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

尽管超声波变幅杆装置（例如，在化学、工艺工程、生物学或材料测试中，还不了解在下文中被称为水声空化（HydroacousticCavitation）的侵蚀性空化流场的细节。 作为一个例子，重点是间接方法的材料测试，其中一个固定的材料样本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空蚀轮廓，这些空蚀轮廓有效地用作材料传感器并提供流动侵略性的地形图。通过改变振荡变幅杆和固定试样之间的差距宽度，现有的测量数据显示了流动侵略性的特征性再分布，这不能用最近的CFD方法来解释。因此，该项目旨在改进和验证3D流动模拟方法，其应用于超声喇叭装置的水声空化应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，重点是对非冷凝空气的影响及其对侵蚀的阻尼影响的模拟扩展。基于一个均匀的混合物Anchor，一个混合的体积的流体（VOF）/欧拉-2-流体（EE 2F）的计划将解决相界面，如果有足够的空间分辨率是可用的。对于EE 2F部分的离散分布，采用了基于类的种群平衡和气泡相互作用模型。相变是由一个可压缩的计划与热力学空化模型，这意味着假设的热和机械平衡和使用的状态方程。该实现基于OpenFOAM的内部扩展。用相界面解析一维单气泡模型，详细的质量和热传递进行评估的多重性，如果单气泡，并为空气脱和吸收率的比例桥接描述，所得到的速率分布通过源项传递到三维EE 2F求解器。除了超声变幅杆装置的水声空化之外，该方法在孔流处的空化诱导的空气释放上进行了验证，这对应于经典的水力空化。

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.