About the ambiguity of causality correlation results for noise source localization in cold jets

Abstract

Interpreting the coefficient matrix resulting from the correlation between turbulent flow quantities obtained in jet-flows and far-field pressure fluctuations can be ambiguous in regions with coherent structures dominating the flow-topology [8,34]. The aim of the present study is to contribute to the understanding of results from this so-called causality correlation approach. The sound radiated into the far-field is calculated using a free-space Green's function for correlated monopoles convecting with a constant speed along a jet axes. The amplitude of the modeled source is damped by an envelope simulating a strong activity at the end of the potential core region of the jet at x=D ¼ 5 (see Fig. 25, left).The fluctuating part of the resulting pressure-time signal is dominated by truncation effects (see Fig. 25, right). This truncation effect can be avoided by using a numerical model based on jittering wave-packets with varying amplitude and spatial extents [10] (see Fig. 26, left).Here the resulting pressure signal is clearly dominated by the intermittency of the source signal,showing high amplitude burst (see Fig. 26, right).In case of the jittering wave-packets model the correlation between the source signal and the far-field is maximal for time delays, corresponding to the sound travel time from the source to the observer (see Fig. 27). The results verify the approach documented in the literature of analyzing the spatial and temporal evolution of the coefficient matrix in order to identify flow regions which are subject to the same physical phenomenon as the aeroacoustic source [33].