A separator (1) for separating contaminants from a fluid stream having 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 apertures (29) 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 flow down to an oil outlet (23). A diaphragm (31) moves 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 to progressively occlude the apertures. The apertures are spaced so that there is no overlap between them along the actuator axis.