Large Eddy Simulation And Infrared Radiation Characteristics Of Non-isothermal Turbulent Jet Flow

The aero-engine and its heat-affected zone are the main sources of aircraft infrared radiation(IR)and are the key parameters affecting the aircraft’s battlefield survivability,especially under afterburner state where the IR of the jet flow of the engine exhaust system increases significantly.The nozzle jet flow of the engine exhaust system is a typical non-isothermal turbulent jet flow,research around which will help to develop more efficient IR suppression techniques for aircraft engines,and provide reference data for the development of more advanced IR detection techniques.To study the effect of the turbulent fluctuation on the IR of high temperature turbulent jet flow,investigations of large eddy simulation(LES)of the non-isothermal turbulent jet flow and IR calculation method based on LES are carried out.The main research contents and achievements include:(1)Investigation of the application of generating inflow turbulence by using simple recyclingrescaling method(RRM)for unfully development pipe flow is carried out.Based on the RRM proposed by Spalart orgionally for plate flow,a ratio of the velocity at the inlet to the recycling plane is introduced to develop a new RRM that can be applied to generate inflow turbulence for undeveloped pipe flow.By analogzing the velocity boundary layer and temperature boundary layer,the application of this new RRM is extended to generation inflow turbulent temperature.Compared with the popular RRM proposed by Lund,this new RRM reuires less information and is easier to execute.Based on the RRM proposed by Uzun originally for turbulent velocity generation,a RRM used to generate inflow turbulent temperature is proposed by analogy of velocity bounfary layer and temperature boundary layer.This method can control the shape of temperature profile at the inlet more accurately than the RRM developed from Splalart’s method.(2)Investigation of developing inflow turbulent velocity and temperature generation method for high Reynolds number(Re)flow is carried out to decrease the grid cost when executing RRM in high Re number flow.According to that the ratios of the instantaneous fluctuating velocity to its root mean square of different Re number flows in similar geometric are approximately equal,an inflow turbulent velocity and temperature generation method using turbulent data extracted from relatively low Re flow is proposed.This method avoids the significant cost of executing the RRM in high Re numbe flow.By comparing the round turbulent jet flow with inflow turbulence generated by this method and the Vortex method,it is proved that this method can generate inflow turbulence with real turbulence characteristics,and thus more accurate turbulent jet field can be obtained than the Vortex method.(3)Investigation of how a round jet at Re=86000 in an ambient temperature 301K develops with the ratio of the external fluid densityto the jet fluid density(jet temperature ratio,Θ0/Θj)changing from 0.96 to 0.35 is carried out.It is found hat the the half-width of velocity field and temperature field of the non-isothermal round jet are proportion to the axial distance and are inversely proportion to the 1/6th power of Θ0/Θj.The radial profiles of average velocity and temperauture match with the Gaussian distribution whose exponent almost decreases linearly with Θ0/Θj.(4)Based on the flow field calculated by the LES,the influence of the turbulent fluctuation of the IR of the round turbulent jet and the jet field of a S-shaped two-dimensional(2-D)nozzle is investigated.It is found that the average radiation intensity calculated by the instantaneous temtepatur and instantaneous species concentration is greater than the average radiation intensity calculated by the average temperature and average species concentration.For the round jet at Re=86000 and inflow average temperature at 850K,the relative error between the two average radiation intensity in the 90°detection direction is 23.6%,and the local raltive error reaches up to 73.5%.For the jet flow field of the S-shaped 2-D nozzle at Re=2.8×105 and inflow average temperature at 555K,the relative error between the two average radiation intensity in the detection direction from the narrow side is 42.1%,and the local raltive error reaches up to 84.3%,while in the detection direction from the narrow side this two errors are 38.4%and 96.4%respectively.The results obtained in thispaper provide a betterinflow turbulence generation method for the LES of high Renumber jet flow and thus help to carry out other related research.The simulation results of the flow field and IR characteristics are helpful to understand the development of the nozzle jet flow of the aero-engine,and promote the development of IR suppression and detection techniques.