Investigation On Design Method And Performance Estimation Of Low Observable S-shaped Nozzle

Low observable signature is one of the most critical characters for the modern and future stealth fighters.The Infrared Radiation Signature(IRS)and Radio Cross Section(RCS)of aero-engine exhaust could be greatly suppressed by low observable S-shaped nozzle,and the stealth ability of fighters is strengthened.Thus,it has come into the stealth technology all over the world.So this paper emphasizes on the exhaust of stealth fighters,the design method,the complex fluid mechanism,the effect of parameters on the flow characteristics,and the coupled with aero-engine of S-shaped nozzle are investigated.The research contents are as follow:1.Based on the S-shaped structure,circular-rectangular transition,and two dimensional exit of low observable S-shaped nozzle,the design method of transformable section S-shaped nozzle based on coupled parameters was presented.The qualifications to completely shield the high-temperature turbine were built.Different configurations of S-shaped nozzle were designed by the design procedure.The design process included the design of centerline,the design of sections,the build of the qualification to completely shield the high-temperature turbine and the conformation of S-shaped nozzle model.The alterable geometric parameters in the design of low observable S-shaped nozzle included the number of S-shaped centerline,the streamwise length of each S-shaped centerline,the offset distance of each S-shaped centerline,the distributions of each S-shaped centerline,the area,the width,the height,the exit shape and their distributions of each S-shaped passage.The qualifications to completely shield the high-temperature turbine of low observable S-shaped nozzle was that the common tangent of the upside and underside longitudinal curves should cross either exit upper point or inlet lower point.2.The suitability of turbulence model for the simulation of S-shaped nozzles was validated by model experiment and CFD.Based on the detailed flow features,the complex flow characteristics and fluid mechanism of the low observable S-shaped nozzle were analyzed detailedly.Results showed that,SST k-ω turbulence model could well predict the flow features of low observable S-shaped nozzle.Flow parameter distributions of low observable S-shaped nozzle were non-uniform.Local acceleration regions were induced by bend structure.Stremwise negative pressure gradient was induced near the center of curvature behind the inflection,and the shear layer thickness was increased.The vortical secondary flow was formed as the flow near wall moved righrabout under the transverse and normal pressure gradient as compared to core flow.The acceleration loss,impact loss,secondary loss,shock wave loss,and friction loss were shown in low observable S-shaped nozzle,the nozzle performance were decreased due to these loss.3.The effects of the pneumatic and geometric parameters on the flow characteristics and performance of low observable S-shaped nozzle were investigated under uniform incoming flow,such as the nozzle pressure ration,the number of S-shaped centerline,the distributions of centerlines,the exit parameters of first S-shaped passage,the length of each centerline,the exit aspect ratio and the length-diameter ratio.The essential reason of the effect of each parameter on performance was given.Results showed that,the low observable double S-shaped nozzle was better to fulfill the demand of nozzle performance and space position as compared to the single S-shaped nozzle and tree S-shaped nozzle.In the design process of low observable double S-shaped nozzle,centerline curves with a gentle turning at each inflection should be chosen,the large exit area of first S-shaped passage should the chosen,the exit width of first S-shaped passage could be large under the require of space,the small offset distance of first S-shaped passage should be chosen,the exit aspect ratio was recommended to be chosen from 4 to 6,the length ratio of two S passages was recommended to be chosen from 2:3 to 1:1,the length-diameter ratio was recommended to be chosen from 2 to 3.The effects of the double ducts(bypass and core flow),the tail cone,the struts,the inlet swirl flows from the engine turbine,and the change of aero-engine state on the complex flow characteristics of the low observable S-shaped nozzle was investigated.Results showed that,the performance of low observable S-shaped nozzle was mainly induced by its inner flow loss.The nozzle performance decreased slightly with the increase of swirl angle and the increase of strut setting angle.4.The matching characteristics of low observable S-shaped nozzle and aero-engine was conducted.Based on Design of Experiment(DOE),Response Surface Methodology(RSM)approximate model and aero-engine performance simulation model with components,the matching model and performance estimation of low observable S-shaped nozzle with aero-engine was conducted.Results showed that the attitude-speed characteristics and the throttle characteristics of the aero-engine with low observable S-shaped nozzle were similar with the aero-engine with axisymmetric nozzle.The operating lines on the high compressor characteristic map were coincident for the aero-engine with low observable S-shaped nozzle and the aero-engine with axisymmetric nozzle,while the operating line of aero-engine with low observable S-shaped nozzle on the fan characteristic map moved toward the surge line as compared to the aero-engine with axisymmetric nozzle.Under different flight attitude,speed and rotate speed,the thrust of aero-engine with low observable S-shaped is lower than that of the aero-engine with axisymmetric nozzle,while SFC is larger,0.04%~5.95% decrease in thrust and 1.43%~6.48% increase in SFC.The effect of geometry parameters of the low observable S-shaped nozzle on the thrust and SFC was related to the distribution of flux coefficient and thrust coefficient versus the geometry parameters.The performance of matching model was mainly affected by the length ratio of two S passages,the exit area of first S passage,and exit aspect ratio.The effect of geometry parameters on the performance of matching model was similar when the flight conditions were changed.