Magnetic Wind Tunnel Alernative Idea
 

If the goal is to measure the aerodynamic drag force of a given shape, perhaps a different approach could work better for the DYI / smaller scale application. Depending on how much one cares about what is lost from a true wind tunnel.

At much slower wind speeds there is a measurable aerodynamic force on an object.

The more we minimize other forces we can get better resolution / accuracy.

A gauge to measure the back force of the object as it's aerodynamic drag pushes it away.

A large plate to support the vehicle ... line the plate and a section of floor with magnets to ( mag-Lev ) would remove rolling resistance losses from the equation... use guides around the edge of the plate to prevent the plate from moving out of the control area during testing or when the vehicle is driving into place... don't need much movement of the plate in the control are... just enough for the gauge to be able to measure the amount of wind pressure / force pushing the car.

Link
100 for $8+Shipping gives a combined ~267 Lbs repulsive force ... ~$160+Ship for magnetic repulsive force of ~5,340 Lbs should be good for many cars including the plate.

Of course there would be other costs as well... but it would be a less expensive way to measure the aerodynamic drag force for a given object at a given wind speed.

Doable in a normal sized garage... and should be able to get it reasonably accurate resolution at measuring the Aerodynamic Drag / Pushing force... it should be repeatable as well... to be able to retest after making small or temporary changes.

I know it's a crazy idea ... but aside from being crazy ... what to people think?

Perhaps you could do it in Kansas but I don't have that much wind in my garage.

what gauge would you use?

We wouldn't want to rely on random 'natural' wind or air flow we would want something we have more control over.

We wound need to generate some wind / air flow ... but with the friction / forces needed to move the object reduced by the fo 'mag-lev' we shouldn't need massive hurricane force winds.

I suspect the significantly reduced air flow requirements would make them far easier & cheaper to achieve.

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There are a variety of styles that could be used ... and a variety of options for each style.

In general I see two major categories.

Pull style:
placed in front of the vehicle ... as the aerodynamic drag pushes the vehicle away from the pull style gauge it could read the force being applied ... could be in grams , kg, pounds , whatever... there are a variety of options even in the 'pull' style.

Compression style:
placed behind the vehicle ... as the aerodynamic drag pushes the vehicle backward it will apply more force to the compression style gauge ... kitchen scales measure into the grams, bathroom scales measure in kg and pounds ... and of course any number of more accurate scales could be chosen.

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Ideally the smaller the units the gauge can accurately measure the less wind / air flow would be needed ... if it measures in grams far less air flow / wind would be needed compared to if it measures in kg or pounds.

If there is a specific kind of aerodynamic range upper limit and lower limit CdA of 5.0 or CdA of 20.0 or whatever... that gives us a ball park of the kind of forces we would be seeing from various amounts of air flow / aerodynamic drag force.

If there is a specific kind of resolution of the aerodynamic force we would like to be able to quantify and identify say CdA of 5.00 vs 5.10 ... or 5.01 ... the better the resolution the smaller the steps the gauge / scale would need to be able to support.

We can trade some scale for some resolution ... 10 gram resolution would be a better resolution if it is in a kg type scale than a 100 gram type scale.

If we have scale and resolution demands / desires ... that lets us narrow down the air flow / wind requirements ... and if our garage and budget put air flow / wind requirements or restrictions we can use those to help narrow down out gauge / scale requirements.

wind tunnel
 

I believe it was Alan Pope who wrote a book,Low Speed Windtunnel Design.You ought to check it out.\
There are some primary criteria which must be honored for a tunnel of any design and the short of the long of it, is that you'd never be able to achieve meaningful results in any tunnel you could do in a garage.
This stuff has been researched to death.You can't cheat in any way and expect results.
Models could end up costing you more than a full-scale build.Ford spent $60,000 for a 3/8th-scale Taurus.
You could do a small tunnel to rough out gross alterations but a homes 200-AMP service would not provide enough electric power for a fan capable of flow velocity and volumes you'd need for drag measurements.
Consider tuft testing.All you're looking for is attached flow and that's easy to ascertain.

My old employer at one time thought they'd put a big prop on an Olds 455 and blow that across our vehicles for "testing". Yeah. :(