Scale effect on mean pressures: study of two reduced models of a ski jump spillway

ABSTRACT The ski jump type spillway is present in large Brazilian dams. After the operation of this type of spillway, a scour hole is originated downstream of the spillway toe, complementing the energy dissipation system, because the jet is cushioned by the waterbed. As the excavation of the scour hole evolves, its shape changes, with the induction of different behaviors of the jet flow within the waterbed. The importance of this scientific study is due to the fact that scour holes downstream of ski jump spillways are in constant transformation, which can cause changes in the patterns of dynamic pressures caused on the bottom. Unexpected erosions can cause instability in the submerged slopes of the pit, jeopardizing the foundation of the dam. In this work it was proceeded physical experiments, by sectional modeling (2D), in two reduced models of 1:50 and 1:100 scales, considering two stages of erosion depths in solid bottom (fixed). It was analyzed the impact site of the jet flow on the bottom, which has the major mean dynamic pressure. As result, it was possible to identify the scale effect in the pressure coefficient between the two models tested, which allowed the adjustment of trend lines that aid to estimate the pressures that occur in prototypes, increasing safety in operation of ski jump spillways.

摘要：挑流式溢洪道（ski jump type spillway）广泛应用于巴西大型水坝中。该类溢洪道运行后，其鼻坎下游会形成冲刷坑，可辅助完善消能系统，因射流可通过水垫实现消能缓冲；随着冲刷坑的发育演变，其形态会发生改变，进而引发水垫内射流流态的变化。本研究的重要意义在于，挑流式溢洪道下游的冲刷坑始终处于动态演变过程中，这会导致底板动压分布模式发生改变，非预期的冲刷作用会使坑内淹没坡体失稳，进而威胁大坝地基安全。本研究通过二维（2D）断面建模的方式开展物理模型试验，采用1:50与1:100两种缩尺比，设置了实体底板（固定）下的两个冲刷深度工况，针对底板上动压均值最大的射流冲击点位展开了分析。试验结果表明，可明确两个试验模型间压力系数的比尺效应，据此可拟合得到趋势线以估算原型工况下的动压值，进而提升挑流式溢洪道的运行安全性。