Large-eddy Simulation Of The Internal Flow In Supersonic Straight Duct

The investigation of supersonic internal flow is of primary interest owing to its extensive fundamentals and applications.In this dissertation,large-eddy simulation is employed to study several internal flows to reveal the complex flow phenomena and behaviors in a constant area duct and in a straight duct as well as in the straight duct with constant backpressure and periodic backpressure.The results and conclusions are briefly given as follows:(1)Numerical investigation of the normal shock-train in a constant area duct was carried out.The shock-train phenomenon observed experimentally was well sim-ulated.Various fundamental mechanisms dictating the intricate flow phenomena,including shock-train structure,shock/boundary layer iinteraction,turbulent s-tatistic features and unsteady characteristics,were investigated.The shock-train structure and the separation shear layer exhibit an obvious influence on the tur-bulencc features.Due to the unsteady character of the shocks,the turbulence intensities and turbulence kinetic energy(TKE)are enhanced.Meanwhile,the unsteady behaviors of the shocks have a contribution to the enhancement of the pressure fluctuation.With the analysis on the transport equation of the TKE,it was found that the unsteady behaviors of the shocks and the separation shear layer have a significant effect on the production of the TKE.The investigation on the transport equation of the pressure fluctuation reveals that the shocks have a major contribution to the production of the pressure fluctuation.Furthermore,by means of the proper orthogonal decomposition analysis on the pressure fluc-tuation field,it is found that the oscillation of the shocks and the evolution of the separated shear layers mainly contribute to the unsteady features of the flow field.(2)The supersonic internal flow in a straight duct and the behavior of the local flow topologies were investigated.Three typical flow regions are considered with dif-ferent pressure gradients in the duct,i.e.the adverse pressure gradient(APG),favorable pressure gradient(FPG)and zero pressure gradient(ZPG).The be-haviors of local flow topologies are studied in detail for the different pressure gradients.It is found that the most probable topology in the inner layer is the unstable node/saddle/saddle(UN/S/S)topology in the APG region and a focal topology in the FPG and ZPG region.The characteristics of conditional average dissipation and enstrophy based on various topologies are clarified in the three pressure gradient regions.Further,the features of the strain rate and the strain rate/vorticity interaction are analyzed.An important correlation between the intermediate principle strain rate and the strain-dominant topologies is found.The influence of APG prefers to generate sheet-like structures.The existence of FPG is favorable to the alignment of the vorticity and the intermediate strain rate eigenvector.(3)Numerical investigation of the internal flow inside the supersonic straight duct with high backpressure was carried out.The results of this study provide phys-ical insight into the flow features and turbulence characteristics caused by the high backpressure.Significant pressure fluctuation has been observed and a deep insight into its formation based on the local flow topology analysis has been con-ducted.It is found that the pressure fluctuation is mainly caused by the unsteady node/saddle/saddle topology.The focal topologies induced by the compressibility also have a remarkable influence on the pressure fluctuation.The investigation of the strain rate tensor and vorticity reveals that the strain has a significant effect on the pressure fluctuation in comparison with the rotation.Furthermore,the compressibility also has a major influence on the pressure fluctuation,especially via the strain-dominant topologies.(4)Numerical investigation of the supersonic straight duct with periodic backpressure was conducted.The complex flow phenomena caused by the periodic backpressure arc investigated.The periodic backpressure with low frequency leads to obvious periodic oscillation of the separation shock and the enhancement of turbulence intensities.Significant differences were observed for the phase averaged flow struc-tures for low frequency backpressure.The discussion of the transport equation of the TKE reveals that the low frequency backpressure has an effect of enhancement on the production of the TKE.Furthermore,the pressure fluctuation is discussed.The low frequency backpressure also enhances the pressure fluctuation,indicat-ing that the low frequency backpressure contributes to the production of pressure fluctuation.The coherent structures mainly concentrate in the middle position of the duct for low frequency backpressure,and downstream for high one.