Near-Field Flow Measurements of a Cavitating Jet Emanating From a Crown-Shaped Nozzle

Abstract

The effect of a crown-shaped nozzle on cavitation is studied experimentally in the near-field of a 25 mm diameter (D) water jet at ReD=2×105 using particle image velocimetry (PIV) and high speed shadowgraphy recorded with a 5000 fps digital camera. The objectives are to passively control the jet flow structure and to examine its consequences on the physical appearance of cavitating bubbles. The experiments are performed in a closed-loop facility that enables complete optical access to the near-nozzle region. The cavitating and noncavitating mean velocity fields are obtained up to three nozzle diameters downstream and compared to those of a companion round nozzle. PIV measurements are taken in two distinct azimuthal planes passing through the tip and bottom points of the crown nozzle edge. The data include shear layer momentum thickness and vorticity thickness, spanwise vorticity distribution and streamwise normal Reynolds stress. Significant deviation from an axisymmetric shear layer is observed in the noncavitating flow consistently up to one diameter downstream, after which identical asymptotic conditions are achieved in both round and crown-shaped nozzles. Maximum magnitudes of spanwise vorticity and streamwise normal Reynolds stress are the highest downstream of the nozzle tip edges under noncavitating conditions. Significant modifications in trends and magnitudes are observed for the shear layer momentum thickness under cavitating conditions up to one diameter downstream. Qualitative flow visualization reveals that bubble growth occurs at different conditions depending on azimuthal location. Bubbles, in the form of elongated filaments, are the dominant structures produced downstream of the valley edges of the nozzle with an inclination of 45 deg with respect to the direction of the flow, and are observed to persist with significant strength up to two diameters downstream. These filaments are stretched between periodic larger-scale, spanwise bubbly clusters distorted in the shape of the nozzle outlet. The tip edges produce cavitating bubbles under conditions similar to that of a classical round nozzle. In summary, it was demonstrated that passive control of turbulent structures in the jet does impact the cavitation process.