Sample Wind Tunnel Test

[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



   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.

Cp is the Coefficient of Pressure.  (P Local - P Static)/q

[A2WindTunnel]

  .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.

   
I get A LOT of emails where people don’t seem to understand that when you are not comparing identical prepared cars you can’t assume that you know the Cd since the car from factory is .XXX, but now there are all kinds of major modifications (wheels, grill, hood, spoilers, solid hub caps, mirrors removed, scoops, lower heights, rake, bumper vailance, cooling mods, etc....  There are too many variables that change Cd & Cl that you would be just shooting in the dark.  In this case, the car is set up to run road races and he has an after market rear spoiler (much larger than stock), wider tires, plus some other mods where faster corner speeds is his goal and not so much drag reduction because he is not running the higher speeds and has plenty of HP (more downforce = faster corner speeds but increased drag).  His goal is to make some modifications and try to balance out the car with more front downforce at his second trip to the wind tunnel.

As JL222 says, a 1982 Camaro has Cd = .350 from factory, but I guarantee it makes no where near the rear downforce this car makes and will have lift if there is not a rear spoiler (on the 82’).  These things will increase the drag when compared to the HOT ROD Camaro which had modifications to reduce the drag to get to the "low .2's" and still have a good aero balance.  And I bet the hot rod camero started out around .350 when it rolled off the factory floor and did not have any modifications.

When looking at Aerodynamics there is a MUCH MUCH bigger picture then just the Cd, and it is just as important to look at Clf and Clr (Lift front/rear).  The majority of the time there is no such thing as a free lunch.  If you increase downforce you will more then likely increase drag (there are some exceptions).  Aerodynamics in race cars is more about finding a good aero balance of front and rear downforce with the least amount of drag penalty (Lift/Drag ratio) for the type of racing that car is used for.  If you have an aero imbalance there is no way you will reach the full potential of your racecar no matter what type of racing.

Looking at Cd only would be like taking your engine to a Dyno and saying Im only interested in HP and don’t really care about Torque or RPM.

Found this posted on web (http://www.mayfco.com/chevy.htm) to illustrate how a Camaro varies in Cd over a pretty small year range.  They say the values came from the book “Racecar Design”. There is .140 (counts) or 33% difference from the highest value to the lowest value.

'94 Camaro Calloway C-8    0.320
'94 Camaro Convertible      0.400
'88 Camaro IROC - Z         0.340
'89 Camaro IROC - Z         0.340
'89 Camaro IROC-Z Conv.   0.420
'93 Camaro V6                 0.340
'93 Camaro V6 Auto          0.340
'92 Camaro Z28                0.380
'93 Camaro Z28                0.340
Camaro Trans Am Racer    0.280

[Rex Schimmer]

Dave,
Just keep making comments to our questions, some of which may be a little less than informed, becasue I learn lots from every one of your posts!!

It is a real tow from Nor. Cal to Mooresville but when I do get something together I will probably do it, first for the aero info, second just to say that I did it and third I have a long time close friend that is with Yates Racing, Max Jones, and I am sure I can stay at his place for free.

Rex

[gazza414]

A2, what benefits ( aero insight ) would customers see if your wind tunnel was capable of say 270 to 300km/hr.

I have seen published specs from many of the F1 teams where their tunnels are all around this test velocity.

[gazza414]

ping A2 or anyone else?

[A2WindTunnel]

A2, what benefits ( aero insight ) would customers see if your wind tunnel was capable of say 270 to 300km/hr.

I have seen published specs from many of the F1 teams where their tunnels are all around this test velocity.

As I understand the F1 rules right now, the teams are limited to a test speed of 50m/s and 50% scale models or 111.8 mph and 50% models.  The equivalent Full Scale speed by FIA rules (to test a full size F1) would be 56mph. They scale the data to what ever speed they want to look at the forces.  Was it test speed or calculated forces you saw published at 168-186mph?

To elaborate on your question:

It really depends on the shape that you are testing in the wind tunnel.  Cars are bluff bodied objects, so most production based cars are not Reynolds sensitive at speeds above (around) 40mph. And most all NASCAR teams test in AeroDyn at 130mph even though they race at closer to 200mph.  Teams will however go to Lockheed in Atlanta GA every now and then to test their cars at 200mph.  Now they don’t expect the same absolute #'s as AeroDyn because Lockheed does not spin the wheels or have a Boundary Layer control system like AeroDyn (I think they just have a suction slot ahead of the car). But at those high speeds you can now observe deflections in the body, windows, and other paneling, and they will work on bracing things up to make the car more solid.  From what I have heard, most of the aerodynamicists testing here are fine with the 130mph testing and don’t feel there is a great added benefit to 200mph testing.  When you are looking at Coefficients they are dimensionless numbers so that don’t vary with speed unless you have a Reynolds # sensitivity or major deflection of the body which is changing the actual shape of the car.  From those numbers (CD, CL) you can calculate lbs and HP at x speed.  Plus at those higher speeds you now have to make solid test pieces riveted securely that won't get blown off where in A2 you can get away with duct taping many things on, which means less time fabricating a piece that might end up in the trash.

