Study On The Influence Of Constraints And Aerodynamic Loads On Pressure Oscillation And Structure Vibration

Solid rocket motor(SRM)need to work reliably and controllably under the multicompound environment of wide temperature range and strong vibration.However,in recent years some SRMs both at home and abroad work stably in the ground static test,but there are frequent problems of thrust,combustor pressure and strong structural vibration in flight test.This difference between in-flight and ground-test condition has become a common bottleneck problem in the survival and operational effectiveness of SRMs,it is an urgent problem to be solved.No relevant studies have been able to explain the pressure oscillation and structural vibration with difference between in-flight and ground-test condition comprehensively.Therefore,it is of great significance to study the instability mechanism of SRM under in-flight environment to provide basis for recurrence of the pressure oscillation and structural vibration on ground test.Based on the strong constraint conditions on ground tests,the weak constraint conditions and the aerodynamic thermal load conditions in flight tests,the structural dynamic and combustion chamber acoustic characteristics of SRMs are studied by modal analysis,the user-defined function(UDF)and two-way fluid-structure interaction simulation method were developed to study the pressure oscillation and structural vibration characteristics of the SRM after excitation.In this paper,the differences of SRM structural modes on ground and in flight are discussed,the different characteristics of flow field oscillation and structural vibration are analyzed,the mechanism of flow field oscillation and structural vibration in flight triggered by pulse is revealed,it provides some reference for solving the pressure oscillation and structural vibration with difference between in-flight and ground-test condition.The main work of this paper is as follows:1.The complex phenomena involved in the interaction of structure and internal flow field are deeply analyzed,corresponding mathematical and physical models are established to describe compressible gas flow,structural vibration and acoustic oscillation.The accuracy and applicability of the numerical model are verified by the simulation of acoustic modes of cylindrical cavity,vibration modes of aluminum plate,shock wave evolution and vibration process of flexible plate when shock waves impact.2.The physical models of the SRM including early stage(T1),middle stage(T2)and late stage(T3)were established.The acoustic modes of SRMs at each working stage are analyzed,the structural modes of SRMs at different working stages under strong,weak constraints and aerodynamic thermal loads are compared.The results show that,compared with the strong constraint condition on ground test,each order structural frequency decreases significantly under the weak constraint condition in flight,showing the different dynamic characteristics of the motor structure between in-flight and ground-test condition.Under the weak constraints condition in flight,combined effects of propellant retreat and aerodynamic thermal load condition change,the frequencies of the first order acoustic mode of the SRM and the stretching mode of structural vibration tend to approach each other and then move away from each other.When the frequencies are close to a certain extent,the triggering condition of structural resonance will be satisfied.3.The interaction between the flow field and the structure of the SRM under strong,weak constraint and aerodynamic loads is studied numerically,the characteristics of pressure oscillation and structure vibration under pulse excitation are analyzed through the process of pressure wave propagation and reflection along axial direction,it is found that the SRM instability is difficult to be excited on ground test,when the SRM on the edge of resonance instability is subjected to aerodynamic force with a frequency close to the first order acoustic frequency under weak constraints in flight,the thrust and pressure show limit cycle oscillation and the structural vibration will be amplified,the mechanism of the nonlinear coupling oscillation between the flow field and structure was clarified.