The size of the wake decreases, indicating that the boundary layer separation on the cylinder or sphere occurs further along the surface than before. What has happened? The phenomenon is related to ...

The tunnel operating conditions were chosen to give a Reynolds number range so that at the lowest Reynolds number the flow will be laminar (even on the tunnel walls), and at the highest Reynolds ...

If we could see laminar flow, it would look like very thin sheets of air sliding smoothly over each other at increasing speeds, starting from the boundary layer, which has zero velocity.