Wind Tunnel results
11-08-2011, 01:58 AM
#16
Rennlist Member
Awesome!! very cool
too bad we couldnt find out the effects of hood vents. We have some general ideas by the use of pressure sensors, but it would be cool to see their effects with the smoke arm too!
nice job carl
too bad we couldnt find out the effects of hood vents. We have some general ideas by the use of pressure sensors, but it would be cool to see their effects with the smoke arm too!
nice job carl
11-08-2011, 11:44 AM
#17
Developer
Thread Starter
Lots of questions, let me answer what I can.
Cost: I was there for 3 hours and we screamed to make the changes in short order. I got 9 different "pulls" done, and about 15 minutes of smoke-wand time. Cost was about $1300. This wind tunnel is not as expensive as those with higher air sopeeds and rolling roads - which are flatly out of my price range.
Top speed; Brian your calculation is close, but you are using the stock frontal area. Our frontal area has been reduced to 20.4 sq ft. Also, be sure you calculate our use 28.5" tall rear tires.
We thought that the front chin splitter was too effective at Bonneville too, and it was lifting the rear end via a cantilever effect. At the windtunnel we found out that the front downforce was WAY higher than we needed, but that it was not making the *** lift like we thought. The rear end was lifting via its own aerodynamics, irrespective of the front downforce.
We did test the effect of the louvers in the hood - this was another item that surprised the wind tunnel operator as he was sure they were having a negative effect on aero. We closed them off and discovered almost no change whatsoever.
Cost: I was there for 3 hours and we screamed to make the changes in short order. I got 9 different "pulls" done, and about 15 minutes of smoke-wand time. Cost was about $1300. This wind tunnel is not as expensive as those with higher air sopeeds and rolling roads - which are flatly out of my price range.
Top speed; Brian your calculation is close, but you are using the stock frontal area. Our frontal area has been reduced to 20.4 sq ft. Also, be sure you calculate our use 28.5" tall rear tires.
We thought that the front chin splitter was too effective at Bonneville too, and it was lifting the rear end via a cantilever effect. At the windtunnel we found out that the front downforce was WAY higher than we needed, but that it was not making the *** lift like we thought. The rear end was lifting via its own aerodynamics, irrespective of the front downforce.
We did test the effect of the louvers in the hood - this was another item that surprised the wind tunnel operator as he was sure they were having a negative effect on aero. We closed them off and discovered almost no change whatsoever.
11-08-2011, 01:25 PM
#18
Instructor
You were still getting lift over the rear with that spoiler cranked all the way up? How much? Traction is limiting you on the salt, so turning lift into downforce will yield real improvement in your trap speed.
11-08-2011, 05:35 PM
#19
Developer
Thread Starter
Our CLr in the rear was .220 of lift at our basiline test. By the end, we had redicuced the rear lift to CLr = .122 so we were still getting lift, just less of it.
As I said, the control surface iof this spoiler is too small (now we know) and we think that once we maximize its size we may get into some down-force numbers.
Glenn, do you want to do the MPH computation for what the reduction in Cd from .30 to .276 might mean? In other words, I WAS at .30 Cd and attained 216.635 MPH. Same car now at .276 Cd would attain what MPH?
Here is one chart my son put together for me, and he incorporated my FDR and tire diameter already.
As I said, the control surface iof this spoiler is too small (now we know) and we think that once we maximize its size we may get into some down-force numbers.
Glenn, do you want to do the MPH computation for what the reduction in Cd from .30 to .276 might mean? In other words, I WAS at .30 Cd and attained 216.635 MPH. Same car now at .276 Cd would attain what MPH?
Here is one chart my son put together for me, and he incorporated my FDR and tire diameter already.
