A separator (1) for separating contaminants from a fluid stream which includes entrained particulate contaminants comprises a cylindrical dividing wall (28) concentrically arranged within an impaction surface (35). The cylindrical dividing wall defines a first chamber (42) into which a fluid stream enters and flows axially through. The dividing wall has a number of apertures (29) in it through which the fluid stream passes towards the impaction surface. As the fluid impacts the impaction surface, the contaminants are separated from the fluid and then flow down to an oil outlet (23). A diaphragm (31) is arranged to move along an axis to adjust the open cross-sectional area of the apertures in the dividing wall according to a pressure differential between fluid pressure in the first chamber and a pressure reference by moving along the dividing wall so as progressively to occlude the apertures. The apertures are spaced so that there is no overlap between them along the actuator axis.