give you that floaty feeling. The air traveling over the top is at a lower pressure than the bottom because it must travel faster. (Try it yourself with a small strip of paper. Hold one end at your bottom lip so you can blow over it. Blow and you'll see the strip of paper rise because the faster moving air is at a lower pressure.)
I can see how what I've done, coupled with the pipey exhaust, could cause some more turbulence since there are not any venturis or even a flat undertray. But, giving the same amount of air more area to occupy and escape cannot create lift if its on the high pressure side. Air cannot curl under, this is because both sides curl towards the dead area right behind the trunk and that the air coming off the top is at a lower pressure than the bottom.
Lol. And no, I didn't get hit with an egg.
"Pop tha hood"?
I hope you realize you can't possibly compare open wheel aero to closed wheel full body aero. I hope the diffusers on your FSAE car are bordering on ridiculous because the speed you're going at in FSAE you need cartoonish aero bits just to make it work. That said, I've heard that in the last few years the gains in aero has shown the aero cars are starting to pull away from the non-aero cars in the autocross. I have also heard that those cars have gigantic front and rear multi element wings with end plates that look like the armor on the sides of a tank.
This thread actually made me break open a book yesterday, I'm not done yet, but you can't even begin to talk about diffuser effects unless you have a flat undertray. More cross section for the air to occupy = lower velocity = higher pressure. The whole point of a diffuser is to keep the air from going turbulent in order to keep the flow under a flat floor flowing. You got to keep the velocities under the flat part of a tray higher i.e. lower pressure.
I don't think you can prove anything just by looking at it on a street car without a flat floor, especially if you have no idea where the boundary layer is forming. Ground effect also seems to be a huge pain in the ***. The air could have gone turbulent and choked the flow off long before it even got to the tail where you chopped it all up. You just don't know unless you model it in CFD or you just take that variable out entirely by making a smooth undertray, then ramp in (feed nozzle) and then ramp out (diffuser).
I didn't really want to discuss this till I had a better understanding, this is just people thinking their eyes are as good as a wind tunnel of CFD. Which in my mind is absolutely ridiculous.
09-16-2010, 03:20 AM
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09-16-2010, 04:30 AM
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I'm part of Oregon State University/DHBW Ravensburg's Global Formula Racing, the first collaborative team in the series. Our team won 3 events this year including the championship race in Michigan as well as taking first in design in at least 5 of the 6 events we attended.
My position on the team is within the composites and chassis team. We were responsible for designing and making the two carbon monocoques as well as the diffuser and many other miscellaneous parts.
I'm still learning fluid dynamics so I have good idea of how it works, but I'm no expert on this... yet. I will be making a model of the RX-8 and running some CFD simultion as one of my projects this year in my classes. When I'm finished I'll post the results of what cutting my bumper actually does to the airflow. I was using the formula car's aero as an example to show that even large changes on such a small car yield in very minute results whereas similar changes done to our cars would yield in even smaller returns.
My position on the team is within the composites and chassis team. We were responsible for designing and making the two carbon monocoques as well as the diffuser and many other miscellaneous parts.
I'm still learning fluid dynamics so I have good idea of how it works, but I'm no expert on this... yet. I will be making a model of the RX-8 and running some CFD simultion as one of my projects this year in my classes. When I'm finished I'll post the results of what cutting my bumper actually does to the airflow. I was using the formula car's aero as an example to show that even large changes on such a small car yield in very minute results whereas similar changes done to our cars would yield in even smaller returns.
09-16-2010, 08:32 AM
#54
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Your front bumper and undertray matter more than your rear, if you are serious about high speed (and please stop quoting the GFR cars, they are open wheel and completely different), then you want a nearly sealed front end and flat under tray, why? Coanda effect and angle of attack.
You're goal is a net force towards the ground, seal up the bottom, and have a rear diffuser with a final path that points around 10 degrees for a race car and 30 degrees for road cars. If you watch a proper race car in the rain (le mans car for example) you'll witness a vortex behind the car. For example you stand on the left side of a road facing the road, the car passing from your right to your left, you will witness (in the rain) a visible vortex behind the car about 4-5 feet off the ground spinning counter clockwise (and if you are on the right side of the road, car passing left to right, the vortex will spin clockwise to you), that vortex is due to the rear diffuser's angle and the bottom of the car's "control" of the passing air.
Faster the air moves under the car the lower the static pressure, the diffuser then alters the momentum slowing the air in addition to adding a y -axis (front of car to back - x axis, back of car then skyward - y axis.) Simple terms, air goes up, car goes down.
