J. A. Ferré

Three-dimensional large-eddy motions and fine-scale activity in a plane turbulent wake

A pattern recognition technique has been applied to simultaneously sampled multipoint hot-wire anemometry data obtained in the far wake of a circular cylinder. Data from both the streamwise fluctuating velocity field and the temperature field have been analysed employing a computer code that uses a correlation approach to automatically detect and ensemble average flow patterns and patterns for mean-square fluctuations

Organized motions in a jet in crossflow

An experimental study to identify the structures present in a jet in crossflow has been carried out at a jet-to-crossflow velocity ratio U / U cf = 3.8 and Reynolds number Re = U cf D / v = 6600. The hot-wire velocity data measured with a rake of eight X -wires at x / D = 5 and 15 and flow visualizations using planar laser-induced fluorescence (PLIF) confirm that the well-established pair of counter-rotating vortices is a feature of the mean field and that the upright, tornado-like or Frics vortices that are shed to the leeward side of the jet are connected to the jet flow at the core. The counter-rotating vortex pair is strongly modulated by a coherent velocity field that, in fact, is as important as the mean velocity field. Three different structures – folded vortex rings, horseshoe vortices and handle-type structures – contribute to this coherent field. The new handle-like structures identified in the current study link the boundary layer vorticity with the counter-rotating vortex pair through the upright tornado-like vortices. They are responsible for the modulation and meandering of the counter-rotating vortex pair observed both in video recordings of visualizations and in the instantaneous velocity field. These results corroborate that the genesis of the dominant counter-rotating vortex pair strongly depends on the high pressure gradients that develop in the region near the jet exit, both inside and outside the nozzle.

The Use of Pattern Recognition and Proper Orthogonal Decomposition in Identifying the Structure of Fully-Developed Free Turbulence

The eigenvectors obtained from proper orthogonal decomposition (POD) are used as the selection criteria to classify the individual events contained in data files of two-component velocity signals recorded in the non-homogeneous (vertical) and homogeneous (horizontal) planes of a fully-developed turbulent cylinder wake. This procedure uses the dominant eigenvectors from POD as initial templates to perform a pattern recognition (PR) analysis of the signals so that the individual coherent events appearing randomly in the signals can be educed more objectively. The prototype or ensemble average of the group of events classified as the large-scale structure in the vertical plane has a circulatory motion with strong negative streamwise and outward lateral velocity fluctuations. In the horizontal plane, the average structure is a double roller with negative streamwise velocity fluctuations in its centerplane. The class of instantaneous events selected contribute significantly to the variance in the outer intermittent region, but much less in the fully turbulent core.

Structure and flow patterns in turbulent wakes

A pattern recognition technique is used to detect and identify patterns and structures embedded in the velocity field of a turbulent wake behind a cylinder. Two component velocities u and ν or u and w are measured with eight X‐wire anemometers aligned with the vertical and spanwise directions in the wake at x/D=420 and Re=1600. The velocity patterns educed as averaged patterns or instantaneous structures are consistent with the presence of double‐roller eddies, whose legs are continuously stretched by the rate of strain. Experimental u and ν data suggest that the rollers are connected at the top, with vorticity parallel to the cylinder axis, forming a horseshoe vortex. The fine‐scale activity, estimated by the envelope of the second derivatives, with respect to time of the velocity signals, confirms that this kind of structure can account for both the uν correlation and the continuous entrainment of potential flow into the wake.

Three-dimensional structure and momentum transfer in a turbulent cylinder wake

Simultaneous velocity and temperature measurements were made with rakes of sensors that sliced a slightly heated turbulent wake in the spanwise direction, at different lateral positions 150 diameters downstream of the cylinder. A pattern recognition analysis of hotter-to-colder transitions was performed on temperature data measured at the mean velocity half-width. The velocity data from the different ‘slices’ was then conditionally averaged based on the identified temperature events. This procedure yielded the topology of the average three-dimensional large-scale structure which was visualized with iso-surfaces of negative values of the second eigenvector of [S2+Ω2]. The results indicate that the average structure of the velocity fluctuations (using a triple decomposition of the velocity field) is found to be a shear-aligned ring-shaped vortex. This vortex ring has strong outward lateral velocities in its symmetry plane which are like Grants mixing jets. The mixing jet region extends outside the ring-like vortex and is bounded by two foci separated in the spanwise direction and an upstream saddle point. The two foci correspond to what has been previously identified in the literature as the double rollers.The ring vortex extracts energy from the mean flow by stretching in the mixing jet region just upstream of the ring boundary. The production of the small-scale (incoherent) turbulence by the coherent field and one-component energy dissipation rate occur just downstream of the saddle point within the mixing jet region. Incoherent turbulence energy is extracted from the mean flow just outside the mixing jet region, but within the core of the structure. These processes are highly three-dimensional with a spanwise extent equal to the mean velocity half-width.When a double decomposition is used, the coherent structure is found to be a tube-shaped vortex with a spanwise extent of about 2.5l0. The double roller motions are integral to this vortex in spite of its shape. Spatial averages of the coherent velocity field indicate that the mixing jet region causes a deficit of mean streamwise momentum, while the region outside the foci of the double rollers has a relatively small excess of streamwise momentum.

Analysis of Turbulent Signals

The fundamentals of principal component analysis and pattern recognition techniques are presented and their ability to analyze features of turbulent signals is compared. Both techniques are developed in a general framework, independent of the turbulent flow being investigated. Their performance is evaluated from the analysis of a single synthetic signal as well as of the velocity field measured by multiple probes in a fully developed turbulent wake behind a cylinder.