Identification Of Coherent Structures Research Articles

Conditional sampling and state space reconstruction

This work looks into the existing links between conditional sampling techniques for coherent structure identification and methods for state space reconstruction which come up in the field of nonlinear time series analysis. It is shown that the sampling condition used by the VITA method corresponds to a round hyper-cylinder, having as its axis the main diagonal of an embedding space reconstructed with the delay method. Such a geometrical interpretation allows us to better understand certain typical characteristics of the method and to see bursting events as excursions of the system leaving the high-dimension basic turbulence.

Experimental analysis of intermittent coherent structures in the near field of a high Re turbulent jet flow

Experimental data obtained in the near region of a turbulent jet flow (from x/D=0 up to x/D=19, where D is the jet diameter and x is the axial coordinate) are processed through wavelet decomposition. A wavelet based technique for coherent structures identification is applied to the longitudinal and transverse velocity data series obtained by X probe hot wire anemometry measurements. This methodology permits the evolution with x of the time signature of coherent structures to be analyzed. The statistics of the local turbulent energy magnitude is also discussed and the temporal dynamics is studied by the analysis of the time of appearance of the most energetic structures evolving with x/D.

Identification of coherent structures in turbulent shear flows with a fuzzy artmap neural network.

An implementation of a Fuzzy Artmap neural network is used to detect and to identify (recognise) structures (patterns) embedded in the velocity field of a turbulent wake behind a circular cylinder. The net is trained to recognise both clockwise and anticlockwise eddies present in the u and v velocity fields at 420 diameters downstream of the cylinder that generates the wake, using a pre-processed part of the recorded velocity data. The phase relationship that exists between the angles of the velocity vectors of an eddy pattern is used to reduce the number of classes contained in the data, before the start of the training procedure. The net was made stricter by increasing the vigilance parameter within the interval [0.90, 0.95] and a set of net-weights were obtained for each value. Full data files were scanned with the net classifying patterns according to their phase characteristics. The net classifies about 27% of the recorded signals as eddy motions, with the strictest vigilance parameter and without the need to impose external initial templates. Spanwise distances (homogeneous direction of the flow) within the centres of the eddies identified suggest that they form pairs of counter-rotating vortices (double rollers). The number of patterns selected with Fuzzy Artmap is lower than that reported for template matching because the net classifies eddies according to the recirculating pattern present at the core or central region, while template matching extends the region over which correlation between data and template is performed. In both cases, the topology of educed patterns is in agreement.

Coherent structure identification from the analysis of hot-wire data

This paper describes methods for the identification of coherent structures from multipoint measurements made in a three-dimensional wall jet using (a) proper orthogonal decomposition (POD), (b) linear stochastic estimation (LSE), (c) Gram - Charlier estimation (GCE) and (d) wavelet decomposition (WD). The comparison of both POD and LSE show that each captures essentially the same information. The GCE is used to develop a spatially dense mapping of the instantaneous flow. WD is found to offer a more general method for investigating a flow field. These methods are applied and the resulting information analysed to educe structure; it is proposed that the anisotropic growth rate, i.e. faster spreading in the lateral direction in comparison to the transverse direction, found in the wall jet is due to large scale structure.

Wavelet decomposition and coherent structures eduction of low Reλ turbulent hot wire signals

Hot wire signals obtained in grid-turbulence are processed through orthogonal wavelet transform. It is shown that using wavelet decomposition in combination with the form of scaling named Extended Self Similarity, some statistical properties of fully developed turbulence can be extended to very low Reλ flows. Furthermore, based on the wavelet decomposition, a new technique for coherent structures identification is introduced. We present results obtained in grid turbulence data at low and very low Reλ conditions.

The Proper Orthogonal Decomposition, wavelets and modal approaches to the dynamics of coherent structures

We present brief precis of three related investigations. Fuller accounts can be found elsewhere. The investigations bear on the identification and prediction of coherent structures in turbulent shear flows. A second unifying thread is the Proper Orthogonal Decomposition (POD), or Karhunen-Loeve expansion, which appears in all three investigations described. The first investigation demonstrates a close connection between the coherent structures obtained using linear stochastic estimation, and those obtained from the POD. Linear stochastic estimation is often used for the identification of coherent structures. The second investigation explores the use (in homogeneous directions) of wavelets instead of Fourier modes, in the construction of dynamical models; the particular problem considered here is the Kuramoto-Sivashinsky equation. The POD eigenfunctions, of course, reduce to Fourier modes in homogeneous situations, and either can be shown to converge optimally fast; we address the question of how rapidly (by comparison) a wavelet representation converges, and how the wavelet-wavelet interactions can be handled to construct a simple model. The third investigation deals with the prediction of POD eigenfunctions in a turbulent shear flow. We show that energy-method stability theory, combined with an anisotropic eddy viscosity, and erosion of the mean velocity profile by the growing eigenfunctions, produces eigenfunctions very close to those of the POD, and the same eigenvalue spectrum at low wavenumbers.

Detection of temperature ramps and flow structures at a deciduous forest site

Objective methods for the identification of coherent turbulent structures observed in the air layers within and above a forest are described. The techniques are based on the detection of scalar ramps, such as those frequently appearing in temperature traces, and on the detection of ejection/sweep flow structures. Coherent structures are revealed most distinctly by characteristic ramp patterns in scalar traces. A relatively simple multipoint technique to pick up the abrupt decrease of temperature terminating each ramp was successful, as was the variable-interval time-averaging (VITA) method with respect to detection at a single level. Identification of coherent ejection/sweep flow structures was accomplished by examination of the components of the fluctuating velocity, along the major axis of the elliptical joint probability contours of the longitudinal and vertical turbulent velocities. Integration of the absolute value of this rotated velocity over each period encompassing a negative excursion followed by a positive one (approximating an ejection/sweep combination), yielded a detection signal that matched well that from the temperature ramps. Detection of large accelerations in the rotated velocity, indicating rapid transitions from ejection to sweep, was less successful because such velocity changes were not easily distinguished from background turbulence.

Morphological structures produced by mixing in chaotic flows

Fluid mixing is poorly understood. However, some of the main concepts can be captured by both computational and experimental studies of two-dimensional flows. The identification of coherent structures and how they change with variations of the governing parameters gives insight into the behaviour of chaotic systems and can serve as a model for a variety of problems found in nature.