On 25/10/2018 17:11, Nightjar wrote:
They are a Savonius rotor, which uses the wind to rotate a shaft. There
is a fan attached to the shaft, which, unlike a simple hole, actively
extracts air from the vehicle. They are also widely used on boats, to
keep them ventilated while sitting at their moorings.

And they have the advantage that they work with the wind in any
direction. Even when you are parked (or moored!)

Andy