Variable geometry turbine

A variable geometry turbine comprises a turbine housing (4) defining a turbine chamber within which a turbine wheel (16) is mounted for rotation about a turbine axis (17). The turbine chamber has a radially extending annular inlet (18) defined between facing annular side walls (19,20) of the turbine housing, the annular inlet (18) having an axial width defined between the annular side walls (19,20). An annular array of swing vanes (22) is mounted within the inlet (18), each vane (22) having an axial length (L) extending across the inlet (18) between chordal vane edges lying adjacent respective annular side walls of the inlet (18), and a chordal length (W) defined between leading and trailing vane edges extending across the inlet (18). Means (24,25) are provided for pivoting each vane (22) about a respective pivot axis (23) extending across the inlet to adjust the effective cross-section area of the annular inlet. Each vane is pivotable between a first position in which the area of the inlet (18) is a minimum and a second position in which the area of the inlet (18) is a maximum. At least one of said annular side walls (18,19) is configured so that clearance between the side wall and at least a portion of the adjacent chordal edge of each vane (22) varies in a predetermined manner as the vane moves between positions intermediate the first and second vane positions.