Target scale for aero testing models?

[Jonny Hotnuts]

For aero testing is there an ideal or better scale to build a models at?

[hotrod]

If I understand your question, the short answer is build it as large as possible.

Aerodynamics does not scale well if the model is too small. Since you "normally" cannot also scale the air down in size, as the model gets smaller the behavior of the airflow begins to differ from the full scale car, due to changes in reynolds numbers etc.

As I recall many manufactures used 1/8 scale models for basic testing, but a 1/25th scale model car will work for gross evaluation of air flow but getting precise numbers off of it such as lift and drag is really difficult.

Also the smaller the model gets the more difficult it is to get the surface shape exactly right so the uncertainty goes up really fast.

Airflow issues like where the boundary layer trips and begins to become turbulent is extremely difficult to determine if the model is very small as you can never exactly duplicate the roughness and shape of the full size car and unless you are really into mathematics figuring out the effective air speed due to changes in the Reynolds numbers viscosity effects not scaling directly with model size makes any airflow behavior that depends strongly on those effects highly suspect.

When I was in high school I built a small wind tunnel and did a bunch of reading on small scale wind tunnel tests. Models much shorter than 18 inches or so were only useful for really gross evaluation of the air flow as I recall.

I'm sure one of our aerodynamics wizards will step in here and add to or correct the above, but that is my recollection from when I was playing with scale model wind tunnel testing.


That said, some yarn tufts and a good wind will give you a boat load of information with out the expense of doing precision models and finding a way to build or get access to a wind tunnel capable of realistic air speeds for land speed records.

Larry

[interested bystander]

 Speaking only of what is local (California) and available. Swift engineering in San Clemente, which, most importanly, is a modern AUTOMOTIVE tunnel - 45/ 50 percent.

Don't recall what Gurney's in Santa Ana is.

[maguromic]

Gurney also used a water tunnel built by the Roiling Hills Corporation in El Segundo.  It’s been a while and I can’t remember if he told me if they bought one or used one of the universities in SoCal.  But lots of schools have them and this could be an option for you as there is always a kid looking for a thesis project.  This is a list http://www.rollinghillsresearch.com/Water_Tunnels/Installations.htm  of the schools that has the Rolling Hills units. Tony

[wobblywalrus]

The water modeling is something I am familiar with, being a hydraulic engineer.  Larry's comments on the second post apply to hydraulic modeling, too.  It is hard to scale down the fluid properties such as surface tension, viscosity, etc. to match the smaller model.  A full size model is best.

Water is non compressible, unlike air.  This must be considered when evaluating the results of hydraulic modeling to determine airflow.  Personally, if my life depended on getting accurate results, I would not use water to model airflow. 

[1212FBGS]

the best scale size is 100%.... for the cost of building and testing your model you could drive and teat at A2
kent

[Blue]

If I understand your question, the short answer is build it as large as possible.

Aerodynamics does not scale well if the model is too small. Since you "normally" cannot also scale the air down in size, as the model gets smaller the behavior of the airflow begins to differ from the full scale car, due to changes in reynolds numbers etc.

As I recall many manufactures used 1/8 scale models for basic testing, but a 1/25th scale model car will work for gross evaluation of air flow but getting precise numbers off of it such as lift and drag is really difficult.

Also the smaller the model gets the more difficult it is to get the surface shape exactly right so the uncertainty goes up really fast.

Airflow issues like where the boundary layer trips and begins to become turbulent is extremely difficult to determine if the model is very small as you can never exactly duplicate the roughness and shape of the full size car and unless you are really into mathematics figuring out the effective air speed due to changes in the Reynolds numbers viscosity effects not scaling directly with model size makes any airflow behavior that depends strongly on those effects highly suspect.

When I was in high school I built a small wind tunnel and did a bunch of reading on small scale wind tunnel tests. Models much shorter than 18 inches or so were only useful for really gross evaluation of the air flow as I recall.

I'm sure one of our aerodynamics wizards will step in here and add to or correct the above, but that is my recollection from when I was playing with scale model wind tunnel testing.


That said, some yarn tufts and a good wind will give you a boat load of information with out the expense of doing precision models and finding a way to build or get access to a wind tunnel capable of realistic air speeds for land speed records.

Larry

Well said, nothing to add.

[Blue]

the best scale size is 100%.... for the cost of building and testing your model you could drive and teat at A2
kent

Also well said.  Tuft the full scale car and strap cameras all over it.  Full scale testing of a 300 mph car at 60 mph is more accurate than wind tunnel testing of a 1/5th scale model.  Even though the Reynolds numbers are the same, the accuracy of the full scale car is superior to any model.