Highly under-expanded jets in the presence of a transverse density gradient

TL;DR

This experimental study investigates how the morphology and mixing behavior of highly underexpanded jets vary with density ratio and Mach number, revealing that density ratio significantly influences jet characteristics.

Contribution

It provides new experimental data on underexpanded jets with varying density ratios and Mach numbers, and introduces simple scaling laws for jet behavior in different regions.

Findings

Jet morphology depends more on density ratio than Mach number.

Lighter jets exhibit more intense mixing with ambient gases.

Scaling laws effectively describe jet behavior in near and mid-long term regions.

Abstract

An experimental research concerning highly underexpanded jets made of different gases from the surrounding ambient is here described. By selecting different species of gases, it was possible to vary the jet-to-ambient density ratio in the 0.04 to 12 range and observe its effect on the jet morphology. By adjusting the stagnation and ambient pressures, it has been possible to select the Mach number of the jets, independently from the density ratio. Each jet is therefore characterized by its maximum Mach number, ranging from 10 to 50. The Reynolds number range of the nozzle is 1000 to 50000. The spatial evolution of the jets was observed over a much larger scale than the nozzle diameter. The gas densities were evaluated from the light emission induced by an electron beam and the gas concentrations were obtained by analyzing the color of the emitted light. The results have shown that the morphology of the jets depends to a greater extent on the density ratio. Jets that are lighter than the ambient exhibit a more intense jet-ambient mixing than jets that are heavier than the ambient, while the effects of changing the jet Mach number do not seem to be too large in the explored range. These results can be expressed by means of two simple scaling laws relevant to the near field (pre-Mach-disk) and the mid-long term field (post-Mach-disk), respectively.