Structure of underexpanded round air jets submerged in water

Abstract

The structure and mixing properties of round turbulent air jets submerged in still water were studied experimentally, considering subsonic and sonic jet exit conditions with underexpansion ratios as high as 8: 1. Measurements included the following mean properties: static pressure distributions along the axis, void-fraction distributions and entrainment rates. Confirming earlier findings, static pressure measurements showed the presence of a shock-wave-containing external-expansion region for underexpanded air jets in water, similar to the external-expansion region of underexpanded air jets in air. Increasing underexpansion ratios tended to stabilize large-scale unsteadiness observed for subsonic conditions, however, enhanced mixing near the jet exit, attributed to unsteadiness, persisted to the highest underexpansion ratios tested. The fully-developed portion of the flow exhibited widths based on void fractions that were roughly twice as large as widths based on scalar properties in single-phase jets; this behavior is caused by the strong sensitivity of void fractions to mixing levels, due to the large density ratio of the flow. Measurements were compared with predictions based on the locally-homogeneous-flow approximation of multiphase flow and the equivalent-adapted jet approximation for underexpanded jets. Predictions were encouraging but were sensitive to the specification of initial conditions and effects of enhanced mixing due to unsteadiness close to the jet exit.