泄洪洞反弧末端掺气减蚀研究

对高水头、大单宽流量、低佛氏数、小底坡长明流泄洪洞，在反弧段末端采用常规的掺气坎往往难以取得理想的掺气效果，同时，反弧段下游边墙易出现空蚀破坏。本文在大比尺模型试验的基础上，提出了竖向、纵向及横向三维均连续变化的新型掺气坎，并对比分析了掺气坎三维体型的变化对掺气特性的影响。试验表明，空间三维连续变动的V型坎能使挑射水流形成连续变化的空腔长度，较二维掺气坎在上述特点的泄洪洞中能形成稳定的空腔形态、减小空腔回水并提高挟气量。试验同时表明，泄洪洞反弧末端掺气坎后空腔段水流动水压强小、水流空化数低，容易出现空化；同时，反弧段下游水流表面自掺气尚不够充分，底部强迫掺气尚未充分扩散，致使反弧段下游附近边墙存在较大范围的掺气盲区，从而容易导致反弧段下游边墙的空蚀破坏。

Study on air entrainment to alleviate cavitations at the end of anti-arc in spillway tunnel

Based on a large-scale model experiment, a new type of aerator, the three dimensional aerator has been devised in this paper. Aerators are widely used in flood discharge tunnels and spillways to prevent the cavitation damage of the structures. An aerator basically consists of two parts, the flow deflector and the air ventilator. The 3-D aerator means that its deflector is three-dimensional in space instead of two-dimensional as the normal aerator is. It is to say that the shape of the flow deflector, it is usually together with a drop step at the downstream edge, is variable not only in the directions of longitudinal and vertical, but also in the lateral direction. For the flow characterized by high head, large-discharge, and slight slope of the channel, the normal 2-D aerator is frequently failed because of the inadequate aeration of the flow and the submerge of the vent duct. In order to improve the aeration effect, numbers of 3-D reflector schemes together with 2-D scheme have been tested in the model experiment. The results indicate that the V-type reflector is the best. For this type, the sucked airflow rate is larger, the length of the air chamber is longer, and the invert flow is scarcely occurred. The reason of why the V-type is better has been analyzed theoretically. It is mainly because the length of the air chamber is continuously varying along the lateral direction within a range. But for the two dimensional situation, it retains a certain value across the channel. The causes why the sidewalls near to anti-arc in flood discharge tunnels is easy to be damaged by cavitation have been explained in this paper. Under the influence of air chamber resulted from the aerator, the water pressure is small and the flow cavitation index is low at that position. Furthermore, natural aeration is not enough at the surface of flow at that position, and the forced aeration is not sufficiently diffused-all of these brings a large un-airflow area in the sidewalls at the backward of anti-arc, as we know it is easy to be destroyed by cavitations under some kind of conditions. Some methods which can be used in design to alleviate or avoid this problem have been proposed at the end of the paper.