Research Of Vortex-driven Pressure Oscillation In Solid Rocket Motors

Vortex-driven pressure oscillation may cause abnormal operation in solid rocket motors(SRM). Though a lot of experiments and theory study already have been promoted,there are still some details, of fluid characters and acoustic characters, need to be further developed in numerial simulations.This paper is based on ANSYS-FLUENT 15.0, making numerical simulations of fluid field in the SRM model which is a subscal model of Ariane-5 with a submerged nozzle and an obstacle, and analyze its characters of fluid field, acoustic field and vortex-acoustic coupling.Firstly analyze the tube model, as a simple acoustic cavity, with an open end and a close end,its results can be comparison and reference to the results of SRM model.Comparising the results of tube model and SRM model,it can be concluded that pressure field can represent acoustic character, velocity and vorticity field can show fluid character in the large L/D acoustic cavity. We also get some consistent characters in these two models.Regarding acoustic mode,the excited acoustic mode is higher when the velocity is larger; the position of acoustic pressure node and antinode influences the amplitude of pressure power spectial density. Regarding fluid developing, strong shear flow at the obstacle causes obstacle vortex shedding, fluid develops at down stream and becomes fully turbulent near the outlet.Regarding the vortex shedding, frequency of small eddies increases with velocity, nearly liner,but frequency of large eddies is always locked in the vicinity of acoustic modes. Acoustic field can control vortex paring of small eddies when they shed and transmit, adjusting frequency of large eddies to acoustic mode.Besides, we conclude that the acoustic field and vortex acoustic coupling has stronger effects in SRM model, when comparing these two models.Analyzing the velocity, vorticity and pressure field comprehensively, we conclude the rule of self-sustained oscillation in SRM, and conclude the effect of vortex acoustic coupling to pressure oscillation amplitude. There is a new method of changing throat diameter to get different mean velocities. This method can make sure that geometry changing will not influence vortex shedding characters.Influences of the .obstacle position and volume of nozzle cavity to pressure oscillation are concluded. Besides,based on previous study,this paper proposes that the relationship of vortex shedding frequency and acoustic frequency should be taken into account, when studing influence of the obstacle position to pressure oscillation.This paper pays much attention to the accurate representaion of the frequency of vortex shedding. It is concluded that the main prequency of PSD analysis results of vorticity magnitude can represent large eddies frequencies,but frequencies of small eddies can be obtained only by observing vorticity contours at different moments. We also verify that it is not correct to use the main frequency of pressure PSD results to represent frequency of vortex shedding.This paper also clarify the definition of the length of SRM chamber,we conclude that the length of SRM chamber should end by the cavity of submerged nozzle, not by the outlet of the nozzle.