11-09-2011, 10:17 AM
#21
Instructor
Hi Carl,
I haven't been ignoring the thread or your question. When I first posted on your car's likely top speed, I estimated aero drag and rolling resistance versus the output of your engine from your dyno chart. (Note how I diplomatically avoided referring to either power or torque!) :-D
Your descriptions of ploughing the splitter and not being able to use full throttle, even at your best terminal speed, prompted me to look more deeply at traction on the salt; front and rear downforce and unloading the rear wheels (and the effect of this on traction); and a guess on induced drag, and incorporated all this, your engine output, gearing (including the 28.4 inch diameter rear wheels) and the effect of the pressure altitude of the day on lift, drag and engine output into a spreadsheet. With some juggling of the different parameters, I was able to within about 0.1 mph of your times over each mile and terminal velocity.
I assumed that you changed up at 6500rpm and were able to drive right on the limit of traction. Clearly, those assumptions are unlikely to be accurate and neither are my other assumptions; however, my aim was to look at the effect of varying the downforce and weight. What was indicated was you, do get a nett benefit from carrying that ballast and if you simply reduced the front downforce by about 75 per cent, you would break the 231.5 mph ... but barely if the salt was in the same condition as for your 216.6mph run. The car would go faster but not in the distance available to you.
I was, however, assuming reasonable downforce over the rear wheels. Using your rear lift co-efficient of 0.22, and the Cd of 0.30, I can't hit your numbers. The car tends to be too fast over the first timed mile or the terminal velocity at the end of the five miles is too low.
What was your lift co-efficient over the front wheels? Also, do you know what the front/rear weight distribution is? I have assumed 50:50.
As others seem to be, I wondered whether the low clearance of the splitter may have been compromising the downforce which the underbody may generate. On the other hand, that spoiler would have generated a big low pressure region behind it, which should increase the effectiveness of the diffuser. Now I wonder whether your car's flat undersurfaces and diffuser may, in fact, have been developing downforce on the track that was not being developed in the wind tunnel due to its fixed floor. On the track, you would have a turbulent layer adjacent to the flat undersurface (in which the velocity is less than that of the free airflow (ie the velocity of the vehicle), but the air velocity at the track surface is that of the vehicle. With a fixed floor, it also has a boundary layer of slower air. To put this in perspective, you have about four inches total of boundary layer air under the rear of your car with a fixed floor, compared to two on the track. This would significantly reduce the airflow under the car and, consequently, any downforce which otherwise would be generated.
If I can more accurately model the forces on the car in its Bonneville configuration, I will be able to give you a better idea of how your sector and terminal velocities will increase with the revised aerodynamics.
Regards,
Glenn
I haven't been ignoring the thread or your question. When I first posted on your car's likely top speed, I estimated aero drag and rolling resistance versus the output of your engine from your dyno chart. (Note how I diplomatically avoided referring to either power or torque!) :-D
Your descriptions of ploughing the splitter and not being able to use full throttle, even at your best terminal speed, prompted me to look more deeply at traction on the salt; front and rear downforce and unloading the rear wheels (and the effect of this on traction); and a guess on induced drag, and incorporated all this, your engine output, gearing (including the 28.4 inch diameter rear wheels) and the effect of the pressure altitude of the day on lift, drag and engine output into a spreadsheet. With some juggling of the different parameters, I was able to within about 0.1 mph of your times over each mile and terminal velocity.
I assumed that you changed up at 6500rpm and were able to drive right on the limit of traction. Clearly, those assumptions are unlikely to be accurate and neither are my other assumptions; however, my aim was to look at the effect of varying the downforce and weight. What was indicated was you, do get a nett benefit from carrying that ballast and if you simply reduced the front downforce by about 75 per cent, you would break the 231.5 mph ... but barely if the salt was in the same condition as for your 216.6mph run. The car would go faster but not in the distance available to you.
I was, however, assuming reasonable downforce over the rear wheels. Using your rear lift co-efficient of 0.22, and the Cd of 0.30, I can't hit your numbers. The car tends to be too fast over the first timed mile or the terminal velocity at the end of the five miles is too low.
What was your lift co-efficient over the front wheels? Also, do you know what the front/rear weight distribution is? I have assumed 50:50.
