Abstract: In a rotational speed viscous dampening device comprising a housing, a shaft having one end located in the housing and rotatable relative thereto, and a rotor body on the one end of the shaft with viscous fluid at least partially filling the housing, an improvement includes various rotor configurations for managing air within the housing so that the air does not interfere with viscous shearing of molecules of the viscous fluid between the rotor body and an interior wall of the housing. The viscous dampening device may be used to control the speed of rotation of a stream distributor component of a sprinkler.