A gas cleaning separator for separating a flowable mixture of substances of different densities, such as a gas and liquid; the separator comprising: a housing (4',70') defining an inner space, a rotor assembly (78',84') for imparting a rotary motion onto said mixture of substances, the rotor assembly (78',84') being located in said inner space and rotatable about an axis relative to the housing (4',70'), wherein the rotor assembly comprises an inlet for receiving said mixture of substances, an outlet from which said substances are ejected from the rotor assembly during use, and a flow path for providing fluid communication between the inlet and outlet, wherein the outlet is positioned more radially outward from said axis than the inlet; and the rotor assembly (78',84') further comprising fan/turbine unit and a rotary shaft coincident with said axis and mounted to said housing (4',70'), wherein a first end portion of the rotary shaft extends through said housing (4',70') to a position exteriorly of said housing (4',70') and a fluid passageway extends axially through the rotary shaft and has an opening positioned exteriorly of said housing (4',70'); characterised in that the rotor assembly (78',84') further comprises flow control means (364,366) for controlling fluid entry to said shaft fluid passageway from the exterior of said housing (4',70'), wherein the flow control means (364,366) comprise a plate member arranged at the lower part of said fan/turbine unit, spaced from said opening of the shaft fluid passageway, and provided with at least one aperture extending through said plate member and being positioned radially outward from said axis of rotation of the rotor assembly (78',84'). a housing (4',70') defining an inner space, a rotor assembly (78',84') for imparting a rotary motion onto said mixture of substances, the rotor assembly (78',84') being located in said inner space and rotatable about an axis relative to the housing (4',70'), wherein the rotor assembly comprises an inlet for receiving said mixture of substances, an outlet from which said substances are ejected from the rotor assembly during use, and a flow path for providing fluid communication between the inlet and outlet, wherein the outlet is positioned more radially outward from said axis than the inlet; and the rotor assembly (78',84') further comprising fan/turbine unit and a rotary shaft coincident with said axis and mounted to said housing (4',70'), wherein a first end portion of the rotary shaft extends through said housing (4',70') to a position exteriorly of said housing (4',70') and a fluid passageway extends axially through the rotary shaft and has an opening positioned exteriorly of said housing (4',70'); characterised in that the rotor assembly (78',84') further comprises flow control means (364,366) for controlling fluid entry to said shaft fluid passageway from the exterior of said housing (4',70'), wherein the flow control means (364,366) comprise a plate member arranged at the lower part of said fan/turbine unit, spaced from said opening of the shaft fluid passageway, and provided with at least one aperture extending through said plate member and being positioned radially outward from said axis of rotation of the rotor assembly (78',84').