High-Resolution Velocity Field Measurements of Turbulent Round Free Jets in Uniform Environments

Turbulent round free jets are one of the most common jet types, which have been intensively studied in the research community for over 90 years. Due to its characteristics of momentum transport in free shear layers, this type of jet is widely used in several industrial applications varying from nuclear reactor safety analysis to aerospace jet engine designs. Focusing on close-to-jet (near-field) and self-similar regions, the entrainment and momentum transport can be properly described by the Reynolds numbers of the flow fields.

To establish a nonconfined free jet, an experimental facility was built with a jet nozzle diameter of 12.7 mm, located at the bottom of a cubic tank with a 1-m side length. The jet flow is realized by a servo-motor-driven piston to avoid possible fluctuations introduced by other motor options. Nominal jet Reynolds numbers range from 5000 up to 22 500. High-speed and time-resolved particle imaging velocimetry techniques are used to measure the velocity fields in the vertical midplane of the jet for both investigated flow fields. The adopted setup has a spatial resolution of 209 × 209 µm² for near-field regions and 684 × 684 µm² for self-similar regions and thus covers the Taylor microscale for all cases presented in this paper. Experimental results are presented in terms of turbulent statistics and the frequency spectrum of the velocities. The sources of uncertainties associated with the measured velocity field are quantified. The results are in good agreement with previously published data. The obtained energy spectra confirm Kolmogorov’s theory in the inertial subrange. Coherent structures, obtained with two-point spatial correlations of variances of velocities, show growth in penetration depth with increased downstream distance, which is consistent with the analysis of temporal correlation fields.