But your stock front bumper, stock under tray, stock wheel wells, and missing rear bumper will yield nothing, i repeat, nothing, its all in your head (or in this case, your following of a trend within the car culture.)
Sincerely,
Hater
You're goal is a net force towards the ground, seal up the bottom, and have a rear diffuser with a final path that points around 10 degrees for a race car and 30 degrees for road cars. If you watch a proper race car in the rain (le mans car for example) you'll witness a vortex behind the car. For example you stand on the left side of a road facing the road, the car passing from your right to your left, you will witness (in the rain) a visible vortex behind the car about 4-5 feet off the ground spinning counter clockwise (and if you are on the right side of the road, car passing left to right, the vortex will spin clockwise to you), that vortex is due to the rear diffuser's angle and the bottom of the car's "control" of the passing air.
Faster the air moves under the car the lower the static pressure, the diffuser then alters the momentum slowing the air in addition to adding a y -axis (front of car to back - x axis, back of car then skyward - y axis.) Simple terms, air goes up, car goes down.
But your stock front bumper, stock under tray, stock wheel wells, and missing rear bumper will yield nothing, i repeat, nothing, its all in your head (or in this case, your following of a trend within the car culture.)
Sincerely,
Hater
09-16-2010, 09:22 AM
#56
Momentum Keeps Me Going
If anyone wants to do some learning...you could do worse than start with these papers
...by Volvo’s Dr Simone Sebben http://papers.sae.org/2004-01-1307
...also a GM paper http://papers.sae.org/900317
... but it'll cost you
...by Volvo’s Dr Simone Sebben http://papers.sae.org/2004-01-1307
...also a GM paper http://papers.sae.org/900317
... but it'll cost you
09-16-2010, 08:32 PM
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I'm part of Oregon State University/DHBW Ravensburg's Global Formula Racing, the first collaborative team in the series. Our team won 3 events this year including the championship race in Michigan as well as taking first in design in at least 5 of the 6 events we attended.
My position on the team is within the composites and chassis team. We were responsible for designing and making the two carbon monocoques as well as the diffuser and many other miscellaneous parts.
I'm still learning fluid dynamics so I have good idea of how it works, but I'm no expert on this... yet. I will be making a model of the RX-8 and running some CFD simultion as one of my projects this year in my classes. When I'm finished I'll post the results of what cutting my bumper actually does to the airflow. I was using the formula car's aero as an example to show that even large changes on such a small car yield in very minute results whereas similar changes done to our cars would yield in even smaller returns.
My position on the team is within the composites and chassis team. We were responsible for designing and making the two carbon monocoques as well as the diffuser and many other miscellaneous parts.
I'm still learning fluid dynamics so I have good idea of how it works, but I'm no expert on this... yet. I will be making a model of the RX-8 and running some CFD simultion as one of my projects this year in my classes. When I'm finished I'll post the results of what cutting my bumper actually does to the airflow. I was using the formula car's aero as an example to show that even large changes on such a small car yield in very minute results whereas similar changes done to our cars would yield in even smaller returns.
I would be interested in seeing the results of your CFD simulations. How do you plan on creating an accurate model of the RX-8? When do you plan on having them done?
09-16-2010, 08:36 PM
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If anyone wants to do some learning...you could do worse than start with these papers
...by Volvo’s Dr Simone Sebben http://papers.sae.org/2004-01-1307
...also a GM paper http://papers.sae.org/900317
... but it'll cost you
...by Volvo’s Dr Simone Sebben http://papers.sae.org/2004-01-1307
...also a GM paper http://papers.sae.org/900317
... but it'll cost you
The Joseph Katz book and some easy reading from the Simon McBeath book is what I'm referencing right now.
Last edited by LCheung; 09-16-2010 at 08:37 PM. Reason: Michigan not Detroit
09-17-2010, 10:35 PM
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The CFD simulations will be done by Christmas probably. I'm planning on using my car and a tape measure to get any measurements I can't find elsewhere. But mostly it'll be just a lot of time behind the computer. I'll likely be using Solidworks but could be using Katia instead. It won't be a perfect simulation, but it'll be accurate enough to have a good idea of whats going on around the car.
09-18-2010, 01:34 AM
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The CFD simulations will be done by Christmas probably. I'm planning on using my car and a tape measure to get any measurements I can't find elsewhere. But mostly it'll be just a lot of time behind the computer. I'll likely be using Solidworks but could be using CATIA instead. It won't be a perfect simulation, but it'll be accurate enough to have a good idea of whats going on around the car.
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