本文作者提出了用以研究混流式水轮机尾水管中不稳定周期性水流的相似律。给出由尾水管中水流环量引起的低频压力脉动频率与振幅的计算表达式。某些水电厂试验表明,计算表达式是正确的。提出在大型混流式水轮机转轮出口处装设轴流式诱导轮,做为消除由尾水管低频压力脉动引起的水轮机振动的措施。 The author proposed a similar law to study the unstable periodic water flow in the draft tube of Francis turbine. The expression of frequency and amplitude of low frequency pressure pulsation caused by water flow in tail pipe is given. Some hydropower plant tests show that the calculation is correct. It is proposed that an axial inducer be installed at the exit of the runner of a large Francis turbine as a measure to eliminate the vibration of the turbine caused by the low frequency pressure pulsation of the draft tube.