Investigations On Flow Mechanisms And Mixing Enhancement Techniques Of Supersonic Mixing Layer

In present work,by employing nano-based planar laser scatting(NPLS)and particle image velocimetry(PIV)techniques,and direct numerical simulation(DNS)method,the flow mechanisms and flow control methods of supersonic mixing layers are comprehensively investigated and analyzed.The effects of typical structures are revealed,and the efficient mixing enhancement methods are proposed.The mixing layer with Mc=0.2 is experimentally researched with focusing on the flow field and velocity field by employing NPLS and PIV techniques.Fine flow structures with high spatial-temporal resolutions are well captured,and the evolution properties of these large scale structures are analyzed.It has been found that in weakly compressible mixing layers,the pairing and merging process of K-H vortices is still the main contributor for the mixing layer growth.The shocklets are first captured in such low convective Mach number conditions and the mechanisms are analyzed.Apart from that,the standard distributions of velocities and turbulent intensities are presented,which can be a reference for the future experimental and numerical tests.The parallel computer program is developed with applications for the calculation of supersonic mixing layers,and three classical tests are conducted to verify present schemes.By employing the program the highly compressible mixing layer at Mc=1.0 are simulated.The multiple ring-like vortices in transition stage of highly compressible mixing layers are first obtained and the generation mechanisms are revealed by employing Helmholtz first conservation law.Unlike that in weak and moderate compressible mixing layers,in highly compressible mixing layers,the ?-vortices and hairpin vortices are not dominant structures.Apart from that,the relation between the “second ejection” and “second sweep” of multiple ring-like structures and the appearance of “multiple peak values” of turbulent fluctuation intensities are created.Mixing characteristics under different inflow conditions are investigated using DNS methods.The flow evolution process and turbulent statistical properties are obtained.The results indicate that for the only variation of R?,the velocity thickness growth rate does not significantly vary,while the momentum thickness growth rate is larger when R?=1.0.With the increase of only Ru,the mixing layer becomes more stable and the velocity and momentum thickness are both drastically depressed in the whole flow field.As only Mc increases,the mixing layer growth is inhibited in the near field through the transition delay of the flow,while in the far field,the growth rates are nearly the same for different Mc.The spatial correlation analysis of structural topology indicates that the effects of each of the three main flow parameters on vortex topology lead to different mean structure sizes and shapes.The limitation of the Mc evaluating the compressibility is analyzed,and a new non-dimensional parameter called general Mach number(Mn)is proposed and the applicability is assessed.After the clarification of basic methods for mixing enhancements,the research and exploration of passive/positive enhanced mixing methods and mixing enhancement induced by oblique shock waves are conducted.Two kinds of triangular lobed mixers are proposed and designed.The vortex evolution in streamwise,transverse and spanwise are presented,and the interaction mechanisms between large-scale streamwise vortices induced by lobed mixer and K-H structures induced by Kelvin-Helmholtz instability are revealed.The interaction dominates the vortices breakdown and the evolution of threedimensional property.By employing fractal analysis and intermittency analysis,the effects of lobed mixer on mixing enhancement are gained and the optimized configuration are obtained.The influences of inflow disturbances on mixing are researched.The flow evolution properties under typical frequency disturbance are revealed.In high-frequency inflow forcing condition(f=20kHz),the size of uniformly distributed vortices in the mixing layer is approximately equal to the ratio between convective velocity and inflow forcing frequency,and the mixing layer growth is notably suppressed in the whole flow field.For the mixing layer affected by low-frequency inflow forcing(f=5kHz),high clockwise rotation process of the K-H vortex band around the large-scale structures is the way to realize the engulfment.The effects of single/double oblique shock waves on mixing evolution are investigated.Compared with that of single shock,the double shock can induce higher mixing layer growth rate,and the turbulent intensities are notably increased.Based on the analysis of the vortex dynamic equation,it can be found that the sharp increase of compressed vortex vorticity induced by the shock wave is the key mechanism of mixing enhancement.