Interplay of vortex rings and streaks in the near field of turbulent jets

TL;DR

This study investigates the coexistence and interaction of vortex rings and streaks in the near field of turbulent jets, revealing their coupled dynamics and the influence of vortex rings on streak formation.

Contribution

It provides new insights into the spatial and spectral characteristics of streaks and vortex rings in turbulent jets, including the effect of vortex ring strengthening on streak persistence.

Findings

Streaks are present at low Strouhal numbers on the outer jet edge.

Vortex rings and streamwise vortices occur together at St ≈ 0.49.

Strengthening vortex rings does not significantly alter streak shapes or energies.

Abstract

In light of recent renewed interest in streaks in turbulent jets, the current work explores their coexistence with vortex rings in the near field of turbulent round jets. A Reynolds number, $Re=1.5 \times 10^5$ jet is studied at two diameters downstream of the nozzle, using high-speed stereo particle image velocimetry. The spectra of individual velocity components reveal radially localized signatures of the large scale structures. The radial shapes of the spectral proper orthogonal decomposition modes corresponding to the signatures, confirm the presence of high speed streaks in the outer edge of the jet at low Strouhal number, $St \rightarrow 0$. The vortex rings and streamwise vortices are found to occur in the shear layer at $St \approx 0.49$, where they convect together as a system and feed the streaks. The role of vortex rings in the existence of streaks is then studied by strengthening the rings through axisymmetric excitation. The streaks are observed to persist, retaining their shapes and show only slight changes in their energies.