A gas turbine engine disposed about a longitudinal axis, the engine comprising: a compressor; a turbine; a fan including a plurality of blades rotatable about the axis; a core case that extends rearward from the fan along the axis and generally surrounds the compressor and the turbine, the core case including an outer surface, a core inlet and a core flow path disposed within the core case, the core flow path configured to receive airflow from the fan and to convey the airflow to the compressor, the core inlet disposed at the entrance to the core flow path; a fan case that generally surrounds the fan blades and a portion of the core case, the fan case including a fan inlet and an inner surface, the fan inlet disposed upstream of the fan blades; a bypass flow path disposed downstream of the fan between the outer surface of the core case and the inner surface of the fan case, the bypass flow path configured to receive air drawn in by the fan through the fan inlet and to convey bypass airflow from the fan rearward; and a fan flow path that extends from the fan inlet to the rear exhaust outlet of the bypass flow path, wherein a portion of the fan flow path proximal to the fan is non-axisymmetric with respect to the axis, characterised in that: the inner surface of the fan case includes a first perturbation downstream of the fan; and the outer surface of the core case includes a second perturbation downstream of the fan and a third perturbation downstream of the fan, wherein the third perturbation is a recess that extends inward into the core case. a compressor; a turbine; a fan including a plurality of blades rotatable about the axis; a core case that extends rearward from the fan along the axis and generally surrounds the compressor and the turbine, the core case including an outer surface, a core inlet and a core flow path disposed within the core case, the core flow path configured to receive airflow from the fan and to convey the airflow to the compressor, the core inlet disposed at the entrance to the core flow path; a fan case that generally surrounds the fan blades and a portion of the core case, the fan case including a fan inlet and an inner surface, the fan inlet disposed upstream of the fan blades; a bypass flow path disposed downstream of the fan between the outer surface of the core case and the inner surface of the fan case, the bypass flow path configured to receive air drawn in by the fan through the fan inlet and to convey bypass airflow from the fan rearward; and a fan flow path that extends from the fan inlet to the rear exhaust outlet of the bypass flow path, wherein a portion of the fan flow path proximal to the fan is non-axisymmetric with respect to the axis, characterised in that: the inner surface of the fan case includes a first perturbation downstream of the fan; and the outer surface of the core case includes a second perturbation downstream of the fan and a third perturbation downstream of the fan, wherein the third perturbation is a recess that extends inward into the core case. the inner surface of the fan case includes a first perturbation downstream of the fan; and the outer surface of the core case includes a second perturbation downstream of the fan and a third perturbation downstream of the fan, wherein the third perturbation is a recess that extends inward into the core case.