Open AccessExperimental investigation of vortex ring formation as a consequence of spiral vortex re-connection
Author(s) -
David Štefan,
Pavel Rudolf,
Martin Hudec,
Václav Uruba,
Pavel Procházka,
Ondřej Urban
Publication year - 2019
Publication title -
iop conference series. earth and environmental science
Language(s) - English
Resource type - Journals
eISSN - 1755-1307
pISSN - 1755-1315
DOI - 10.1088/1755-1315/405/1/012033
Subject(s) - vortex , vortex ring , starting vortex , physics , mechanics , horseshoe vortex , spiral (railway) , burgers vortex , vortex shedding , vortex stretching , vortex generator , classical mechanics , turbulence , engineering , mechanical engineering , reynolds number
The unsteady shape variation of a cavitating spiral vortex is experimentally studied pointing out the repetitive creation of vortex ring (or vortex loop). To generate the spiral vortex structure, a newly designed vortex generator is employed. This generator is used to study the decelerated swirling flow leading to the spiral vortex formation as a consequence of spiral vortex breakdown, which is considered to be the main triggering mechanism for the occurrence of the coherent vortex rope structure in the Francis turbine draft tube operated at part load conditions. Thanks to its design, the vortex generator enables to change the ratio between fluxes of axial momentum and tangential moment of momentum of generated swirl. Using this set-up, the behavior of the vortex structure changes in a similar way as the flow rate variation in the draft tube of Francis turbine. At certain flow conditions, the spiral vortex movement is characterized by sudden spiral entanglement leading to disconnection of a vortex ring and followed by spiral reconnection. Thanks to the transparent diffuser of the swirl generator apparatus, both high-speed camera recording of the cavitating vortex and PIV measurements of velocity fields at cavitationfree conditions are employed. The main aim of this paper is to link the visual observation of the above described vortex dynamics with the velocity fields measured in one longitudinal and one cross-sectional planes.