Nonlinear spectral dynamics of a transitioning flow

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Abstract
The nonlinear spectral dynamics of a transitioning flow in the wake of a flat plate is experimentally studied at different downstream positions with a two‐point method. The measurement setup consists of two sensors, which are separated in the downstream direction. The quadratically nonlinear transfer function between the two points is then estimated from the digitized fluctuation data. Such transfer functions permit one to quantify the quadratically nonlinear spectral dynamics occurring between the two sensor points. The method used to estimate the transfer functions and local bicoherency for non‐Gaussian input and output signals, by means of digital spectral analysis techniques, is briefly discussed. The measured quadratic transfer function of the experimental data changes gradually with downstream distance, but its main features are unchanged. The observed appearance of progressively higher harmonics of the fundamental mode and the filling in of the spectral valleys over short downstream distances are, thus, mainly caused by spectral redistribution of energy that is available in the interacting modes and not caused by abrupt changes in the coupling properties. This result is supported by the local bicoherency measurements.