Characteristics Of Coherent Structures Research Articles

Three-dimensional evolution of vortices and early features of coherent structure in the turbulent wake behind a 2-D circular cylinder

3-D evolution of Karman vortex filaments and vortex filaments in braid regions in the turbulent wake of a 2-D circular cylinder is investigated numerically based on inviscid vortex dynamics by analyzing the response of the initially 2-D spanwise vortex filaments to periodic spanwise disturbance of varying magnitude, wavelength and initial phase angles. Our results reveal a kind of 3-D vortex system in the wake which consists of large scale horseshoe-shaped vortices and small scale γ-shaped vortex filaments as well as vortex loops. The mechanism and the dynamic process about the generation of streamwise vortical structure and the 3-D coherent structure are reported.

Near-field flow structures and sideband instabilities of an initially laminar plane jet

The intrinsic characteristics of coherent structures in the near field of a plane jet are extensively studied by hot-wire measurements. The instability modes which are responsible for the dynamics of the coherent structures are found to exhibit distinct evolution characters at different transverse positions of the shear layer along downstream direction. The occurrence of multiple peaks in the energy spectra depicts the formation of the sideband instabilities in the early stage of the jet flow field. These sideband instabilities are investigated to be induced by the mechanisms of the nonlinear interactions between neighboring fundamental and subharmonic instabilities, and the feedback effects of the preferred mode near the end of the potential core. Also, from the spatial distributions of the instability modes over the jet flow field, Ho's subharmonic evolution model (1982) is further examined with more interpretations.

Modeling of individual coherent structures in wall region of a turbulent boundary layer

Models for individual coherent structures in the wall region of a turbulent boundary layer are proposed. Method of numerical simulations is used to follow the evolution of the structures. It is found that the proposed model does bear many features of coherent structures found in experiments.

The characteristics of coherent structures in the near-wall region of turburent channel flow changing with Reynolds number

The characteristics of coherent structures in the near-wall region of turburent open channel flow changing with Reynolds number are investigated using flow visualization. The flow visualization were done in the streamwise view using fluorescent dye illuminated by a sheet of Argon laser light.Some new aspects of the characteristics are inffered from this experiment. The ratio of the inner length to water depth in turbulent open channel flow is governed by Reynolds number. The greatest number of streamwise vortices are formed in the buffer layer. Futher non dimensional vertical and spanwise scale of streamwise vortices are constant changing Reynolds number from 2000 to 7000.

The ‘preferred mode’ of the axisymmetric jet

The ‘preferred mode’ of an incompressible axisymmetric free jet has been organized through controlled perturbation, and spatial distributions of time-average as well as phase-average flow properties in the near field are documented. The excitation produces noticeable changes in the time-average measures of the jet, although these changes are less dramatic than those for the excitation producing stable vortex pairing. For different stages in the evolution of the preferred-mode coherent structure, the phase-average vorticity, coherent Reynolds stress, and incoherent turbulence intensities and Reynolds stress have been educed through phase-locked hot-wire measurements, over the spatial extent of the structure and without invoking the Taylor hypothesis. For a particular stage of the evolution (i.e. when the structure is centred at x/D ≃ 3) the distributions of these quantities have been compared for both initially laminar and fully turbulent exit boundary layers, and for four jet Reynolds numbers. The relative merits of the coherent structure streamline and pseudo-stream-function patterns, as compared with phase-average velocity contours, for structure boundary identification have been discussed. The structure shape and size agree closely with those inferred from the average streamline pattern of the natural structure educed by Yule (1978).These data as well as τ-spectra show that even excitation at the preferred mode cannot sustain the initially organized large-scale coherent structure beyond eight diameters from the jet exit. The background turbulence is organized by the coherent motions in such a way that the maximum rate of decrease of the coherent vorticity occurs at the structure centres which are the saddle points of the background-turbulence Reynolds-stress distributions. The structure centres are also the locations of peak phase-average turbulence intensities. The evolving shape of the structure as it travels downstream helps explain the transverse variations of the wavelength and convection velocity across the mixing layer. The coherent structure characteristics are found to be independent of whether the initial boundary layer is laminar or turbulent, but depend somewhat on the jet Reynolds number. With increasing Reynolds number, the structure decreases in the streamwise length and increases in the radial width and becomes relatively more energetic, and more efficient in the production of coherent Reynolds stress.