Evaluation of Boattail Geometry and Exhaust Plume Temperature Effects on Nozzle Afterbody Drag at Transonic Mach Numbers

Book excerpt: An experimental program was conducted to investigate the interaction effects which occur between the nozzle exhaust flow and the external flow field associated with isolated nozzle afterbody configurations at transonic Mach numbers. Pressure data were obtained from three afterbody geometries with boattail angles of 10, 15, and 25 deg at Mach numbers from 0.6 to 1.5 at zero angles of attack and sideslip. Cold (High-pressure air) and hot (Air/ethylene combustion) jet test techniques were used to simulate and duplicate, respectively, the nozzle exhaust flow for a sonic jet installation. Nozzle exhaust temperature was varied from 540 to approximately 2,900 R. The most significant results pertain to those effects on boattail pressure drag caused by exhaust plume temperature and flow asymmetry (Model support strut induced). The differences obtained in boattail pressure drag between the cold jet simulation and hot jet duplication results were significant at nozzle pressure ratios representative for turbofan and turbojet engines at subsonic Mach numbers. Adjusting the cold jet nozzle pressure ratio to correct for changes in the exhaust plume specific heat ratio with temperature did not account for the differences observed. Flow asymmetry effects were Mach number and nozzle pressure ratio dependent and increased in severity as the boattail angle was increased.