On the generation mechanisms of low-frequency synchronous pressure pulsations in a simplified draft-tube cone

This paper focuses on the generation mechanisms of synchronous pressure pulsations featuring a lower frequency than the rotating frequency of a vortex core which are observed in a simplified draft-tube cone through analyzing the results of experiments along with unsteady-state numerical simulations. A Venturi-tube is selected as a simplified geometry of hydro-turbine draft-tube; and a stationary-blades swirl generator is installed upstream to generate a uniform swirling flow in the Venturi cone (diffuser part). The experiments, including the dynamic pressure measurements, are conducted for several flowrates to determine the characteristics of pressure fluctuations in the Venturi cone. Two types of pressure fluctuations are identified, namely convective and synchronous. Both cover wide ranges of different frequencies, revealing that the phenomena responsible for generation of these pressure fluctuations feature a stochastic component. The synchronous pressure pulsations, which feature lower frequencies than the convective fluctuations, are identified as the product of self-excited oscillations of vortex breakdown. High-speed visualization of the vortex shape reveals that the vortex features three states: an axisymmetric vortex, a breakdown, and appearance of several helical vortices generated downstream of the breakdown location; while the location of the vortex breakdown fluctuates in time. By using the results of unsteady CFD simulation, it is confirmed that the location of the vortex breakdown fluctuates with the same frequency (Strouhal numbers 0 to 1) as the low-frequency synchronous pressure pulsations observed in the Venturi cone. Moreover, it is observed that the volume of the stagnation region produced due to the counterflow downstream of the breakdown location fluctuates with the same low frequency. Further analysis of the CFD results showed that the flow field at the location of the vortex breakdown experiences a severe shear layer formation around the center of the Venturi. This shear layer stretches the vortex center spatially on the plane perpendicular to the vortex axis. It is also observed that several intermittent helical vortices are generated downstream the breakdown location and rotate periodically in the Venturi while featuring a stochastic nature, as well.