Direct Numerical Simulation Of Shock-Obstacle Interaction

Shock-obstacle interaction is a common phenomena existing in supersonic gas-solid two-phase flow.It can be seen in the nozzle of solid rocket motors.When the aircraft flying in rain,snow or dusty gas at high speed,there is also interaction.In reactive medium,ignition may occur when shock wave passes obstacles.Thus,it is important to understand the mechanism so that we can take proper measures to ensure the safety during storage and transportation.Besides,shock-obstacle may induce detonation,which can be useful for the Pulse Detonation Engine(PDE).In the present thesis,based on the high-order shock capturing scheme WENO scheme and immersed boundary method,a parallel computing platform for direct numerical simulation(DNS)has been developed.Investigations have been conducted on the variations of flow structure and the unsteady force exerted on the obstacle when shock interacts with cylinder,sphere and triangular wedge,etc.Furthermore,the research has been extended to shock-obstacle interaction in reactive gaseous medium.We have studied the effect of the geometry of the obstacle on the shock-induced ignition,flame accelerating and deflagration-to-detonation transition(DDT).DNS has been conducted for the whole process of shock interaction with stationary and moving cylinders,respectively.Variations of flow structure and the drag force exerted on the cylinder are investigated,and influences of the incident shock Mach number and cylinder diameter are considered.We have found some linear variety in the highly non-linear system.For shock interaction with stationary cylinder,when the incident shock Mach number is fixed,the trajectory of the upper triple point and the time evolution of the vertical distance from the highest point of the primary reflected shock to the centerline of the cylinder(height of the highest point of the primary reflected shock)can both be predicted by linear correlation.For shock interaction with moving cylinder,trajectory of the uper triple point,time evolution of the height of the highest point of the primary reflected shock,and the shock detachment distance can all be predicted by linear correlation.When the cylinder keeps stationary,a negative valley may appear in the time evolution of the drag coefficient when the incident shock is weak.However,such phenomena can never appear for moving cylinders.Taking the effects of incident shock Mach number and cylinder diameter into consideration,we have proposed an exponential-like correlation to predict the maximum drag coefficient for the cylinder.The "shock focusing" phenomena is captured in the process of shock-sphere interaction.For a given incident shock Mach number,the trajectory of the upper triple point and the time evolution of the height of the highest point of the primary reflected shock can both be predicted by linear correlation.While for the shock detachment distance,the linear feature can only be observed when the incident shock Mach number is small.Through investigation of the influence of the sphere diameter,we find that at a given time,both shock detachment distance and height of the highest point of the primary reflected shock decreases with increasing shock Mach number.However,location of the upper trile point in vertical direction varies non-monotonically with shock Mach number.To verify the code's capability in dealing with complex geometry with sharp corners,DNS has been conducted for the interaction between shock and triangular wedge.Two vortices with opposite rotational directions are formed near the vertexes of the wedge.Trajectories of the core of the main vortices are similar to the shape of "V".Besides,it is found that the incident shock Mach number has significant effects on the shape of the reflected bow shock and the structure of the vortices.However,the time when transition of the trajectory of the core of the two main vortices happens has little dependence on the incident shock Mach number.Through investigation of flow around NACA0012 airfoil,it is found that the drag force exerted on the airfoil increases with inflow Mach number.When the inflow is subsonic,variation of the drag force with the inflow speed is apparent.However,when the inflow speed increases to suersonic,influences of inflow speed on the drag force is obviously weakened.What's more,we have observed a phenomena similar to that found in flow around circular cylinder.When the inflow Reynolds number is small,the flow structure is symmetric about the horizontal axis.Increasing the Reynolds number,Karman vortices appear downstream of the airfoil.Further increasing it,the phenomena disappears,and the flow structure becomes symmetric again.DNS has also been conducted for shock-obstacle interaction in reactive gas.The gaseous medium is the mixture of ethylene,oxygen and Argon.We have done some research on the influences of the geometry of the obstacle in the tube on the chemistry reaction.Three different geometries are semi-cylinder,equilateral triangular wedge and square,respectively.It is found that when the incident shock is weak,ignition can't happen for all the three cases.However,the geometry of the obstacle does have very important influence on the flow field.When the incident shock Mach number is medium,the square obstacle is prone to induce reaction most.Increasing the incident shock Mach number to a relatively large one,combustion occurs in all cases.Furthermore,flame acceleration and deflagration-to-detonation(DDT)can be observed.Comparatively speaking,the ignition happens latest when the obstacle is triangular wedge.However,the time when deflagration transforms to detonation is generally similar for all the three cases.The flame speed is not affected by the geometry of the obstacle.The shape of the flame presents distinct wrinkle.