Ensemble-Empirical-Mode-Decomposition method for instantaneous spatial-multi-scale decomposition of wall-pressure fluctuations under a turbulent flow

This work illustrates the possibilities of the Ensemble-Empirical-Mode-Decomposition (E-EMD) technique for a detailed analysis of the time and space characteristics of the wall-pressure fluctuations under a turbulent flow. Pressure fluctuations are measured with a linear microphone array, for the cases of a turbulent boundary layer and for a diffuse airborne acoustic field. The E-EMD technique is shown to be an efficient tool for representing the spatial scales of the turbulent fluctuations at each instant. In particular, this representation is obtained without any particular assumption or a priori information on the data (e.g. temporal or spatial stationarity of the wall pressure data is not required), and acts, when applied to wide-band turbulent signals, as a wavenumber filter. Finally, it is shown how, to some extent, the E-EMD technique can separate at each instant the acoustic (propagative) from the hydrodynamic (convective) energy.