Beyond optimal disturbances: a statistical framework for transient growth

TL;DR

This paper introduces a statistical framework for analyzing transient growth in linear systems, providing mean and probability-based measures that better predict realistic disturbance amplification than traditional maximum-based methods.

Contribution

It develops a novel statistical approach to quantify transient growth, including formulas for mean energy amplification and its probability distribution, improving upon traditional worst-case analyses.

Findings

Mean gain can be much smaller than maximum gain for broadband disturbances.

Large-scale initial disturbances lead to significantly higher expected growth.

Mean energy amplification scales linearly with Reynolds number, unlike the quadratic scaling of optimal growth.

Abstract

The theory of transient growth describes how linear mechanisms can cause temporary amplification of disturbances even when the linearized system is asymptotically stable as defined by its eigenvalues. This growth is traditionally quantified by finding the initial disturbance that generates the maximum response, in terms of energy gain, at the peak time of its evolution. While this bounds the growth, it can vastly overstate the growth of a real disturbance. In this paper, we introduce a statistical perspective on transient growth that models statistics of the energy amplification of the disturbances. We derive a formula for the mean energy amplification in terms of the two-point spatial correlation of the initial disturbance. We also derive an accurate approximation of the probability density function of the energy of the growing disturbance, from which confidence bounds on the growth can be obtained. Applying our analysis to Poisseuille flow yields a number of observations. First, the mean gain can be drastically smaller than the maximum, especially when the disturbances are broadband in wavenumber content. In these cases, it is exceedingly unlikely to achieve near-optimal growth due to the exponential behavior which we observe in the probability density function. Second, the characteristic length scale of the initial disturbances has a significant impact on the expected growth; specifically, large-scale initial disturbances produce orders-of-magnitude-larger expected growth than smaller scales, indicating that the length scale of incoming disturbances may be key in determining whether transient growth leads to transition for a particular flow. Finally, while the optimal growth scales quadratically with Reynolds number, we observe that the mean energy amplification scales only linearly for certain reasonable choices of the initial correlations.