Experimental and numerical analysis of swirling vortex flow and induced cavitation rope

Abstract

Cavitation in swirling flows is a major source of unsteadiness, pressure fluctuations, and material damage in hydraulic and marine systems, where strong vortical motion generates pronounced pressure deficits within the vortex core, making cavitation inception governed primarily by local vortex dynamics rather than global pressure conditions. Studying this phenomenon directly in turbine environments is geometrically complex, costly, and difficult to control experimentally. This study replicates the vortex rope phenomenon observed in hydro turbines using a simplified closed-loop configuration consisting of a tangential swirl chamber followed by a straight circular tube, generating a stable concentrated vortex without blades or rotating components. Experiments were conducted across five operating conditions ranging from Q = 0.458 L/s to Q = 1.302 L/s, allowing the vortex core to be characterised in terms of rope diameter, axial length, and helical structure. Cavitation number analysis confirmed that the observed gaseous core formed through dissolved gas release rather than vaporous cavitation. Numerical simulations using a three-dimensional finite volume solver with the Volume of Fluid method and inlet air content of 48 mg/L successfully reproduced the helical vortex rope structure, with good agreement in rope geometry, helix pitch, and core extent. The frequency of rope generation, section length, and travelling velocity all varied with inlet flow conditions. These findings provide a validated modelling foundation relevant to hydraulic turbines, pump inducers, and marine propellers.