A core duct assembly for a gas turbine engine, the core duct assembly comprising: a core duct comprising an outer wall and an inner wall, the outer wall having an interior surface, the core duct defining a gas flow path; a gas flow path member extending across the gas flow path at least partly between the inner and outer walls (46, 48) and having a leading edge and a trailing edge; at least one compressor rotor blade located downstream of the gas flow path member within the gas flow path, the compressor rotor blade having a radial span extending from a blade platform to a blade tip, wherein an upstream wall axis is defined as an axis tangential to a point on a first portion of the interior surface of the outer wall of the core duct extending downstream from the gas flow path member, the upstream wall axis lying in a longitudinal plane of the gas turbine engine containing the rotational axis of the engine, wherein the upstream wall axis intersects the compressor rotor blade at a point spaced radially inward from the blade tip of the compressor rotor blade, wherein the interior surface of the outer wall of the core duct further comprises a second portion downstream of the first portion, the second portion being formed by a portion of the interior surface having a convex profile and extending along the core duct between an upstream boundary and a downstream boundary, wherein the first portion of the interior surface of the core duct outer wall forms a non-convex portion extending between: an intersection of the trailing edge of the gas flow path member and the interior surface; and the upstream boundary of the second portion; wherein an acceleration distance (a) is defined as: the distance along the interior surface of the core duct outer wall between the intersection of the trailing edge of the gas flow path member and the interior surface; and a point on the upstream boundary of the second portion; wherein the second portion has a centre point midway between the upstream boundary and the downstream boundary, and a separation distance (b) is defined as: the axial distance between the centre point of the second portion and an axial position level with the radial tip of the leading edge of the compressor rotor blade; and characterised in that a ratio defined as: the acceleration distance a the separation distance b is in a range between 0.13 and 2. a core duct comprising an outer wall and an inner wall, the outer wall having an interior surface, the core duct defining a gas flow path; a gas flow path member extending across the gas flow path at least partly between the inner and outer walls (46, 48) and having a leading edge and a trailing edge; at least one compressor rotor blade located downstream of the gas flow path member within the gas flow path, the compressor rotor blade having a radial span extending from a blade platform to a blade tip, wherein an upstream wall axis is defined as an axis tangential to a point on a first portion of the interior surface of the outer wall of the core duct extending downstream from the gas flow path member, the upstream wall axis lying in a longitudinal plane of the gas turbine engine containing the rotational axis of the engine, wherein the upstream wall axis intersects the compressor rotor blade at a point spaced radially inward from the blade tip of the compressor rotor blade, wherein the interior surface of the outer wall of the core duct further comprises a second portion downstream of the first portion, the second portion being formed by a portion of the interior surface having a convex profile and extending along the core duct between an upstream boundary and a downstream boundary, wherein the first portion of the interior surface of the core duct outer wall forms a non-convex portion extending between: an intersection of the trailing edge of the gas flow path member and the interior surface; and the upstream boundary of the second portion; wherein an acceleration distance (a) is defined as: the distance along the interior surface of the core duct outer wall between the intersection of the trailing edge of the gas flow path member and the interior surface; and a point on the upstream boundary of the second portion; wherein the second portion has a centre point midway between the upstream boundary and the downstream boundary, and a separation distance (b) is defined as: the axial distance between the centre point of the second portion and an axial position level with the radial tip of the leading edge of the compressor rotor blade; and characterised in that a ratio defined as: the acceleration distance a the separation distance b is in a range between 0.13 and 2. the distance along the interior surface of the core duct outer wall between the intersection of the trailing edge of the gas flow path member and the interior surface; and a point on the upstream boundary of the second portion; wherein the second portion has a centre point midway between the upstream boundary and the downstream boundary, and a separation distance (b) is defined as: the axial distance between the centre point of the second portion and an axial position level with the radial tip of the leading edge of the compressor rotor blade; and characterised in that a ratio defined as: the acceleration distance a the separation distance b is in a range between 0.13 and 2. the axial distance between the centre point of the second portion and an axial position level with the radial tip of the leading edge of the compressor rotor blade; and characterised in that a ratio defined as: the acceleration distance a the separation distance b is in a range between 0.13 and 2.