Sample Wind Tunnel Test

[interested bystander]

Stan knows what CFD is- he's just an anti tech guy.

In conversations yrs truly has had with the premier Top Fuel Dragster wing builder, the problem, especially with less than near - perfect shapes -vintage for example - the grid pattern may have to be near MOLECULAR in dimesion to get anything other than an approximation.

The computer he has required FBI clearance before purchase.

[JimL]

I sure wish someone could enlighten me about the open wheel effect.  The more I think about it, the more questions I have.

I know the principal of fendered cars, where we try to block air flowing into the face of the tire near the ground (like the spats in front of all four tires on many high mpg cars), but I wonder if there is lift going on over the rear, upper radius of the rotating tire?  Here's something confusing:

I watched a special on global warming research, where they were developing computer controlled sailing ships (without sails) that had to be able to control themselves in any wind.  It seems there is a property of a rotating cylinder, wherein wind/air flow across it at an angle (regardless direction of rotation) increases pressure on the "back" side relative to pressure on the "front" (Bernoulis prinicipal...spelled that wrong ).  They were able to power the ship with two vertical rotating tubes, spinning about 300 RPM, in very little wind.  It was amazing to watch, and the ships could handle much wider range of windspeed without attention...but I digress....

So....does that have any relation at all to a tire rotating in fast moving air?

Here's another weird one:

When Dan and I were running the mod roadster (Bob Sights Jr. car now), we found it would plaster HARD salt deposits into the paint, just ahead of the rear wheels, about 45 degrees up from the axle line.  The salt was so hard I had to soak it off with water after a run.  Everywhere else, the salt wiped off easily.  I was told that Indy cars have air diverting body work in front of the rear wheels because of "high pressure ahead of the top of the tire, against the body", but I don't know if that's the whole reason.  I was told it can be the highest pressure area on the whole car (?!?)

Sooooo....are we dealing with high pressure ahead of the tire (and in what area), or lift upward and behind, or both??  Should we try to keep the open tires clear of the nose, to make the car run straighter?

I'd like to hear some ideas or entertaining guesses!  thanks!!
Regards, JimL

[interested bystander]

The Flettner affect for boat propulsion was used methinks by Costeau on a research ship, by the way.

As is known, there's been  considerable research into the aero effects of wheels, both open and covered, both in books and on the net that I'm sure many smarter than I can expand on.

Simplisticly a rotating wheel on the ground at speed generates lift, (and maybe 50% of the vehicle's drag) but generates a high pressure wave forwards towards the ground  and- see "TIRE WIPERS" on F1 and Indy Cars for instance - that force can be used to generate downforce.

Observing salt patterns and El mirage dust on vehicles is interesting, for sure. Especially how the lakebed dust can migrate INSIDE into the strangest places.

[A2WindTunnel]

Quote from: A2WindTunnel on January 28, 2009, 03:38:25 PM
I don't recall ever saying that tape has little effect (could you help me remember the topic).  The front end is very sensitive on cars and there are big gains to be made.  All cars differ in how well things help and hurt the aerodynamics and that is why a lot of times you can't generalize because one change that helps one car might hurt another.


I may be wrong, but I thought  some time ago their was a discussion on here about smoothing the joints on a body.  Thanks for posting the information. Its been very educational for a non aero guy like me.

Yes... hood seams vary on all cars.  With production cars like this, the seams are fitted well and this car has a rubber seal around the hood seam so there might not be an effect when taped because there is no air "leaking".  The second run of the test, the hood seams AND headlights were taped.  The the headlights accounted for the majority of the reduction in drag in this configuration if that was your question.  I have seen hand built cars where the seams don't fit as well and might be "leaking" air so tape would help in that situation.

In other words...  If the seams are tight and built right then you would see little or no effect with taping vs. a car with "bad" fitting seams.

[Rex Schimmer]

Dave,
I notice in your spread sheet that you measure the Cp and that you have 18 values per run. Related to the car aero surface where are you measuring the Cp? and how do you measure it? Since a plot of the Cp along a "streamline" would define the pressure gradient along that streamline and would also provide a good indication of airfow along that line and also boundry layer thickness.

