Aerodynamic And Infrared Radiation Characteristics Of Low Observable Double S-shaped Nozzles

A parametrical design method based on super-elliptical transition and infrared radiation shield for double S-shaped nozzle was introduced.The complete shield of high temperature components in the S-shaped nozzle was achieved by the method mentioned in this paper.Numerical simulations and experiences were performed to investigate the effects of design parameters S/D,L/D and W/H on the aerodynamic performances.The effects of these three parameters and nozzle inlet temperature on the infrared radiation characteristics of 2D S-shaped nozzle were also analyzed.In addition,the RBF neural network and genetic algorithm were used to optimize the structure of 2D S-shaped nozzles.Finally,the integrated model of nozzle and aircraft was built to obtain the influence of nozzle type and trailing edge cooling on the aerodynamic performance and infrared radiation characteristics of the whole aircraft.The main conclusions are as follows:1.Local high velocity zones exist at the inflection of the first turning and the second turning of the nozzle;A obvious recirculation zone is observed in a shorter nozzle(L/D=3),which causes a lot of aerodynamic losses;The total pressure recovery coefficient and the thrust coefficient increase firstly,and then decrease by the increase of the offset ratio.The nozzle thrust coefficient of S/D=0.25 increases by 5.6% and 15.2% respectively compared with S/D=0 and S/D=1 nozzles.The aerodynamic performances increase sharply,and then trend to const with the increase of the ratio of length to diameter for nozzles.When L/D increases to more than 3.5,the total pressure recovery coefficient and thrust coefficient are basically unchanged,and the range of variation is not more than 0.5%;A vortex pair exists at the nozzle exit.With the increase of aspect ratio,the influence of vortex is gradually reduced,while the friction loss is increased.There exists a optimum aspect ratio for double S-shaped nozzle.2.The double S-shaped nozzle can effectively shelter high temperature components in the nozzle and reduce its infrared radiation intensity.Both aspect ratio W/H and offset ratio S/D have significant impacts on the infrared radiation intensity and it is reduced by the increase of aspect ratio and offset ratio.The maximum depression is more than 50%.The change of ratio of length to diameter has little influence on the infrared radiation intensity of nozzles.When the inlet temperature of the nozzle has a increase of 100 K,the infrared radiation intensity can be enhanced by double.3.The final optimization result is W/H=6.67,S/D=0.250,and the thrust coefficient has the maximum value of 0.932 when the ratio of length to diameter(L/D)was 3.Recirculation zone disappeared in the first section of the optimized nozzle.The range of recirculation zone in the second section and the separation zone at the end section of the optimized nozzle are both reduced.Compared with the reference nozzle(W/H=3.3,S/D=0),the total pressure recovery coefficient of the optimized nozzle increase by 5%,the thrust coefficient increase by 9%,and the total pressure distortion coefficient of outlet decrease by 6%.4.The nozzle type has less influence on the aerodynamic performance of the aircraft.Due to the unique shielding structure of the 2D S-shaped nozzle,the overall infrared radiation intensity of the aircraft with 2D S-shaped nozzle is weakened compared with that with the conventional 2D nozzle exhaust system.Adding the cooling structure at the trailing edge of the aircraft can effectively reduce the wall temperature of the trailing edge,thereby reduce the radiation intensity of the rear hemisphere.