Mottyll Stephan, Skoda Romuald
Chair of Hydraulic Fluid Machinery, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany.
Ultrason Sonochem. 2016 Jul;31:570-89. doi: 10.1016/j.ultsonch.2016.01.025. Epub 2016 Jan 25.
As a contribution to a better understanding of cavitation erosion mechanisms, a compressible inviscid finite volume flow solver with barotropic homogeneous liquid-vapor mixture cavitation model is applied to ultrasonic horn set-ups with and without stationary specimen, that exhibit attached cavitation at the horn tip. Void collapses and shock waves, which are closely related to cavitation erosion, are resolved. The computational results are compared to hydrophone, shadowgraphy and erosion test data. At the horn tip, vapor volume and topology, subharmonic oscillation frequency as well as the amplitude of propagating pressure waves are in good agreement with experimental data. For the evaluation of flow aggressiveness and the assessment of erosion sensitive wall zones, statistical analyses of wall loads and of the multiplicity of distinct collapses in wall-adjacent flow regions are applied to the horn tip and the stationary specimen. An a posteriori projection of load collectives, i.e. cumulative collapse rate vs. collapse pressure, onto a reference grid eliminates the grid dependency effectively for attached cavitation at the horn tip, whereas a significant grid dependency remains at the stationary specimen. The load collectives show an exponential decrease towards higher collapse pressures. Erosion sensitive wall zones are well predicted for both, horn tip and stationary specimen, and load profiles are in good qualitative agreement with measured topography profiles of eroded duplex stainless steel samples after long-term runs. For the considered amplitude and gap width according to ASTM G32-10 standard, the analysis of load collectives reveals that the distinctive erosive ring shape at the horn tip can be attributed to frequent breakdown and re-development of a small portion of the tip-attached cavity. This partial breakdown of the attached cavity repeats at each driving cycle and is associated with relatively moderate collapse peak pressures, whereas the stationary specimen is rather unfrequently stressed at the end of each subharmonic oscillation cycle by the violent collapse of the complete cavity.
为了更好地理解空蚀机制，将具有正压均质液-气混合物空化模型的可压缩无粘有限体积流动求解器应用于有无固定试样的超声变幅杆装置，这些装置在变幅杆尖端呈现附着空化。与空蚀密切相关的空泡溃灭和冲击波得到了解析。将计算结果与水听器、阴影成像和侵蚀试验数据进行了比较。在变幅杆尖端，蒸汽体积和拓扑结构、次谐波振荡频率以及传播压力波的振幅与实验数据吻合良好。为了评估流动攻击性和侵蚀敏感壁区，对变幅杆尖端和固定试样应用了壁载荷统计分析以及壁相邻流动区域中不同溃灭次数的统计分析。将载荷集合（即累积溃灭率与溃灭压力）后验投影到参考网格上，有效地消除了变幅杆尖端附着空化的网格依赖性，而在固定试样上仍存在显著的网格依赖性。载荷集合显示随着溃灭压力升高呈指数下降。对于变幅杆尖端和固定试样，侵蚀敏感壁区都得到了很好的预测，并且载荷分布与长期运行后侵蚀的双相不锈钢样品的实测形貌分布在定性上吻合良好。对于根据ASTM G32 - 10标准考虑的振幅和间隙宽度，载荷集合分析表明，变幅杆尖端独特的侵蚀环形形状可归因于尖端附着空泡一小部分的频繁破裂和重新发展。附着空泡的这种部分破裂在每个驱动周期重复出现，并与相对适中的溃灭峰值压力相关，而固定试样在每个次谐波振荡周期结束时，由于完整空泡的剧烈溃灭而受到的应力相当不频繁。
