Abstract: A wind powered (e.g. roof-mounted) device contributes to ventilation and/or air circulation for vehicles (including road vehicles and boats), trailers, buildings e.g. portable buildings, containers, or any (relatively) flat roofed structure requiring ventilation and/or air circulation. The device for ventilation and/or air circulation includes a rotor assembly with a fan and a rotor operatively connected to the fan and capable of being rotated by moving air to drive the fan to rotate. The rotor has a base plate and at least two rotor blades generally curved at least in part and extending upwardly from the base plate in spaced overlapping relation with their concave, or partly concave, internal surfaces in opposition to define an air passage between the rotor blades. A portion of the external surface of one or both rotor blades includes a plurality of raised and/or recessed surface features.