CFD|自由液面(Free Surface)分析
液体压强中与大气联通（即与空气接触）的液面叫做自由液面（free surface），即船舶倾斜时船内舱柜中能自由变动的液体表面。自由液面分析可以模拟两个不混溶流体之间的自由移动界面，如气体-液体和液体-液体。 The model covered in this analysis pertains to the fuel tank used for transporting gasoline. Inside the tank, baffles are installed to suppress the sloshing of gasoline. Since the interior of the tank is not completely filled with gasoline, it has a free surface to separate liquid and gas. To incorporate the influence of this free surface on the flow in the analysis, the free surface module should be utilized. In this technical note, we will delve into the details of the free surface functionality and the method of analyzing the flow inside the tank The free surface function is a module that calculates the free surface. The level set algorithm, using the ODDLS (overlapping domain decomposition level set) function, is advantageous when the variation of the free surface is significant and complex. In this technical note, we utilized the level set function to analyze changes in the surface due to the tank's oscillations.
Generally, free surface analysis interprets the interface between gas and liquid. In such cases, the density and viscosity of gas and liquid differ by about a factor of 1000, making it possible to ignore the influence of gas to some extent. In these situations, analysis can be performed using the single-phase (monophase) level set technique, offering the advantage of reducing analysis time while maintaining the accuracy of the free surface. The level set algorithm applies the ODDLS (Overlapping Domain Decomposition Level Set) technique to minimize errors in the analysis results caused by discontinuities in properties at fluid interfaces. The fundamental concept of the ODDLS technique involves decomposing the analysis domain, which shares the interface, into separate analysis domains that include the respective interfaces for analysis.
水平集法属于欧拉框架下的界面捕捉方法，这种方法是将运动界面定义为一个函数的零等值线或等值面，然后直接求解界面运动方程。每一个时间步都要重新初始化LS方程，这些初始化的过程中总伴随着界面位置的移动，会造成质量损失，导致质量不守恒。采用的是光滑的距离函数来捕捉相界面。
优点：复杂界面处理能力强。缺点：质量不守恒，计算时间较VOF法长。
水平集法的应用范围：
分层流、晃动、液体中的大气泡流动、溃坝时的水流、船舶动力学中的水面扰流、大变形自由液面、三相流。
水平集法的局限性：
所有控制容积必须充满单一流体相或者相的联合，水平集法中不允许待求流体域中没有任何流体的存在。
NFX CFD求解多相流问题时的求解策略只需要在边界条件设置和求解设置中额外增加两个步骤即可完成。第一步：定义多相模型中的相材料，VOF支持两种相的添加，水平集模型支持三种相的添加。
第二步：定义多相体积分数在进出口边界或计算域内部的进入或生成方式Explain below exampleThe example features a gasoline tank with baffles installed, as illustrated in the following diagram.
*When using the hybrid (accuracy) method, it is advisable to apply it after achieving a certain level of convergence using the standard (stability) method.
The gasoline inside the tank oscillates in the y-axis direction with a frequency of 4Hz. The acceleration experienced by the gasoline follows the function below.10*sin(8*𝛑*t）
