Investigations On Flow Structures And Mixing Characteristics Of Supersonic Mixing Layer Induced By Forced Vibration

The investigations are based on the research background of efficient mixing of fuel and airflow in dual combustion ramjet(DCR)and the object of supersonic mixing layer in DCR.Nanoparticle-based planar laser scattering(NPLS),particle image velocimetry(PIV)and large eddy simulation(LES)are employed to investigate the flow and mixing characteristics of supersonic mixing layer under different forced vibration conditions.Former research progress of theoretical analysis,experimental studies and numerical simulation of supersonic mixing layer are summarized.Then the characteristics of different improving mixing methods are analyzed from the active and passive control techniques aspects.Finally,the research status of flow control induced by vibration is analyzed as well.Combining the latest research findings of promoting mixing induced by vibration around the world,the thin metal plate utilized in the present study is designed.The thin metal plate is connected to the double-nozzle and the experiment is conducted in the supersonic suction mixing layer wind tunnel.Through calibrating the flow field,the Mach numbers of upper and lower flows are 2.12 and 3.18 respectively,with a convective Mach number of 0.22.A vibrating controller system is adopted to excite the thin metal plate.Through controlling the vibration of different frequencies and amplitudes and adopting edge detection technology,the generation and development of large scale structures in supersonic mixing layer are studied experimentally.In addition,the motion of thin metal plate is described by using User Define Function(UDF),and the flow process is analyzes by employing LES.Fine flow structures under different vibration conditions are obtained by adopting NPLS.It is revealed that the development of supersonic mixing layer experiences laminar region,transition region and complete turbulent region.Meanwhile,the vortex structures in supersonic mixing layer are characteristic of rapid shift and slow distortion along the flow direction under different vibration conditions.Besides,the flow structures under different vibration conditions reveal that forced vibration has an important effect on the flow structures and mixing efficiency.Compared with no vibration,forced vibration enhances three-dimensional characteristics and advances the instability position of mixing layer.Meanwhile,forced vibration promotes the mixing of upper and lower streams and enhances the mixing efficiency.By comparing the spanwise flow structures under different vibrations,it is shown that with high forcing frequency of 4000 Hz,the ordering of vortex structures is better,and the spanwise structures exhibit characteristics of banding distributions.Besides,the horn vortex is clearly obtained.The velocity distribution is studied by employing PIV technique.The results show that forced vibration has little effects on the streamwise velocity distribution.Whereas,the transverse velocity distribution is remarkably influenced and the peak values of speed are larger,indicating that the ability of entrainment of large scale structures is stronger and the momentum and scalar are more fully mixed.To quantitatively analyze the effect of forced vibration on mixing characteristics of supersonic mixing layer,image edge detection technique,pseudo-color processing technique and fractal dimension technique are adopted,and the indexes of supersonic mixing layer growth rate,momentum thickness,scalar mixing level and mixing layer vortex thickness measuring mixing efficiency are defined and utilized.Compared with the condition without vibration,the mixing layer vortex thickness,mixing layer growth rate and momentum thickness dramatically increase.In addition,with the increase of forcing frequency,the mixing of scalar becomes better.Meanwhile,the scalar mixing level vary linearly under different vibration conditions.The large scale vortex structures induced by forced vibration(especially high forcing frequency vibration)contribute much to the mixing process of upper and lower flows.