Numerical and Experimental Study on Cavity Flow

本研究的结果可以总结为：1。采用气泡两相流模型和有限差分技术，对有限跨度叶片和旋转叶片的非定常粘滞空化流动进行了计算。由于只对低雷诺数进行了计算，因此没有得到与实验数据定量一致的结果。然而，计算结果很好地表达了实验的趋势，表明了该流动模型计算此类流动的有效性。叶尖涡的结构并不像朗肯涡那样简单，而是由箔上、下表面的边界层分别形成的两个核心组成。这就是尖涡腔具有扭带形式的原因。增大箔上的空腔尺寸可减弱叶顶涡。但叶尖涡的旋转促进了叶尖附近的分离，导致叶尖附近的空腔较大。一个空腔抬升了剪切层，导致更厚的粘性尾迹。由于离心力的作用，旋转叶片上的气流流向叶尖。因此，流动模式变得完全不同于在一个直箔。研究表明，除采用合适的湍流模型外，网格生成技术是实现高雷诺数计算的关键。利用有限跨度箔片模型进行了精确的实验，测量了箔片上的压力分布和箔片周围的流场，并观察了空化现象。特别对叶顶涡的位置和速度分布进行了详细的测量。这些结果对上述流动结构进行了澄清，并对计算结果进行了详细的分析。

The results of this study can be summarized as follows :1. Computations were made for unsteady viscous cavitating flows around a finite span foil and a rotating blade, using Bubble Two-phase Flow model and a finite difference technique. Since the computations were performed only for low Reynolds numbers, a quantitative agreement with the experimental data was not obtained. However, the computed results expressed the trends of the experiments well, showing the effectiveness of the present flow model for the computation of such flows.2. The tip vortex does not have a simple structure like a Rankine vortex but consists of two cores caused by the boundary layrs on the upper and lower surfaces of the foil, respectively. This is the reason why a tip vortex cavity has a form of a twisting ribbon.3. Increasing cavity size on the foil weakens the tip vortex. But the rotation of tip vortex promotes the separation near the foil tip, resulting in larger cavity near the tip area.4. A cavity lifts up the shear layr, resulting in thicker viscous wake.5. Flow on a rotating blade goes toward the tip because of centrifugal force. As a result, the flow pattern becomes completely different from that on a straight foil.6. It has been shown that a grid generation technique is a key for a high Reynolds number computation in addition to adopting an appropriate turbulence model.7. Accurate experiments were performed using a finite span foil model to measure pressure distribution on the foil and flow field around the foil as well as to observe cavitation. Particularly, the positions and velocity distributions of the tip vortex were measured in detail. These results have been used to clarify the above-mentioned flow structure, together with the detailed analysis of the computed results.