Identification and reconstruction of high-frequency broadband fluctuations in a turbulent flow exhibiting low-frequency deterministic motions

The turbulent flow around a squared-section cylinder at $Re=22000$ exhibits both a low-frequency Vortex-Shedding (VS), a common feature of all bluff-body flows, and high-frequency Kelvin-Helmholtz (KH) structures evolving on the shear layers formed at the top and bottom side of the cylinder. The VS motion triggers a periodic movement of these shear layers, inducing oscillations of the strength of their velocity gradients. The high-frequency KH structures respond to these low-frequency oscillations of the base-flow gradients in a quasi-steady manner. In this paper we will propose a general framework to capture and reconstruct stochastic high-frequency fluctuations (the KH structures) evolving on top of a low-frequency deterministic motion (the periodic VS). Based on a generic triple decomposition, that allows separation of the stochastic component from the deterministic one, we propose a Phase-Conditioned Localized Spectral Proper Orthogonal Decomposition (PCL-SPOD), which isolates high-frequency spatio-temporal structures within the stochastic component at different phases of the VS motion, with a short-time Fourier transform. Finally, we compare these structures to those obtained with a Phase-Conditioned Localized Resolvent formalism (PCL-Resolvent), which consists in a Resolvent analysis around instantaneous snapshots of the periodic VS.

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