火箭发动机喷管中流固耦合的力学行为及其影响

During rocket engine start-up and shut-down, the feeding pressure changes to a large extent. The rocket nozzles must experience ambient environment from sea-level to high altitude, also the sea-level environment in ground test. The nozzle designed cannot work well for all these situations. Therefore, transient over-expansion and internal flow separation will certainly appear. This phenomenon causes an asymmetric pressure distribution which can result in side loads and induce dynamic loads. These side loads are of a large enough magnitude to damage the nozzle itself as well as engine support structures. Though the problem of flow separation induced side loads has investigated for decades. The mechanics and the affecting parameters which result in the asymmetric separated flow in the nozzle have not been understood yet. By analyzing the computation approaches in the literatures published in recent years, the fluid/structure interaction play an important role in the side loads generation. In present project, a numerical method for simulating the nozzle fluid/structure interaction will be established. The actual and more detailed ambient environments will be considered and involved. The characteristics of fluid/structure interaction under different feeding pressure and different ambient environment parameters will be studied. More attention will be paid to effects of fluid/structure interaction on the shock structure, shock/boundary interaction and asymmetric separation in the internal flow of nozzle. It is believed that all the analysis will be helpful to understand the side loads generation. The investigation is hoped to be used for future rocket engine nozzle design.

火箭发动机启动和关闭过程中内部气流总压会有变化，还会面临从地面到高空的不同外部环境，尤其还存在地面试验环境的情况，而发动机喷管外形的设计不可能包括这些变化，因此发动机喷管会面临喷管气流过膨胀和气流分离的状况，它会引起喷管的侧向载荷，是火箭发动机使用和地面试验过程中的巨大威胁。尽管经历几十年来的不断研究，仍然对喷管非对称分离导致的侧向载荷机理没有认识清楚，通过对国外已有工作的分析，发现喷管流固耦合的动力学过程会对喷管侧向载荷有很大的影响，本项目从喷管流固耦合的精确数值模拟出发，建立一套发动机喷管流固耦合的模拟方法，更全面、真实再现发动机喷管的实际工作环境，研究在不同内部总压、不同外部环境下喷管的流固耦合动力学特征，分析和揭示该动力学过程对喷管内流场激波系结构、激波/附面层干扰、非对称分离涡的影响机制，从而认识和理解喷管流固耦合过程对侧向载荷形成的影响机理。此工作对航天科技发展具有较大意义。

本项目通过发展数值计算方法，研究了火箭发动机喷管的流动演化问题、流固耦合问题、侧向力预测问题、侧向力抑制方法、侧喷中的流场模拟精度问题和冲压发动机的壁板流固耦合问题。主要研究成果包括：建立两种数值模拟火箭发动机喷管流固耦合问题的分析方法；提出一种新的适用于格心形式和满足几何守恒的高精度有限体积方法；得到了振动状况下幅度和频率对喷管流场及侧向力的影响规律，发现当喷管振动的频率接近流场的分离流的非定常主要频率时，会激发出更大的侧向力；当喷管外部流动速度达到一定值时，可以导致自由激波状态演化为受限激波状态，而且通过提出一种动量测定法，对该演化过程的机理进行了分析；研究了壁面温度变化对喷管内部的激波形态的影响，传统计算中以绝热方式计算喷管的内部流动就会导致结果上的明显差异；提出了采用变形喷管的方法抑制侧向力，研究结果表明，当挤压喷管变形达到一定幅度时，可以有效地抑制侧向力；通过对侧喷流场研究，证明模拟喷管内部流动比常规的边界条件给定方法更加符合实际流动情况，计算结果具有更好的模拟精度。