Experimental investigation of a twin vortex breakdown on the draft tube models

This work is devoted to the experimental study of the flow structure in the hydro turbine draft tube models. A precessing helical vortex rope was formed at the flow with high swirling degree. Effect of transition between single- and double-vortex rope was revealed in different laboratory models. Frequency characteristics of the precessing structures were measured in the range Re = 5 · 10⁴ - 5 · 10⁵. From the analysis of high-speed camera records, the scenario of full transition between single and double vortex was proposed. The study of this phenomenon is of particular interest for the design and safe operation of hydraulic turbine equipment, because powerful pressure pulsations with unexpected frequency and amplitude may be generated in the flow.