An inlet diffuser for a jet engine, the inlet diffuser comprising: a diffuser body defining an internal volume facing outboard walls and having an inlet to the internal volume for receiving incoming ambient air and an exit from the internal volume for delivering the air to a downstream portion of the jet engine, wherein the diffuser body has an internal surface facing the internal volume, the internal surface comprising a fore region proximate to the inlet and an aft region downstream of the fore region and proximate to the exit, a pair of elongate structures extending longitudinally along the internal surface from the fore region of the internal surface to the aft region of the internal surface, wherein the pair of elongate structure defines a channel of the internal volume inboard of the elongate structures, wherein the pair of elongate structures is configured to split a lower boundary of the air into a first portion and a second portion of boundary layer air such that the first portion of the boundary layer air flows within the channel between the pair of elongate structures from the fore region to the aft region, wherein a respective elongate structure of the pair of elongate structures comprises a wall face extending substantially perpendicular to the internal surface of the inlet diffuser and extending along a longitudinal length of the respective elongate structure, wherein the wall face is an inboard surface of the respective elongate structure and is configured to restrict the first portion of boundary layer air from flowing across the respective elongate structure, wherein the wall face comprises an apex portion opposite the internal surface, and wherein said respective elongate structure of the pair of elongate structures comprises a ramp face, said ramp face is configured to: (i) extend from the apex portion of the wall face to a portion of the internal surface of the inlet diffuser that is outboard of the respective elongate structure, and (ii) extend along a longitudinal length of the respective elongate structure. a diffuser body defining an internal volume facing outboard walls and having an inlet to the internal volume for receiving incoming ambient air and an exit from the internal volume for delivering the air to a downstream portion of the jet engine, wherein the diffuser body has an internal surface facing the internal volume, the internal surface comprising a fore region proximate to the inlet and an aft region downstream of the fore region and proximate to the exit, a pair of elongate structures extending longitudinally along the internal surface from the fore region of the internal surface to the aft region of the internal surface, wherein the pair of elongate structure defines a channel of the internal volume inboard of the elongate structures, wherein the pair of elongate structures is configured to split a lower boundary of the air into a first portion and a second portion of boundary layer air such that the first portion of the boundary layer air flows within the channel between the pair of elongate structures from the fore region to the aft region, wherein a respective elongate structure of the pair of elongate structures comprises a wall face extending substantially perpendicular to the internal surface of the inlet diffuser and extending along a longitudinal length of the respective elongate structure, wherein the wall face is an inboard surface of the respective elongate structure and is configured to restrict the first portion of boundary layer air from flowing across the respective elongate structure, wherein the wall face comprises an apex portion opposite the internal surface, and wherein said respective elongate structure of the pair of elongate structures comprises a ramp face, said ramp face is configured to: (i) extend from the apex portion of the wall face to a portion of the internal surface of the inlet diffuser that is outboard of the respective elongate structure, and (ii) extend along a longitudinal length of the respective elongate structure. (i) extend from the apex portion of the wall face to a portion of the internal surface of the inlet diffuser that is outboard of the respective elongate structure, and (ii) extend along a longitudinal length of the respective elongate structure.