以計算流體力學工具驗證加壓桶槽中之流體晃動現象

Abstract

火箭飛行時，因引擎運作、外界氣動力等環境會造成燃料桶槽及氧化劑桶槽的振動，進而影響內部液體產生晃動現象(Sloshing) ，此現象的發生會使液體重心偏移產生慣性力矩，在火箭飛行時是不樂見的。為了保持飛行的穩定性，GNC (Guidance, Navigation and Control)系統在控制上常將流體晃動轉換為機械模型，自然頻率為其中一個重要參數。在過去此參數常藉由簡單的幾何外型之解析解與進行實驗所得到，為了因應不同任務的環境條件，桶槽尺寸增大及幾何外型趨向複雜，實驗變得非常耗時、需要更大的實驗設備且沒有對應之解析解，因此本研究將使用數值模擬方式分析液體晃動現象。 近年來計算流體力學技術迅速發展，並結合固體力學分析流體與固體之互制作用(Fluid-Structure Interaction, FSI) ，已普遍應用於許多工程上之模擬分析，為揭示數值模擬之可靠性，本研究使用商用模擬軟體LS-DYNA建立氧化劑桶槽之模擬分析模組及重要參數，利用ALE (Arbitrary-Lagrangian-Eulerian)方法針對不同幾何外型之桶槽、填充液體表面之高度及加入擋板(Baffle)進行模擬，配合邊界條件的設定，輸出自然頻率並與文獻數據驗證比對，藉此提高模組可靠性，達到以數值模擬方式提供GNC所需之參數。

Sloshing phenomenon becomes important in the fuel and oxidizer tanks due to vibration of engine and the external aerodynamic effect during rocket flight. Sloshing makes the center of gravity of the liquid, and thus the moment of inertia, in the tank oscillate transversely. For the control of a rocket, it is a highly unexpected situation which should be avoided during flight. In the GNC (Guidance, Navigation and Control) system of a rocket, the physical sloshing model is often converted to a mechanical one. Natural frequencies are among the most important parameters. This parameter was obtained by analytical solution of simple geometrical shapes and was validated through experiments in the past. The tank size can be large and the geometry can become complex in order to cope with the environmental conditions of different missions. This leads the experiments very expensive and time-consuming. Most of the time, there is no corresponding analytical solution; instead, numerical simulation becomes important. In this study, the goal is to investigate the sloshing phenomena by means of a CFD commercial package, LS-DYNA. In recent years, the rapid development of computational fluid dynamics combined with solid mechanics analysis of fluid-structure interaction has been widely used in many engineering analysis. To understand the reliability of numerical simulation, the ALE (Arbitrary-Lagrangian-Eulerian) method is adopted to model the sloshing with different depths of the liquid surface with the different geometries of the tank with or without baffle. In this study, the results show that the simulated natural frequencies are in the excellent agreement with the experimental and simulation data in the literature.