Measurement of Nonlinear Transfer Functions for Transitioning Flows

A method for estimating the linear and quadratically nonlinear transfer functions between two points in a transitioning or turbulent flow is presented. The technique does not require a Gaussian input signal, and is demonstrated on data measured in the nonlinear stages of the transitioning flow of a wake behind a thin flat plate. The results indicate that in the early nonlinear stages of transition, interactions between modes of similar frequency and a low frequency component are most efficient in redistributing spectral energy. As transition progresses, interactions involving the first harmonic frequency become efficient as well in the spectral energy transfer process. In general, nonlinear transfer function techniques allow a quantitative characterization of spectral energy transfer dynamics.