A comparison of Helmholtz oscillators with differently shaped petal nozzles

Abstract

This study analyzes the effect of nozzle geometry on the dynamic characteristics of water jets. The flow characteristics of Helmholtz oscillators with three different nozzle shapes (four, six, and eight petals) are investigated using large eddy simulations. The relationship between coherent structures and cavitation is illustrated using the vortex transport equation. The results show that the evolution of cavitation clouds in a Helmholtz cavity results in a periodic change in the jet pressure and the formation of pulsed water jets from petal-shaped nozzles. In addition, the cavitation bubbles inhibit the stretching and expansion of the vortex structure, and the convergence of the turbulent kinetic energy is conducive to maintaining its stability. The four-petal nozzle has the highest velocity at the central axis of the jet at 1.76% higher than that of the six-petal nozzle. When the number of petals in the nozzle is increased from six to eight, the velocity decreases by 7.96%. The streamwise vortex strength of the jet with six petals is enhanced by 61.31% compared to the four-petal case, while the eight-petal nozzle jet is only 11.75% higher than that of the six-petal nozzle. The six-petal nozzle significantly improves the mixing characteristics by slightly reducing the velocity. This study provides guidance for reducing the stagnation pressure loss when using nozzles with special shapes to enhance jet mixing.