涂层对绕水翼云状空化的运动特性及动力特性影响的试验研究

Experimental Study of Coatings’ Effect on Kinetics and Dynamics in Cloud Cavitating Flows

采用多种试验技术方法讨论了不同涂层水翼云状空化的空穴形态、运动特性和动力特性。三种涂层分别为光滑环氧涂层(模型A)、光滑氟碳涂层(模型B)和表面粗糙氟碳涂层(模型C)。研究表明:(1)涂层的粗糙度和材质共同影响绕水翼云状空化流动特性;(2)不同涂层水翼的空穴形态不同:附着在模型A的空穴厚度更小,空穴长度随时间变化更快,空穴脱落周期更短。且模型A透明空穴区透明性更好,变化范围最小,模型B其次,模型C最大。(3)涂层影响云状空化运动特性:绕不同涂层水翼的局部低速区和涡量区位置的各不相同导致透明空泡堆积位置也不同,局部正涡量区位置的不同导致出现旋涡水汽混合区位置的差异,模型A的旋涡水汽混合区位置距离水翼头部最近,B其次,C最远;(4)与粗糙度相比,涂层材质对绕水翼云状空化的动力特性影响更为明显。不同涂层水翼升力系数急剧下降区不同,升力的主导频率也不同,但与云空化流动结构中空化的变化周期相一致,此外绕模型B空穴转变为云状空化的空化数比模型A、C的要小,因此模型B涂层能够抑制云状空化发生。

Different experimental techniques are combined to carry out the flow patterns, kinetic and dynamic characteristics of cloud cavitation around three different coating hydrofoils and discuss the influence of different coatings. Three different coatings are smooth epoxy coating(Hydrofoil A), smooth fluorocarbon coating(Hydrofoil B) and rough fluorocarbon coating(Hydrofoil C). The results show four main phenomena. Firstly, coatings’ roughness and material both effect the flow characteristics of cloud cavitation. Secondly, different coatings have different flow patterns. The comparison among cavities over hydrofoils with three different coatings verifies that Hydrofoil A has thinner thickness of attached cavity, more transparent cavity region, larger growth rate of cloud cavity and shorter period. Thirdly, coating effects the kinetic characteristics of cloud cavitation. For different coatings, positions of local low-speed region are different, resulting in positions of accumulation of transparent bubble are different, the same to the local positive vorticity region for the reason of the vortex vapor mixing region. In terms of variation range of transparent cavity, Hydrofoil A has smaller range than Hydrofoil B and C. In terms of positions of vortex vapor mixing region, Hydrofoil A has the shortest distance to the leading edge. Fourthly, compared with roughness, material of coating has more obvious effect on the dynamic characteristics of cloud cavitation. The dominant frequency of lifts of hydrofoils with different coating is different in cloud cavitation, which consistent with changes of cavitation flow structure. When develops into cloud cavitation, Hydrofoil B has the lowest cavitation number among three hydrofoils. So restriction on cloud cavitation generated by Hydrofoil B is stranger than Hydrofoil A and C.