Coherent structures in axis-switching elliptical jets

TL;DR

This study uses DNS and spectral POD to analyze coherent structures in elliptical jets undergoing axis switching, revealing how forcing levels influence the dominant flow modes and their spectral characteristics.

Contribution

It identifies and characterizes new flapping and wagging modes associated with axis switching in elliptical jets, linking forcing levels to flow structure dynamics.

Findings

Higher forcing causes earlier axis switching.

The flapping mode decays faster at high forcing levels.

A new flapping mode dominates in the post-axis-switch region.

Abstract

Coherent structures in aspect ratio 2, axis-switching elliptical jets are studied using direct numerical simulation (DNS). Three different datasets are studied with varying near-nozzle forcing levels. Increasing the forcing level causes the jet to axis switch at an earlier streamwise location. Spectral proper orthogonal decomposition was applied to the dataset to extract the most-energetic coherent structures in the flow, and modes associated with the main symmetries of the flow were identified. The flapping mode was found to decay faster at the high forcing level, a feature that was linked to the axis-switching behavior. The axis-switching phenomenon causes the flapping mode to become a wagging mode relative to the new axis, lowering the growth rate of the structure. Two different coherent structures were found in the SA (dihedral group $D_2$) symmetry for the axis-switching cases: the wagging mode which was dominant in the pre-axis-switch region and a new flapping mode which is dominant in the post-axis-switch region. The new flapping mode was dominant in the low-frequency region of the full-field SPOD spectrum which was overtaken by the wagging mode at $St\approx 0.2$ for the medium-forcing case and at $St\approx 0.4$ for the high-forcing case. This new flapping mode is likely a flapping mode relative to the axis-switched mean flow, which develops due to the slower growth of the shear layer in the major axis.