Prandtl Meyer expansion fan starts around the 35 second mark where you see the normal shockwave appear in the tunnel. At this point the upstream pressure has reached double the pressure at the throat and we have a choked condition. You can see how the flow following the wall, squeezes past the normal shockwave and is accelerated further. This is known as an expansion fan, and all venturi tunnels experience this phenomenon when the pressure ratio reaches ~2:1, at least with air at room temperature and at sea level. The middle venturi with increased cross sectional area but same AOA, creates a vortex that avoids the shockwave phenomena(starts at ~1:00), this is because the vortex core lowers the fluid's pressure, reducing the pressure ratio between the inlet and throat. The same expansion fans are present in the middle venturi. The bottom one takes much longer to both choke and the expansion fans are much weaker, perhaps if you were trying to create maximum flow, this venturi tunnel would be a smidge too large for purpose. Something like the middle one would be ideal because of the vortex that forms at the throat mitigating the shockwave and accelerating flow at the same time.