As others seem to be, I wondered whether the low clearance of the splitter may have been compromising the downforce which the underbody may generate. On the other hand, that spoiler would have generated a big low pressure region behind it, which should increase the effectiveness of the diffuser. Now I wonder whether your car's flat undersurfaces and diffuser may, in fact, have been developing downforce on the track that was not being developed in the wind tunnel due to its fixed floor. On the track, you would have a turbulent layer adjacent to the flat undersurface (in which the velocity is less than that of the free airflow (ie the velocity of the vehicle), but the air velocity at the track surface is that of the vehicle. With a fixed floor, it also has a boundary layer of slower air. To put this in perspective, you have about four inches total of boundary layer air under the rear of your car with a fixed floor, compared to two on the track. This would significantly reduce the airflow under the car and, consequently, any downforce which otherwise would be generated.
If I can more accurately model the forces on the car in its Bonneville configuration, I will be able to give you a better idea of how your sector and terminal velocities will increase with the revised aerodynamics.
Regards,
Glenn
11-09-2011, 11:20 AM
#23
Developer
Thread Starter
Excellent response, Glenn. I knew you'd get into it right up to your ankles head-first.
Our weight distribution is 2130 lbs on front axle and 2503 lbs on rear axle.
When we started, our lift coefficient on the front was -.226 so we had tons of nose down force, and by the time we had tuned the car better, it was now -.250 on average.
Note we were not trying to reduce downforce on the nose, but rather, our goal was to tune the front and rear forces to put the car into a good aerodynamic tune. As raced, the front was getting more and more downforce and the rear was getting more and morelift. The car was unbalanced and hellish to drive. Our goal was to balance the car, and find some downforce.
We mistakenly believed that the front down force was causing lift on the rear tires because of a cantilever effect. This theory was proven wrong, the lift we were getting ion the rear was all aero and irrespective of the amount of front down force.
Glen, would you like to see my Excel spreadsheet on this? PM me and I can send it over.
What was your lift co-efficient over the front wheels? Also, do you know what the front/rear weight distribution is? I have assumed 50:50.
When we started, our lift coefficient on the front was -.226 so we had tons of nose down force, and by the time we had tuned the car better, it was now -.250 on average.
Note we were not trying to reduce downforce on the nose, but rather, our goal was to tune the front and rear forces to put the car into a good aerodynamic tune. As raced, the front was getting more and more downforce and the rear was getting more and morelift. The car was unbalanced and hellish to drive. Our goal was to balance the car, and find some downforce.
We mistakenly believed that the front down force was causing lift on the rear tires because of a cantilever effect. This theory was proven wrong, the lift we were getting ion the rear was all aero and irrespective of the amount of front down force.
Glen, would you like to see my Excel spreadsheet on this? PM me and I can send it over.
11-09-2011, 12:13 PM
#25
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Would a small row of micro vortex generators just prior to the rear hatch glass help energize that airflow and allow it to adhere longer so it strikes the wing better?
Or are those illegal in your class?
Perhaps cut a slightly longer piece of lexan for that rear hatch so it sits flush with that lip?
Also, tape the hatch/body seem on the roof. Small amount of energy is being lost there also i bet
http://www.nasa.gov/centers/langley/...06-52-LaRC.pdf
Or are those illegal in your class?
Perhaps cut a slightly longer piece of lexan for that rear hatch so it sits flush with that lip?
Also, tape the hatch/body seem on the roof. Small amount of energy is being lost there also i bet
http://www.nasa.gov/centers/langley/...06-52-LaRC.pdf
11-09-2011, 12:40 PM
#26
Developer
Thread Starter
Tony, you are right on the money with that.
I have long known that the recess that the 928 hatch glass is in causes our rear spoilers to be so ineffective because the of the boundry layer separation it causes right at the top of the glass.
In 2009 i had vortex generators like you describe on the roof just forward of the depression and it gave me 5 mph on the main straight from corner 14 to 1 at Road America!
Unfortunately, the rules dont allow me to do use them in this class at Bonneville.
You can see them in these pictures from 2009:
I have long known that the recess that the 928 hatch glass is in causes our rear spoilers to be so ineffective because the of the boundry layer separation it causes right at the top of the glass.
