[aerohead]

I'm guessing, they're below 20-mph airspeed to prevent dilution of the smoke, while compromising the fixed-Cd, creating the erratic image in the process.
Running at 20-mph, or faster would have provided the 'steady' smoke filaments ( like VW has used for their XL1 ), but that'd require a long-exposure, still-photograph in order to 'show' them.
Imaging the baby template car at DARKO was similarly challenging. White on white ,low contrast smoke flow, at supercritical Reynolds number.
Centerline-only illumination, in an otherwise, darkened test section, or something like the A2 Wind Tunnel's darker test section surfaces would have provided more visual contrast.