Numerical Study On The Characteristics And Its Influencing Factors Of Shock Train In The Isolator

In this thesis,numerical simulation is used to study the characteristics of shock train in the isolator of scramjet engine,and the influence of factors such as back pressure ratio,Mach number,wall expansion angle,cross-sectional shape,wall cavity,etc.are analyzed.The change law of structure and important flow field parameters are obtained.First,the flow field of the rectangular isolator under the conditions of different inlet and outlet back pressure ratios,different incoming flow Mach numbers and different expansion angles is simulated and analyzed.The results show that the length of the shock train and the number of nodes increase,and the shape and spacing of the shock waves remain unchanged.The shock train advancing at the entrance becomes longer.At the same time,the wall pressure and average temperature in the flow field increase,the shock wave intensity at the center of the flow field is greater than the shock wave intensity at the wall surface,and the mass-weighted Mach number decreases.When the incoming flow Mach number increases,the distance that the shock wave train in the flow field advances toward the entrance decreases,the length and the inclination angle decrease,and the isolator’s anti-backpressure ability increases.The relationship between the inlet and outlet mass weighted Mach number of the isolator and the average temperature with the back pressure ratio under the condition of incoming Mach number can be approximately fitted to the form of a quadratic polynomial.In the symmetrical expansion isolator,as the expansion angle increases,the shape of the shock train does not change significantly,and the distance to the entrance gradually becomes longer;the length of the shock train becomes shorter,and the number of visible shock nodes decreases.For the unilateral expansion isolator,the flow field presents an asymmetric flow field,showing the phenomenon of shock wave impacting the wall surface reflection,and the separation degree of the upper and lower wall boundary layer separation areas at different expansion angles is opposite.Compared with the symmetrical isolator with the same expansion angle,the shock wave in the unilateral expansion angle isolator has a longer distance to the entrance,and the anti-backpressure ability is reduced.The change of various parameters in the flow field has nothing to do with the expansion form,and the expansion angle is the decisive reason.Secondly,the shock train and parameter characteristics in the flow field with different aspect ratios and circular cross-section isolators are studied.The simulation results show that: the aspect ratio increases,the shape of the shock train does not change significantly,the advancing distance to the entrance increases,and the isolation antibackpressure ability of the segment decreases,the shock wave spacing of the shock in the flow field becomes larger,and the number of visible shock waves decreases.The performance of the circular isolator is significantly better than that of the rectangular isolator with different aspect ratios,the total pressure loss in the flow field is the smallest,and the anti-backpressure ability is the strongest.Finally,the influence of the position and structure of the wall cavity on the shock train characteristics in the isolator is studied.The results show that the mutual coupling of the cavity position and the back pressure ratio will cause subcritical cavities and supercritical cavities models.In the subcritical cavity model,the shape of the shock train in the isolator is slender,and the distance between the nodes is short and unclear.At this time,the parameters in the flow field are the same as those in the isolator without a cavity structure.In the supercritical cavity model,the shock train in the isolator of the shock train structure and the non-cavity structure is similar,the distance of the shock train advancing to the entrance is greatly advanced,and the antibackpressure ability of the isolator decreases.In the study of the different structures cavity isolation,it is found that the long cavity isolator generates obvious shock waves in the flow field at the cavity,and the wall pressure and temperature in the flow field are higher than that of the open cavity.The mass-weighted Mach number and the total pressure value is lower,the final value is the same;while the flow field of the shallow cavity isolator is closer to the open cavity flow field,but the shock wave intensity at the cavity is greater,and the shock train shape after the cavity is more consistent,it is similar to that in the open cavity.The temperature,mass-weighted Mach number and total pressure oscillations in the flow field are weaker than the open cavity,indicating that the change of the cavity length is the key to changing the flow field and antibackpressure performance in the isolator,depth changes will not have a big impact.