Velocity dependence of damage due to spot cavitation

许多研究者对半球形圆柱体上的各种空化现象进行了研究。当雷诺数小于临界值时，半球形体从层流分离向下游转变。在这种情况下，在身体周围观察到片状空化。然而，当雷诺数增加到临界值以上或通过在阀体上安装起扣消除层流分离时，片状空化被清除，但偶尔会观察到附着的点空化。它发生在最小平均压力附近的固定位置，并长成一个三角形的楔形。认为斑点空化是由于表面局部的孤立粗糙度造成的。在最近的研究中，点状空化是由安装在半球形圆柱体上的边界层起下钻下游直径0.3 mm的空心人为产生的。在空化数为0.91时，通过瞬时照片和高速摄影观察到更多的空化现象。在主腔的闭合区，观察到逆u型涡腔的周期性释放或腔云分离。与流速无关，基于空腔厚度、流速和涡空化释放或空腔云分离频率的斯特劳哈尔数约为0.41。在25 ~ 50 m/s流速范围内，空化数为0.91时，通过计算铝试样上的损伤坑数，得到了损伤击打的频率。点蚀率在两个周向位置的最大值约为1。间距为3mm。最大点蚀率的轴向位置在平均空腔尾缘或略高于平均空腔尾缘。最大损伤区域的点蚀率变化约为5^< >次流速，除反u型涡空腔和主空腔下游空腔云的崩塌外，空腔闭合区域的重入流导致空化气泡的崩塌被认为是造成严重损伤的原因。少

Various types of cavitation on a hemispherical cylindrical body have been studied by many investigators. When the Reynolds number is smaller than the critical value, the hemispherical body exhibits transition downstream from laminar separation. In this tuation, the sheet cavitation is observed around the body. However, when the Reynolds number increases to and above the critical value or the laminar separation is eliminated by a trip installed on the body, the sheet cavitation is swept away but the attached spot cavitation is observed occasionally. It occurs at fixed places in the vicinity of the minimum mean pressure and grows into a triangular wedge. It is considered that the spot cavitation is due to isolated local roughness on the surface. In the resent investigation, the spot cavitation is artificially generated from a hollow of diameter 0.3 mm downstream from a boundary-layer trip installed on a hemispherical cylindrical body. The behavior of the artificially generated spot cavit … More ation at the cavitation number of 0.91 is observed by instantaneous photographs and high-speed photography. In the closure region of the main cavity, periodic release of inverse U-shaped vortex cavities or cavity cloud separation is observed. The Strouhal number based on the cavity thickness, the flow velocity, and the frequency of the vortex cavitation release or the cavity cloud separation is approximately 0.41 irrespective of the flow velocity. The frequency of damaging blows in the range of the flow velocity of 25 to 50 m/s at the cavitation number of 0.91 is obtained by counting the number of damaged pits on an aluminum specimen. The pitting rate has maximums at two circumferential locations at approximately 1 .3 mm away from each other. The axial position of the maximum pitting rate is at the mean cavity trailing edge or slightly upstream of the mean cavity trailing edge. The pitting rate at the maximum damage zone varies roughly 5^<th> power of the flow velocity, In addition to the collapses of the inverse U-shaped vortex cavities and the cavity clouds downstream of the main cavity, the collapses of cavitation bubbles by the reentrant flow in the cavity closure region are considered to be the causes of the serious damage. Less