Comparative analysis of twin vortex ropes in laboratory models of two hydro-turbine draft-tubes

Sergey Skripkin*, Mikhail Tsoy, Sergey I. Shtork, Kemal Hanjalić

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

38 Citations (Scopus)


We report on parallel experimental studies of twin vortex ropes in laboratory models of an elbow and a conical hydroturbine draft tube focusing on the unstable cyclic switch-overs between the single and twin helices. The measurements involve high-speed visualization, pressure pulsation recordings and laser-beam detecting of the precessing helices. The Fourier transform of the pressure probe signals accompanied by optical detection provided the frequencies and amplitudes of the detected single and twin ropes, which were compared between the two draft tubes in a range of Reynolds number from 105 to 5 × 105. The experiments showed constant, though different, Strouhal numbers both for the single and twin ropes. The single-rope Strouhal numbers normalized with the swirl number for the two draft tubes collapse onto a unique value of 1.1 independent of Reynolds number. The ratio of the helix frequencies in the twin- and single rope regimes was 1.15 and 1.4, respectively, for the elbow and the conical model.

Original languageEnglish
Pages (from-to)450-460
Number of pages11
JournalJournal of Hydraulic Research
Issue number4
Publication statusPublished - 2016


  • Hydroturbine draft tube
  • precessing vortex core
  • swirling flows
  • twin vortex
  • vortex dynamics


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