| Shock reflection is a physical phenomenon that occurs when a shock propagating in a medium with given acoustic impedance obliquely interacts with another medium with different acoustic impedance.Shock reflection phenomenon is of great significance in academic researches and engineering applications such as blast wave propagation and supersonic aircraft design.In recent years,the unsteady shock reflection phenomenon in curved shock circumstance has attracted extensive attention in academic community.However,wedge angle usually changes continuously during the unsteady curved shock reflection process.The coupling of the continuous variation of wedge angle with the curved shock and its induced flow makes the physical process extremely complicated and difficult to analyze,therefore the current research is progressing very slowly,lack-ing the understanding of the physical nature of the unsteady curved shock reflection.To explore the shock reflection phenomenon in curved shock circumstance,three issues,including the reflection of cylindrically curved shock over a single wedge with constant wedge angle,the reflection of cylindrically converging shock over a double wedge and the unsteady regular reflection(RR)to Mach reflection(MR)transition under complex conditions,are studied in the present work.The main contents are summarized as fol-lows:1)To isolated study the unsteady effects introduced by the curved shock and its induced flow,specially curved wedges are designed for the converging and diverging shock circumstances,respectively,to eliminate the unsteady effects introduced by the wedge angle variation.First,the reflection of cylindrically converging shock over a single wedge with constant wedge angle is studied experimentally and numerically by using a cylindrically converging shock tube,which is designed by the shock dynamic-s method,and vas2d program.Subsequently,the reflection of cylindrically diverging shock over a single wedge with constant wedge angle is studied numerically by using the vas2d program.It is found that the behavior of the propagation of disturbance in curved shock circumstance is the same as that in planar shock circumstance in infinitely short reflection process.Under the condition that the shock intensity variation is limit-ed and the wedge angle is constant,the theoretical prediction of the trajectory of triple point in curved shock circumstance is realized.Based on the disturbed shock front de-fined in the present work,the propagation of disturbance in curved shock circumstance is measured more effectively,and a better understanding of the geometrical effect of the curved shock is realized.It is found that the geometrical contraction effect of the converging shock increases gradually with shock convergence,thus the growth rate of the length of disturbed shock front decreases continuously during the reflection process of a converging shock over a single wedge with constant wedge angle.The geometrical expansion effect of the diverging shock increases gradually during shock divergence,thus the growth rate of the length of disturbed shock front increases continuously dur-ing the reflection process of a diverging shock over a single wedge with constant wedge angle.2)To study the effect of wedge angle variation on the unsteady converging shock reflection process,a specially double wedge,which can ensure that the wedge angle changes only at the intersection point of double wedge during shock convergence,is designed.By using the cylindrically converging shock tube and vas2d program,the reflection of cylindrically converging shock over a specially double wedge is studied experimentally and numerically.The main conclusions can be summarized as follows:(1)For the RR?RR process,the type of the wave behind the reflection over the second wedge is dominated by the flow-induced waves while the effect of the shock-induced waves is almost negligible.(2)For the MR?MR process,the variation of wedge an-gle will alter the strength of the disturbance propagating along the shock front,and the variation trend of the disturbance intensity depends on the variation trend of the wedge angle.(3)For the MR?MR process and the RR?MR process,the history of the reflec-tion over the first wedge has an effect on the Mach reflection over the second wedge.Besides,if the process of the Mach reflection over the second wedge is long enough,the Mach reflection over the second wedge will asymptotically approach the Mach reflec-tion of converging shock over a single wedge.(4)Converging shock circumstance may change the included angle between shocks and result in wave configurations different from that in planar shock circumstance.3)To explore the physical nature of the unsteady RR?MR transition,a systematic investigation of the unsteady RR?MR transition in inviscid perfect air under complex conditions is carried out.First,according to the type of shock and the range of shock Mach number,the possibilities of the unsteady RR?MR transition are discussed.Fur-ther,the shape of wedge corresponding to different possibilities is clearly distinguished.Based on the view that the effect of the shock intensity variation on the RR?MR transi-tion is a small quantity,five types of reflection processes,which the RR?MR transition is possible,are classified without considering the reflection on concave wedge,includ-ing a planar shock over a convex wedge,a converging shock reflecting over a convex wedge with constant or reduced wedge angle,a diverging shock reflecting off a con-vex or straight wedge with reduced wedge angle.For a planar shock reflection over a convex wedge,the mechanism of the disturbance propagation is interpreted in detail,it is found that the flow-induced rarefaction waves always exist between disturbances generated from neighbouring positions and isolate them.For a curved shock reflection over a convex wedge,the continuous variation of shock intensity during the reflection process will affect the variation trend of the pressure behind the reflected shock,there-fore the distributions of the flow regions of rarefaction wave and compression wave are different from those in the reflection of a planar shock over a convex wedge.Howev-er,as long as the wedge is convex,the isolation effect of the flow-induced rarefaction waves always exists,thus the flow-field analysis for the reflection of a planar shock over a convex wedge can be extended to the reflection of a curved shock over a con-vex wedge.Flow-induced rarefaction waves are absent in the reflection process of a diverging shock over a straight wedge,therefore the shock-induced compression waves generated earlier can overtake the ones generated later and the superposition of them will generate a stronger pressure wave.However,even in the most extreme case,the pressure wave generated by the compression wave superposition cannot overtake the reflection point before the pseudo-steady criterion is reached.The flow-field analysis shows that in all the five reflection processes considered,the flow in the vicinity of the reflection point is transient pseudo-steady.In other words,whether the unsteady RR?MR transition will occur only depends on the local flow conditions,i.e.,local wedge angle and shock intensity.Therefore,the unsteady RR?MR transition can be predicted by pseudo-steady criterion,regardless of shock intensity,the wedge curvature and the initial wedge angle.Extensive inviscid numerical simulations are performed to verify the conclusion of the flow-field analysis,and all the numerical results show the reliability of the pseudo-steady criterion for predicting the unsteady RR?MR transition. |
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