Rex

[ol38y]

Great info, thanks for taking the time.

You once posted that you would comment on the use of broad-coverage front fenders on motorcycles, but probably got distracted with "working" or some such.  Can you share any insight, for a guy on an unfaired bike?

Dean

Agreed. I also would like to hear your thoughts on fenders as well as the use of a headlight and its effect on aero.

Thanks, Larry

[A2WindTunnel]

You once posted that you would comment on the use of broad-coverage front fenders on motorcycles, but probably got distracted with "working" or some such.  Can you share any insight, for a guy on an unfaired bike?

Dean

Also, is it possible to operate the wind tunnel with the rider aboard the bike? How is it supported?

Dean,

I will try to go back and look at the fender and headlight questions you guys had. 

Yes it is possible, and I highly recommend you test with the rider on the bike.  When you put a rider on the bike you change the flow field and the path the air travels to get towards the rear of the bike.  The bike and rider work as a system (aero) and should be tested with the variables as close as you can to how you race.  This means using the the same helmet, suit, gloves. boots etc...(in other words don't test in jeans and a t-shirt)  And because of body shape size, you cannot assume that a motorcycle has a constant Cd=X.XXX since every rider is different and will more than likely not yield the same Cd, so the Cd is a function of bike+rider.  Example: the same hayabusa with a 120lb rider will not have the same Cd as a 210lb rider. 

Just like in the bicycle article I posted for the motorcycle guys, a riders position and equipment become important and could have a big influence on the Cd for a motorcycle.  See Video about bicycle rider position for those who have not seen:  This can help the motorcycle guys see how important positioning/equipment can become. (Click on link to see Video): http://www.testrider.com/fly.aspx?layout=player&video=1

"How is it supported?" There is a front and rear fixture we mount the motorcycles in the tunnel where the front is stationary and the rear can adjust to the wheel base of the bike.  The BUB bike had special fixtures made so skip the first few pictures in the motorcycle section of the A2 website. (Click to see website) http://www.a2wt.com/Motorcycle.htm

or here is a single picture:

[A2WindTunnel]

Dave,
I notice in your spread sheet that you measure the Cp and that you have 18 values per run. Related to the car aero surface where are you measuring the Cp? and how do you measure it? Since a plot of the Cp along a "streamline" would define the pressure gradient along that streamline and would also provide a good indication of airfow along that line and also boundry layer thickness.

Rex

There are 18 channels available for the customers use in A2 along with radiator, oil coolers, and tube anemometers (flow meters).  In the sample Camaro test there were no Cp's or anemometers hooked up, and that is why none of the #'s will make much sense in those section of the spreadsheet.  You could set them longitudinally along the body to see the pressure gradient.  Some of the stockcar teams might put them on the surface of a rear spoiler to see the pressure across the face (side to side), on A-B-C pillars, rear glass, front bumper, valiance, cowling etc....  There really is and endless number of things you could measure with Cp's.  At AeroDYN there are 50 pressure taps available to teams and a 2D pressure grid (240+Cp's) built into the floor under the car to show under body pressure from in front of the nose to the rear of the car.

I will have a short video soon of how to install pressure taps on a surface along with hopefully many other 101 wind tunnel testing Q&A’s?  Let me know the interest of something like that?

Dave

[1 fast evo 2]

Dave,
I notice in your spread sheet that you measure the Cp and that you have 18 values per run. Related to the car aero surface where are you measuring the Cp? and how do you measure it? Since a plot of the Cp along a "streamline" would define the pressure gradient along that streamline and would also provide a good indication of airfow along that line and also boundry layer thickness.

