A gas turbine engine (20) includes fan, compressor, combustor, and turbine sections (28, 30, 32, 34), an outer casing (43), an outside core annular region (68), and a fan air circulation system (52). The outer casing (43) is disposed radially outside of the compressor, combustor, and turbine sections (28, 30, 32, 34). A core gas path extends through the compressor, combustor, and turbine sections (28, 30, 32, 34), and is disposed radially inside of the outer casing (43). A fan bypass air duct (62) is defined by inner and outer radial flow path boundaries (66, 60). The outside core annular region (68) is disposed radially between the outer casing (43) and the inner radial flow path boundary (66). The fan air circulation system (52) is configured to receive fan bypass air from the fan bypass air duct (62) and selectively pass the received fan bypass air into the outside core annular region (68).