Effect Of Mass Ejection On Boundary Layers Stability

During the flight of the hypersonic vehicle,the thermal protection layer on the surface will be ablated due to high temperature.During the ablation process,the thermal protection layer will generate a large amount of gas and other ablation products to be ejected into the boundary layer.We call this phenomenon mass ejection.Mass ejection will affect the flow stability of vehicle.In this paper,the hypersonic blunt plate with Ma = 6 is taken as the research object,using direct numerical simulation(DNS)and linear stability theory(LST).The effect of mass ejection on flow stability is quantitatively studied by defining N-value correction,as?N.The effect of different factors of the two-dimensional mass ejection on the evolution of disturbances is analyzed,such as the amplitude of ejection velocity,ejection width,location,combination and volume flowrate,and the influence of three-dimensional disturbance wave is studied;the effect of the three-dimensional mass ejection with specific width and ejection velocity amplitude on the flow stability is aslo analyzed.Through the analysis of numerical simulation results under different working conditions,the following conclusions are obtained:1.Mass ejection will lead to the local concave of the neutral curve of the second mode,and the degree of concave increases with the increase of the amplitude of the ejection velocity and the width of the ejection region;mass ejection destabilizes the unstable disturbance waves,and its volume flowrate is an important factor to determine the effect on the disturbances.The instability effect on the disturbance wave is the same when the volume flowrate is identical;when the ejection position is close to the lower bound of the neutral curve of a disturbance,the instability effect of mass ejection on it is the strongest;the combined mass ejection has the superposition effect when the volume flowrate is small;mass ejection also destabilizes the three-dimensional disturbance waves,and the effect is the strongest for a specific spanwise wave number,while the effect is weaker when ? is 0 or infinity;2.?N can qualitatively describe the effect of mass ejection on flow stability,?N>0 indicating that mass ejection makes the disturbance wave more unstable,while?N<0 indicating that mass ejection makes the disturbance wave stable;When q = 10,?N can reach to 3 for a certain range of frequency,which should be considered in the transition prediction.3.The three-dimensional mass ejection on a blunt plate will result in an obvious vortex structure at downstream of the ejection region,and the three-dimensional mass ejection also destabilizes the unstable disturbance waves.4.The value of N predicted by LST is close to the DNS result for all conditions in this paper,implying that LST can be used to quantitatively determine the effect of the mass ejection on the stability of boundary layer.