Investigations of eddy coherence in jet flows by

Abstract

In turbulent shear flow the term “Coherent Structures” refers to eddies which are both spatially coherent, i.e. large eddies, and also temporally coherent, i.e. they retain their identities for times which are long compared with their time scales in fixed point measurements.In certain cases, for example transitional flows, the existence of such structures is evident from flow visual isations. However in many other flows, and in the complex flows usually found in practical situations, such structures are not so evident although some indications of their possible existence have been found. In this paper an evaluation of the reasons for the existence of these two classes of flows is first given. Attention is then focused upon the more difficult flows, particularly the round turbulent jet, where coherent structures are not so evident, and upon techniques by which the existence (or non-existence) of such structures in these flows, can be established from point measurements, backed up by flow visualisations. A major difficulty is the need to discriminate between real losses in eddy coherence and apparent losses in coherence introduced by ‘phase scrambling’ effects which ‘smear’ multipoint correlations. The analysis of multiprobe time dependent data in cold and reacting round turbulent jets is described and it is shown how evidence of strong eddy coherence can be extracted from data in spite of small values of the classical statistical cross-correlations.