Below is a sample test done with a NASCAR Truck at inspection height (splitter 3.5” off ground) in AeroDyn at 130mph and then in A2 at 85mph.  You can see that the added speed did not change the numbers very much. Delta Cd = .007 (7 counts) out of a total over Cd=  .500 (500 counts) was delta Cd=1.27% difference from the two tunnels (130mph vs 85mph).  And the same gurney lip change they made in the two tunnels was in the same direction and magnitude on all force data.  Absolute values are important to make sure the flow field is as close to real world as you can make it, but in wind tunnel testing and development the deltas quantifies the results and helps interoperate what is going on.  If I do this change to the car what is the effect? In this case adding a gurney lip added drag and rear downforce to the truck.

There is a reason A2 was designed to have a max speed of 85mph and that comes down to cost savings that are passed on to the customer.  I am not advocating that A2 is the solution to every ones aero needs, but as the test speeds, and capability of a wind tunnel increase so does the cost to build which is reflected in the hourly rate.

A2: $390/hr
-85mph
-No Spinning Wheels.
-BL Control: Passive floor ahead
-No Yaw

Lockheed: around $1400-$1500/hr (not for certain)
-200mph
-No Spinning Wheels
-BL Control: Suction slot ahead of model
-Yaw (not sure on max for cars, but I think the table rotates 180 deg??)

AeroDyn: $1650/hr
-130mph
-BL Control: Continuous blowing/suction groundplane ahead and under the car
-Spinning Wheels
-Automated Ride Height Control System
-+/- 3 Deg Yaw

Wind Shear: $4500/hr
-180mph
-BL Control: Rolling Road
-+/- 8 Deg Yaw

The learning curve is pretty steep in wind tunnel testing and most all first time teams that test in A2 have learned very valuable information. If spinning wheels, active BL control and yaw are a must for your test, then AeroDyn and Wind Shear are the only other full scale options in North America for racecars.


Dave

[A2WindTunnel]

Also... The NASCAR teams test Full Scale at 130mph and look at calculated force data at 200mph.


Correction to Lockheeds Pricing is $1300/hr

[Blue]

I sure wish someone could enlighten me about the open wheel effect.  The more I think about it, the more questions I have.
...............
I'd like to hear some ideas or entertaining guesses!  thanks!!
Regards, JimL

OK, Jim.  This is a lot of good empirical data and no one has answered you.

The fact is that if you see something on your car, there is a rational and physical explanation for it.  It is NOT the responsibility of the aero engineer to say "well, it was different in CFD and the wind tunnel".  Full scale Rn effects that are not easily models or seen at low Rn are where I make my living.

You need to tuft your car.  I know, I'm beating a dead horse here.  Here's the thing:  putting yarn tufts on (and motorcylcel guys, PLEASE put them on the rider!) and running at 40, 80, and 100+ (the limit of your chase car, photography platform) will give everyone an understanding of what separates at low Rn and how stagnation grows and separation shrinks at high Rn.

Since I have not seen your pictures, I will speculate on your "hard salt" deposits.  This is an informed opinion, nothing more.  As air flows over exposed wheels it accelerates more on the upper portion than the lower half due to the higher relative velocity at the top of the wheel.  This high velocity should create low pressure on top of the tire (lift) and help adhered salt spray at the 10 o'clock and 2 o'clock positions (looking inboard at the right wheel) while preventing it from spraying at the 11 to 1 o'clock arc.  Bicyclists get a stripe up their a-- in the rain because the upper half of the rear wheel in enveloped in the separated flow from the rider, lakesters do not have this.  It would be rational for the wheel to spit the salt off at a specific 9 to 10 o'clock and 2 to 3 o'clock points with enough centrifugal force to compress it on the impacted surface.

My experience with salt and grading a course (yes, ONLY one season and we didn't use the course) is that the greater the force of the salt spray the harder the deposit became.  At the speeds you are running, the salt becomes ballistic.  The soft salt deposits are most likely random spray caught in disturbed air flow.  The hard salt is coming from the spinning gun of your wheels.  There are specific aerodynamic and mass transport effects causing it, tuft it and post pictures.