Numerical and Experimental Study on Cavity Flow

1.采用气泡两相流模型和有限差分技术，对有限跨度翼片和旋转叶片的非定常粘性空化流动进行了数值模拟。由于计算仅针对低雷诺数进行，因此无法得到与实验数据定量一致的结果。然而，计算结果较好地反映了实验的趋势，表明了本文流动模型对此类流动计算的有效性。尖端涡不具有朗肯涡那样的简单结构，而是由两个核心组成，分别由翼面上和下表面的边界层引起。这就是为什么叶尖涡腔具有一种扭曲的带状结构的原因。增加翼片上的空腔尺寸会削弱尖端涡流。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。空穴抬升切变层，产生更厚的粘性尾迹。旋转叶片上的气流由于离心力而流向叶尖。其结果是，流型与直箔上的流型完全不同。研究表明，除了采用合适的湍流模型外，网格生成技术是高雷诺数计算的关键。使用有限跨度翼片模型进行了精确的实验，测量了翼片上的压力分布和翼片周围的流场，并观察了空化现象。特别是对叶尖涡的位置和速度分布进行了详细的测量。这些结果被用来阐明上述流动结构，并对计算结果进行了详细分析。

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.