Organized motions in a fully developed turbulent axisymmetric jet

Abstract

An experiment has been conducted to study the occurrence, configuration and dynamics of large-scale coherent vortical motions in the fully developed region of a turbulent axisymmetric jet. The key idea is to use vorticity signals from a spatial grid to detect and sample large-scale vortical structures and then use the (smoothed) vorticity peaks of spatial vorticity patterns to align and ensemble average successive realizations to determine structure configuration and dynamics. Measurements were made in an air jet at ReD = 69000 by employing a radial rake of seven × -wires to obtain the azimuthal vorticity map. Two additional conditioning probes were placed ± 90° away from the rake to determine the three-dimensional phase and hence the structure configuration. Structures with axisymmetric, helical and double helical configurations have been educed. Among them, the helical structures are far more dominant than the others, and the jet dynamics are thus discussed in terms of these helical structures. Helical structures move radially outward as they advect downstream. This radial movement, in conjunction with simultaneous local ejection of turbulent fluid and subsequent entrainment of the ejected fluid with ambient fluid, appears to be a major means of jet spreading. The shear strain rate is strong on the downstream side of the structure, causing intense small-scale turbulence production and mixing there.

Statistical analysis of the velocity field in a mechanical precessing jet flow

“…39-42). The far-field coherent structures are mainly in the helical mode 40 and present almost all the time. 42 The MPJ flow may also be divided into three regions: the near-field, transition, and far-field regions.…”

Wavelet diagnostics for detection of coherent structures in instantaneous turbulent flow imagery: A review

“…It became clear that despite its apparent disorder, a turbulent shear flow may possess considerable order, to a greater or lesser degree depending on the flow. More quantitative studies have used two-point space-time correlations of velocity fluctuations (Tso & Hussain 1989). However, the averaging process inherent in the correlation technique may underestimate the degree of order present, especially as there is considerable jitter in the structures, and both temporal and spatial scales can be highly variable.…”

Development of turbulence in subsonic submerged jets

“…The processes in the jet main part are studied in considerable detail (see, e.g. [85][86][87][88][89][90][91][92][93]). We will consider only the processes in the initial part of a jet.…”