Rex

There are 18 channels available for the customers use in A2 along with radiator, oil coolers, and tube anemometers (flow meters).  In the sample Camaro test there were no Cp's or anemometers hooked up, and that is why none of the #'s will make much sense in those section of the spreadsheet.  You could set them longitudinally along the body to see the pressure gradient.  Some of the stockcar teams might put them on the surface of a rear spoiler to see the pressure across the face (side to side), on A-B-C pillars, rear glass, front bumper, valiance, cowling etc....  There really is and endless number of things you could measure with Cp's.  At AeroDYN there are 50 pressure taps available to teams and a 2D pressure grid (240+Cp's) built into the floor under the car to show under body pressure from in front of the nose to the rear of the car.

I will have a short video soon of how to install pressure taps on a surface along with hopefully many other 101 wind tunnel testing Q&A’s?  Let me know the interest of something like that?

Dave

As a one time user of your great facility I would be very interested in more knowledge of basic wind tunnel testing.

          Mike Reichen

[A2WindTunnel]

In case you didn't understand Stan Back's query -- the "spinning wheel" situation is our thoughts that there might be turbulence caused by the spinning wheel (mostly front).  We've got some rules about the size and shape of the front fender -- so the question is mostly one of whether we should use a custom front fender to achieve best aero efficiency, and, if so -- what aspects should we try to design in to maximize the gain in efficiency.

Thanks -- and Stan, maybe he knows what you meant -- but I thought that as the head handlebar holder of this site I should make it clearer (I hope I did so!) to be sure that the casual reader/lurker/surfer would know whatcher talkin' 'bout.

It took me a while to find the question….

Slim.  This is a good question but very vague when it comes to the different style bikes, wheel combo, fork spacing, rake, set ups etc... that are in LSR.  One fender might work well on one bike but not another so there is not a simple “this fender/design works” for all bikes.

I’m not sure if anyone out there has done any development work on motorcycle front fenders they are willing to share or chime in on for this post?  I remember one guy said that Texas A&M had spinning wheel capability for motorcycles in that tunnel or maybe it was San Diego?  Has anyone spent time trying different fenders in a tunnel that spins wheels or even one that does not spin?  The few motorcycles that have tested fenders here and found a fender combo that showed positive results for their bike did show an improvement at the track (from what they told me).   And of coarse I can't divulge any of the information they learned here in the tunnel without their permission.

[sabat]

Thanks for taking the time, we appreciate it.  Reading around the internet (sometimes dangerous) leads me to think that a broad coverage front fender would reduce the turbulence at the top of wheel, where it's going twice the speed of the bike. But I've seen several pictures of very fast LSR bikes with small or absent front fenders, - so I wonder what gives. These folks put a lot of thought into their fairing, so I wonder if experience has shown them something. I know I can just try it myself, and I will, but I like to ponder these things on cold winter nights, and you probably have considerable knowledge on this topic.

I wonder if anyone has put fenders on a lakester (I know it's not legal) to see the effect?

Thanks,
Dean

[Stainless1]

Dean, check the old picture thread, several of the streamliners are lakester conversions.  You can put fenders on a lakester, but then its a liner. 

Bikewise, we picked up 2 to 3 MPH with our fender changes, 197 to 199 GSXR to Busa and 199 to 202 Busa to Aero fender.  Or the kid was getting better with his tuck.... 

[Speed Limit 1000]

Or the kid was getting better with his tuck.... 

No that is what got the extra 6 for the red hat. 202 to 208

[jl222]

Dave,
I notice in your spread sheet that you measure the Cp and that you have 18 values per run. Related to the car aero surface where are you measuring the Cp? and how do you measure it? Since a plot of the Cp along a "streamline" would define the pressure gradient along that streamline and would also provide a good indication of airfow along that line and also boundry layer thickness.

Rex

   Rex I take it your referring to  Cp as center of pressure? I did.
   .493 cd for a 97 Camaro? A 1982 has a .35 from the factory and the HOT ROD MAGAZINE even earlier style camaro got down to a low .2s go figure?

                                JL222 
to the low .2s go figure.
 

[sabat]

Bikewise, we picked up 2 to 3 MPH with our fender changes, 197 to 199 GSXR to Busa and 199 to 202 Busa to Aero fender.  Or the kid was getting better with his tuck.... 

Thanks SS, that's a big difference. Do you have any pics of the aero fender?  -Dean