Coherent structure analysis of cavitation waterjets using dynamic mode decomposition

Abstract

This study analyzes the influence of nozzle geometry on the vortex and cavitation cloud structures. The differences between the coherent structures of the Helmholtz nozzle, organ pipe nozzle, and venturi nozzle jets are investigated through large eddy simulation. The vorticity transport equation is used to investigate the relationship between the cavitation cloud and diagonal pressure torque terms. The cavitation and vortex structure shedding frequencies of the jets are investigated using the dynamic mode decomposition method. Three distinct stages of the cavitation bubbles are illustrated: priming, expansion, and collapse. The nozzle structure determines the shape of the primary cavitation bubbles. Moreover, turbulent kinetic energy convergence facilitates the maintenance of the coherent structure. Organ pipe nozzle jets have a high peak velocity at the center axis. Their vortex structure only exhibits a stretched state in the downstream and collapses later than the vortex structures of other nozzles. Advantageously, organ pipe nozzles maintain the stability of the coherent structure. The jets generated by the three nozzles have similar static modes. Helmholtz nozzles produce jets with higher energy and periodically shedding small-scale vortex structural modes. These modes are coupled to the static flow field, resulting in quasi-periodic oscillations of the Helmholtz nozzle jets. The periodic oscillation effect of the Helmholtz nozzle jets is superior to that of the other nozzle jets. The high-energy modes of the venturi nozzle jets have anisotropic and small-scale vortex structures. Furthermore, the venturi nozzle jets exhibit good dispersion and cavitation properties. This study provides guidance for the use of jets with different properties in the respective engineering fields.