Large-scale organized structures in the turbulent plane wake of a circular cylinder are investigated in air up to a downstream distance of 40d at a Reynolds number of Red = 1.3 × 104; d is the cylinder diameter. Velocity signals from a linear transverse rake of 8 X-wires are sampled simultaneously to calculate the instantaneous span wise vorticity. We have appropriately smoothed the temporal traces of vorticity to obtain time evolutions (including the transverse displacement, sign, strength and size distributions) of organized structures identified by vorticity contour maps. The periodicity of the initial structures is rapidly lost: dispersion in streamwise spacing, transverse displacement, strength and size of structures increases with increasing downstream distance.Particular emphasis is placed on examining alternative general schemes for educing coherent structures in natural or unexcited turbulent shear flows, especially in their fully developed states. The optimal eduction scheme employed involves centring the rake at the most probable transverse location of centres of advected structures and accepting those structures that: (i) are centred at the midpoint of the rake, (ii) have a peak value of (smoothed) vorticity of a given sign above a specified level, and (iii) have streamwise and transverse extents of the (smoothed) vorticity contours above a specified size. From successive accepted structure signatures the instants of occurrence of structure centres (i.e. smoothed vorticity peaks) are identified. Un-smoothed signals are then time-aligned with respect to these instants and ensemble averaged to educe coherent structure and incoherent turbulence characteristics. Further enhancement is achieved by iteratively improving the time-alignment by maximizing the cross-correlation of individual structure vorticity with the ensemble-averaged vorticity and by discarding structures that require excessive time shifts or that produce significantly weak peak correlation values.Following this optimal scheme, large-scale coherent structures have been educed in the fully turbulent wake. The average structure centre is found to be closer to the wake centreline than the half-width location, and the structure size does not increase in proportion to the wake width, suggesting that transverse wandering of structures (including their three-dimensionality) increases significantly with increasing downstream distance. The various flow properties associated with coherent and incoherent turbulence, and the coherent structure dynamics, in particular the role of vortex stretching (at the saddle) in turbulence production and mixing, are discussed.

This paper describes a quantitative study of the three-dimensional nature of organized motions in a turbulent plane wake. Coherent structures are detected from the instantaneous, spatially phase-correlated vorticity field using certain criteria based on size, strength and geometry of vortical structures. With several combinations of X-wire rakes, vorticity distributions in the spanwise and transverse planes are measured in the intermediate region (10d [les ] x [les ] 40d) of the plane turbulent wake of a circular cylinder at a Reynolds number of 13000 based on the cylinder diameter d. Spatial correlations of smoothed vorticity signals as well as phase-aligned ensemble-averaged vorticity maps over structure cross-sections yield a quantitative measure of the spatial coherence and geometry of organized structures in the fully turbulent field. The data demonstrate that the organized structures in the nominally two-dimensional wake exhibit significant three-dimensionality even in the near field. Using instantaneous velocity and vorticity maps as well as correlations of vorticity distributions in different planes, some topological features of the dominant coherent structures in a plane wake are inferred.

The formation mechanism of streamwise vortices in the near field of the three-dimensional wall jet discharging from a circular nozzle along a flat plate is studied experimentally using a conditional sampling technique. Ensemble-averages of the lateral velocity component indicate the presence of large-scale horseshoe-like structures, whose legs are inclined and stretched to form the streamwise vortices in the mixing region of the jet. Based on the present result, a coherent structure model for the near field of the wall jet is proposed.

Conventional hot-wire measurements have been made in the turbulent developing wake of a thin flat plate. The data obtained are primarily concerned with turbulence characteristics in the near-wake region. Turbulence properties in the inner wake first increase up to a streamwise distance of 200–300 times the initial wall-layer length scale, followed by a gradual decay farther downstream. The mechanism of this overshoot phenomenon is inferred from space-time correlation data. Quasiperiodic motion similar to that in the far wake is observed as early as near the end of the near wake.