@shortbus

Changing any internal structure in a tunnel usually means a lot of time to de-rig, re-rig and re-calibrate.

I'm not sure about one of your earlier comments -
Since wind tunnels usually suck/pull, not blow, the air through them.

Yes - I do expect to have a contraction stage, a test section, a diffuser and a fan, in that order, so the air will be sucked through. I had to imagine how you would position an orifice or baffle in the downstream diffuser, so you may have an approach that I had not thought of. But keeping a predictable airflow through the test section depends a lot on maintaining a fixed flow which adheres reliably to the walls of the diffuser. 'Off-centre' suction would result in a gradient across the test section, and baffles could cause boundary layer separation. This would be likely to result in surges, which would probably propagate upstream, and into the test section...

 

Not a job for a stepper, they produce their max torque when holding--they are not good at providing torque to move loads.

Not quite true.
With the right controller maintaining the motor plate rated coil current, the torque can be maintained constant throughout the useful range.
.

 

There is a problem driving a PSC motor with a VFD: the capacitor’s reactance varies with frequency.
Which in practice means that the split phase winding won’t see the correct electric phase to generate the rotating magnetic field, it would become a pulsating field.....

This is what I would do: since you already have that AC motor, at least give it a try with a Triac based control. Those are inexpensive, and it might just work.
But if it doesn’t, then you’ll have to bite the bullet and obtain the correct motor- drive combo.
Either a 3 phase with a VFD or a DC one with a phase control.

 

60RPM is gearhead and/or DC motor territory. A sufficiently massive DC motor with closed-loop speed control would provide plenty of low speed torque, broad speed range and adequate speed regulation for this sort of job. DC treadmill motors on the order of 1-4HP can be had for cheap and usually come fitted with heavy flywheels to aid in speed regulation. Speeds on the order of 20-5,000 RPM depending on loading and cooling. You would simply need to furnish an encoder/tachometer and a closed-loop DC speed controller. Or if the load is static, adjust the input voltage until a photo tac reads the desired speed.

 

In this model the fan turns high rpm

 

This one is even faster 2000RPM