In 2009 i had vortex generators like you describe on the roof just forward of the depression and it gave me 5 mph on the main straight from corner 14 to 1 at Road America!
Unfortunately, the rules dont allow me to do use them in this class at Bonneville.
You can see them in these pictures from 2009:
11-09-2011, 11:37 PM
#27
Rennlist Member
Would a small row of micro vortex generators just prior to the rear hatch glass help energize that airflow and allow it to adhere longer so it strikes the wing better?
Or are those illegal in your class?
Perhaps cut a slightly longer piece of lexan for that rear hatch so it sits flush with that lip?
Also, tape the hatch/body seem on the roof. Small amount of energy is being lost there also i bet
http://www.nasa.gov/centers/langley/...06-52-LaRC.pdf
Or are those illegal in your class?
Perhaps cut a slightly longer piece of lexan for that rear hatch so it sits flush with that lip?
Also, tape the hatch/body seem on the roof. Small amount of energy is being lost there also i bet
http://www.nasa.gov/centers/langley/...06-52-LaRC.pdf
Tony, you are right on the money with that.
I have long known that the recess that the 928 hatch glass is in causes our rear spoilers to be so ineffective because the of the boundry layer separation it causes right at the top of the glass.
In 2009 i had vortex generators like you describe on the roof just forward of the depression and it gave me 5 mph on the main straight from corner 14 to 1 at Road America!
Unfortunately, the rules dont allow me to do use them in this class at Bonneville.
You can see them in these pictures from 2009:
I have long known that the recess that the 928 hatch glass is in causes our rear spoilers to be so ineffective because the of the boundry layer separation it causes right at the top of the glass.
In 2009 i had vortex generators like you describe on the roof just forward of the depression and it gave me 5 mph on the main straight from corner 14 to 1 at Road America!
Unfortunately, the rules dont allow me to do use them in this class at Bonneville.
You can see them in these pictures from 2009:
look how nice the rear flow off the windo is, even with the separation near the rear hatch. just like the porsche windtunnel tests show, there is only about a 7 degree change of angle, I bet one of the reasons the lower drag coeffcicient, is because of the rear spoiler ,and of course all the clean up on the front of the car.
Interesting stuff for sure!
Mk
11-10-2011, 05:57 AM
#29
Former Vendor
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Carl,
When I went to Octoberfest I had the chance to finally see Mark A.'s and Joe F.'s racers.
IIRC, they had placed aluminum spacers to fill the rear hatch voids around the glass and then placed plexiglass to be flush with the outer edges of the the rear hatch glass surround.
I believe some older NASCAR cars, Superbird or Chargers maybe, had their rear glass areas made flush by factory stylists to help increase their speeds at Daytona.
Would this type of modification be allowed in the rules? This would probably help the laminar air flow hug the rear hatch better to allow more contact with the rear wing.
This is really cool stuff you are doing by the way.
Cheers,
When I went to Octoberfest I had the chance to finally see Mark A.'s and Joe F.'s racers.
IIRC, they had placed aluminum spacers to fill the rear hatch voids around the glass and then placed plexiglass to be flush with the outer edges of the the rear hatch glass surround.
I believe some older NASCAR cars, Superbird or Chargers maybe, had their rear glass areas made flush by factory stylists to help increase their speeds at Daytona.
Would this type of modification be allowed in the rules? This would probably help the laminar air flow hug the rear hatch better to allow more contact with the rear wing.
This is really cool stuff you are doing by the way.
Cheers,
11-10-2011, 11:44 AM
#30
Rennlist Member
Great video Carl.
Is that actually some reversion we're seeing at 3:28 over the rear hatch or is it just the pressure of the smoke coming out of the wand? It's odd to see the smoke spread backward toward the top of the glass.
Is that actually some reversion we're seeing at 3:28 over the rear hatch or is it just the pressure of the smoke coming out of the wand? It's odd to see the smoke spread backward toward the top of the glass.