Numerical simulation of fluid dynamics and flow aggressiveness for hydroacoustic cavitation at ultrasonic horns

尽管超声波喇叭装置在化学、工艺工程、生物学或材料测试等领域有广泛的应用，但侵略性空化流场的细节（下文称为水声空化）尚不清楚。作为一个例子，重点是材料测试的间接方法，其中一个固定的材料样品被放置在对面的振荡喇叭。这种极具侵略性的流动在试样上留下了空化侵蚀剖面，它有效地充当了材料传感器，并提供了流动侵略性的地形图。通过振动喇叭和静止试样之间间隙宽度的变化，现有的测量数据显示了流动侵蚀性的特征重新分布，这是最近的CFD方法无法解释的。因此，本项目旨在改进和验证三维流动模拟方法，将其应用于超声变角装置的水声空化，有助于更好地理解与侵蚀模式相关的流动物理。除了湍流和热效应，重点是模拟的扩展到不可冷凝空气的影响及其对侵蚀的阻尼影响。在均匀混合分析的基础上，提出了一种混合流体体积(VOF) /欧拉-欧拉-2-流体（EE2F）方案，在有足够空间分辨率的情况下，求解相界面。对于方法中EE2F部分的分散分布，采用了基于类的种群平衡和气泡交互模型。相变是用热力学空化模型的可压缩格式来描述的，这意味着热力学和力学平衡的假设和状态方程的使用。该实现基于OpenFOAM的内部扩展。通过相界面解析一维单气泡模型，对单个气泡的多重性进行了详细的质量和传热评估，对于空气脱吸速率的尺度桥接描述，所得速率分布通过源项传递给3D EE2F求解器。除了超声变幅装置的水声空化外，还对孔口流中空化诱导的空气释放进行了验证，这与经典的水动力空化相对应。

In spite of the broad application of ultrasonic horn devices e.g., in chemistry, process engineering, biology or material testing, details of the aggressive cavitating flow field, named as Hydroacoustic Cavitation in what follows, are not understood yet. As an example, the focus is on the indirect method for material testing, where a stationary material specimen is placed opposite to the oscillating horn. This exceedingly aggressive flow leaves cavitation erosion profiles on the specimens, which effectively serve as material sensors and provide topography maps of flow aggressiveness. By a variation of the gap width between oscillating horn and stationary specimen, available measurement data show a characteristic redistribution of flow aggressiveness, which could not be explained by recent CFD approaches. Thus, the project aims at an improvement and validation of 3D flow simulation methods whose application on hydroacoustic cavitation at ultrasonic horn devices should contribute to a better understanding of the flow physics, which is associated to the erosion patterns. Beyond turbulent and thermal effects, the focus is on an extension of the simulation towards the effect of non-condensable air and its damping impact on erosion. Based on a homogeneous mixture ansatz, a hybrid Volume-of-Fluid (VOF) / Euler-Euler-2-Fluid (EE2F) -scheme will be developed which resolves the phase interface if a sufficient spatial resolution is available. For the disperse distribution on the EE2F part of the method, class-based population balance and bubble interaction models are employed. The phase transition is described by a compressible scheme with thermo-dynamical cavitation model which means the assumption of thermal and mechanical equilibrium and the use of an equation of state. The implementation is based on in-house extensions of OpenFOAM. With a phase interface resolving 1D single bubble model, the detailed mass and heat transfer is evaluated for a multiplicity if single bubbles, and for a scale-bridging description of air de- and absorption rates, the resulting rate distribution is passed via source term to the 3D EE2F solver. Beyond hydroacoustic cavitation at ultrasonic horn devices, the method is validated on cavitation-induced air release at an orifice flow, which corresponds to classical hydrodynamic cavitation.