A tile assembly (22) which, in use, is fitted to a base structure to form at least part of a fluid washed surface. The tile assembly comprises a housing (42) with at least one plenum (45) being provided within the housing (42). A wall (44) of the housing (42) is provided with a plurality of flow passages (46) which extend from the plenum side of the wall (44) to an outer surface (48) of the wall. Flow passage closures (50) are provided which are operable to open and close at least some of the flow passages (46).