High-order Accurate Numerical Simulation On Supersonic Mixing Layer And Experimental Study On Mixing Enhancement With Streamwise Vortices

The residence time of the incoming flows in the combined-cycle engines is in the magnitude of millisecond(ms).In addition,the strong compressibility of the incoming flows also suppresses its mixing processes with the fuel.Thus,improving the mixing processes of the incoming flows and the fuel has becoming a key research point for developing of the combined-cycle engines.Based on the present background,the thesis takes the supersonic mixing layer as the research object.The developing processes and mixing enhancement of supersonic mixing layer are studied thoroughly using theoretical analysis,numerical simulations and experiments.The influences of the compressibility,viscosity,velocity ratio and density ratio on the stability characteristics of the mixing layer are studied using liner stability theory(LST).The results indicate that when the incoming convective Mach number(Mc)belows 0.6,the two-dimensional(2D)disturbance waves dominate the developing processes of the mixing layer,while it is the three-dimensional(3D)disturbance waves when Mc exceeds 0.6.The viscosity suppresses the developing of the mixing layer.The mixing layer can get the best growth rate with an appropriate combine of the incoming flows,when the incoming media is different or the same media with different incoming temperature.Interactions between the oblique shock waves and the mixing layer are studied using LES.The inflections of the mean velocity profiles are induced by the large-scale vortices in the mixing layer.The oblique shock waves make the gradient of the velocity becomes higher.Then,hairpin vortices appear in the vicinity of the point where the oblique shock wave interacts with the mixing layer.However,the induced hairpin vortices disappear quickly.The thickness of the mixing layer changes along with the developing of the large-scale vortices in the mixing layer.The energy exchange among the turbulent structures is enhanced by the oblique shock waves,and then the mixing processes of the mixing layer are also enhanced.The thickness of the mixing layer at the interaction point of the oblique shock waves on the mixing layer decreases first and then increases rapidly.In general,the oblique shock waves have positive influences on the thickness of the mixing layer.The precursor simulation method is studied as the inflow conditions for the boundary layer.The Digital-filter(DF)method is used as comparision.As the large-scale vortices develop slowly while the small-scale vortices develop quickly downstream the inflow of the boundaty layer,the precursor simulation method ignores the high-frequency small-scale vortices and emphasizes the low-frequency large-scale vortices.The boundary layer with precursor simulation method as the inflow condition develops turbulence in a quite short distance downstream the inflow.Additionally,it is simpler compared with DF method.The precursor simulation method can also be applied to other wall-bounded flows such as backward-facing step flows,plane mixing layer,et al.Influences of the incoming states on the plane mixing layer are studied using LES.There are more shocklets in the laminar plane mixing layer(LPML)than the turbulent plane mixing layer(TPML).The shocklets are induced by the large-scale vortices in the mixing layer.The turbulent inflow weakens the inflow shocks,leading shocks and shocklets in the mixing layer.2D large-scale vortices induced by K-H instability as well as(43)vortices and hairpin vortices present in LPML.In comparision,there are no 2D large-scale prenent in TPML.In addition,(43)vortices and hairpin vortices in TPML also break up quickly.In the “self-preserving” stage,the two mixing layers have the same growth rate,which is about 0.0213.Profiles of RS12 for the plane mixing layer have two peaks.However,the lower peak will disappear with developing of the mixing layer.And it is earlier for TPML than LPML.The frequency of the main modes for TPML is higher than that for LPML from DMD(Dynamic Mode Decomposition)analysis.TPML need more modes to reconstruction the flowfields exactly.The mixing enhancement effects of the rectangular lobed mixer and shock-eliminating lobed mixer on supersonic mixing layer are compared by experiments.The pressure difference between the peak and trough regions of the lobed mixers results in the formation of the streamwise vortices,one of which rotates in the clockwise direction and the other two rotate in the anticlockwise direction.The mixing enhancement of the lobed mixers is due to the streamwise vortices and the increased interface area of the two inflows.The streamwsie vortices contribute to about 80% of the mixing enhancement especially in the far field.Though the streamwise vortices in the shock-eliminating lobed mixer present much later compared with the rectangular lobed mixer and in some extreme cases there are no streamwise vortices in the shock-eliminating lobed mixer,the two mixers have almost the same mixing enhancement effects with the same expansion angles.The streamwise vortices downstream lobed mixers make the mixing layer develop earlier instead of faster.The higher the expansion angle is,the better the mixing enhancement would be.The shock-eliminating lobed mixer can eliminate the shocks in the mixing layer,and then reduce the total pressure losses of the flowfields.