Performance of a flapped duct exhausting into a compressible external flow

This paper describes a computational study of the performance of a flapped exhaust duct. The duct curves through 90° so that the exhaust gases are turned into the streamwise direction before passing out into the primary flow. The exhaust port is of rectangular cross-section and a flat plate flap is located on its upstream edge. Mach numbers of 0.4, 0.55, 0.7 and 0.8 are considered and flap angle varied between 15° and 45°; the pressure ratio was varied between 0.64 and 0.97 in order to obtain the range of discharge flow ratio coefficients required. The ratio of boundary layer thickness to orifice length varies between 0.095 and 0.110 and the Reynolds number of the flow at the exhaust leading edge varies between 1.8×10 and 3.5×10. Thrust and discharge coefficients are predicted. The predictions are validated against published data. The flow is shown to comprise a complex mixture of a jet emerging into a compressible flow, longitudinal vortices generated at the flap side edges, a shear layer shed from the flap trailing edge and, depending on pressure ratio, a normal shockwave. The study will inform a larger investigation into the performance of pressure relief doors.