Space–time analysis of actuation transients: Example of plasma-controlled jet flow
Phys. Rev. Fluids 11, 064614 – Published 22 June, 2026
DOI: https://doi.org/10.1103/ktxc-hgjf
Abstract
We outline a workflow for examining statistically transient fluid flows, which eludes most standard modal analysis techniques. As a challenging example, we investigate the forcing-induced transient between statistically stationary and cyclostationary states and discuss the underlying flow physics. The transient dynamics of a turbulent supersonic twin-rectangular jet flow, forced symmetrically at a Strouhal number of 0.9, are studied using synchronized large-eddy simulations. Under plasma-actuated control, the statistically stationary jet evolves towards a cyclostationary state over a transient phase. Forcing-induced perturbations of the natural jet are extracted using synchronized simulations of the natural and forced jets. A database is collected that captures an ensemble of realizations of the perturbations within the initial transient, which is characterized by fast energy growth. The initial pulse of the actuators produces large, impulsive perturbations to the flow field. The mean flow deformation transient reveals that shock cells are deflected by the forcing towards then away from the nozzle, resulting in a subtle but permanent contraction of shock spacings. Despite this, time–frequency analysis shows that jet screech persists throughout the transient. Streamwise wavelet-based filtering of the time-varying mean flow deformation effectively isolates small-scale shock contraction and shear-layer modal instability waves from large-scale but low-amplitude perturbations. The optimal eigenvector from space–time proper orthogonal decomposition is an antisymmetric—or flapping—mode. Applying an analogous filter to the optimal mode, we track the evolution of a large-scale, transient, and dispersive wave-packet structure and observe the coupling of widely disparate scales enabled by spatial inhomogeneity and temporal transience.
