轴对称后向台阶不稳定流动及压强振荡数值研究

使用翼柱形装药的战术发动机,在翼槽烧完时会出现轴对称的后向台阶结构,易产生转角涡脱落引起的声涡耦合现象。根据战术发动机中后向台阶流动特点设计简化模型,从线性稳定性理论出发,应用声涡耦合模态预测方法进行分析。采用湍流流动的大涡模拟(LES)方法,进行了较宽来流Re数条件下后向台阶不稳定流动的数值模拟,获得了转角涡脱落产生的一般过程,分析了不同来流条件下的旋涡多尺度运动规律。通过分析流场中关键位置的压强振荡,研究了气动声学现象。结果表明,声涡耦合模态预测方法可对不同速度下可能诱发的声场固有频率进行合理预测。流场头部压强波动的振幅最大,频率与声场轴向声模态相耦合。再发展区出现低于轴向基频的宽峰振荡主要受大尺度旋涡运动的影响,该频率与基频接近时,会造成基频的多峰现象。当近壁面小尺寸旋涡形成频率接近声场某阶固有频率时,该频率及更低阶频率就有可能被激发。

Numerical study on unsteady backward-facing step flow and pressure oscillations

There was an axisymmetric backward-facing step structure in the burned out tactical solid rocket motor with finocyl grain.The vortex shedding from the step could cause hydrodynamic instability to drive acoustical oscillations.The hydrodynamic stability analysis was illustrated on a simplified motor geometry to predict longitudinal acoustic mode driven by vertex.LES turbulence model was also used to simulate the vortex generation process and acquire pressure data of the key monitoring point in the flow field.The hydrodynamic stability analysis results were consistent with the LES.The maximal pressure amplitude was in the head-end position with the first few frequencies of the chamber longitudinal acoustical mode.In the reattachment region,a new broad pressure peak below fundamental frequency which was influenced by the motion of the large-scale vertex was produced.There was a multi-peak phenomenon of the fundamental frequency if it was close to the broad peak.The pressure oscillations would be excited by the vortex shedding phenomenon when the formation frequency of the small vertex near the step is close to a nature frequency of the sound field.