Aircraft trailing vortices, characterized by high Reynolds ($\mathit{\text{Re}}$) and swirl numbers ($q$), play a crucial role in wake dynamics and pollutant dispersion. The present study investigates transient energy growth in a heavy $q$-vortex with a variable-density core at $\mathit{\text{Re}}={10}^6$ and $q=10$. Unlike modal stability analysis, which looks for asymptotic exponential growths, nonmodal stability analysis is carried out to identify perturbations with an optimal response before 20 vortex timescales. We take advantage of this approach to measure the additional gain expected from the optimal perturbations compared to the adjoint modes $\mathrm{\Delta }{G}_E^{\mathrm{opt}}={\mathcal{G}}_E/G_E^{\mathrm{adj}}$, where ${\mathcal{G}}_E$ is the energy amplification of the optimal perturbation and $G_E^{\mathrm{adj}}$ that of the adjoint mode. The results reveal a threefold mechanism of transient energy growth, driven by intercomponent energy transfers through pressure fluctuations and the self-sustained interaction between radial velocity and density perturbations.

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• Flow instability
• Rayleigh-Taylor instability
• Vortex dynamics