Quantitative three-dimensional imaging and the structure of passive scalar fields in fully turbulent flows

1990 ◽  
Vol 216 ◽  
pp. 1-34 ◽  
Author(s):  
Rahul R. Prasad ◽  
K. R. Sreenivasan
Keyword(s):  
Turbulent Flows ◽  
Three Dimensional ◽  
Scalar Fields ◽  
Passive Scalar ◽  
Working Fluid ◽  
Small Scale ◽  
Main Body ◽  
Two Dimensional ◽  
Scaling Properties ◽  
Two Dimensional Images

The three-dimensional turbulent field of a passive scalar has been mapped quantitatively by obtaining, effectively instantaneously, several closely spaced parallel two-dimensional images; the two-dimensional images themselves have been obtained by laser-induced fluorescence. Turbulent jets and wakes at moderate Reynolds numbers are used as examples. The working fluid is water. The spatial resolution of the measurements is about four Kolmogorov scales. The first contribution of this work concerns the three-dimensional nature of the boundary of the scalar-marked regions (the ‘scalar interface’). It is concluded that interface regions detached from the main body are exceptional occurrences (if at all), and that in spite of the large structure, the randomness associated with small-scale convolutions of the interface are strong enough that any two intersections of it by parallel planes are essentially uncorrelated even if the separation distances are no more than a few Kolmogorov scales. The fractal dimension of the interface is determined directly by box-counting in three dimensions, and the value of 2.35 ± 0.04 is shown to be in good agreement with that previously inferred from two-dimensional sections. This justifies the use of the method of intersections. The second contribution involves the joint statistics of the scalar field and the quantity χ* (or its components), χ* being the appropriate approximation to the scalar ‘dissipation’ field in the inertial–convective range of scales. The third aspect relates to the multifractal scaling properties of the spatial intermittency of χ*; since all three components of χ* have been obtained effectively simultaneously, inferences concerning the scaling properties of the individual components and their sum have been possible. The usefulness of the multifractal approach for describing highly intermittent distributions of χ* and its components is explored by measuring the so-called singularity spectrum (or the f(α)-curve) which quantifies the spatial distribution of various strengths of χ*. Also obtained is a time sequence of two-dimensional images with the temporal resolution on the order of a few Batchelor timescales; this enables us to infer features of temporal intermittency in turbulent flows, and qualitatively the propagation speeds of the scalar interface. Finally, a few issues relating to the resolution effects have been addressed briefly by making point measurements with the spatial and temporal resolutions comparable with the Batchelor lengthscale and the corresponding timescale.

Computing Blunt Body Flows on Coarse Grids Using Vorticity Confinement

2002 ◽  
Vol 124 (4) ◽  
pp. 876-885 ◽  
Author(s):  
M. Fan ◽  
Y. Wenren ◽  
W. Dietz ◽  
M. Xiao ◽  
J. Steinhoff
Keyword(s):  
Boundary Layers ◽  
Three Dimensional ◽  
Scalar Fields ◽  
Passive Scalar ◽  
Navier Stokes ◽  
Small Scale ◽  
Eddy Simulation ◽  
Discrete Equations ◽  
Vorticity Confinement ◽  
Helicopter Landing

Over the last few years, a new flow computational methodology, vorticity confinement, has been shown to be very effective in treating concentrated vortical regions. These include thin vortex filaments which can be numerically convected over arbitrary distances on coarse Eulerian grids, while requiring only ∼2 grid cells across their cross section. They also include boundary layers on surfaces “immersed” in nonconforming uniform Cartesian grids, with no requirement for grid refinement or complex logic near the surface. In this paper we use vorticity confinement to treat flow over blunt bodies, including attached and separating boundary layers, and resulting turbulent wakes. In the wake it serves as a new, simple effective large-eddy simulation (LES). The same basic idea is applied to all of these features: At the smallest scales (∼2 cells) the vortical structures are captured and treated, effectively, as solitary waves that are solutions of nonlinear discrete equations on the grid. The method does not attempt to accurately discretize the Euler/Navier-Stokes partial differential equations (pde’s) for these small scales, but, rather, serves as an implicit, nonlinear model of the structures, directly on the grid. The method also allows the boundary layer to be effectively “captured.” In the turbulent wake, where there are many scales, small structures represent an effective small scale energy sink. However, they do not have the unphysical spreading due to numerical diffusion at these scales, which is present in conventional computational methods. The basic modeling idea is similar to that used in shock capturing, where intrinsically discrete equations are satisfied in thin, modeled regions. It is argued that, for realistic high Reynolds number flows, this direct, grid-based modeling approach is much more effective than first formulating model pde’s for the small scale, turbulent vortical regions and then discretizing them. Results are presented for three-dimensional flows over round and square cylinders and a realistic helicopter landing ship. Comparisons with experimental data are given. Finally, a new simpler formulation of vorticity confinement is given together with a related formulation for confinement of passive scalar fields.

scholarly journals Visual Depth Perception of three dimensional images and two dimensional images

1990 ◽  
Vol 26 (Supplement) ◽  
pp. 248-249
Author(s):  
Am CHO ◽  
Kageyu NORO ◽  
Shinya KOSHIE ◽  
Atsuko HONDO ◽  
Sakae YAMAMOTO
Keyword(s):  
Depth Perception ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Visual Depth ◽  
Three Dimensional Images ◽  
Two Dimensional Images

SBART 2.4: An IEC Tool for Creating Two-Dimensional Images, Movies and Collages

Leonardo ◽  
2002 ◽  
Vol 35 (2) ◽  
pp. 189-191 ◽  
Author(s):  
Tatsuo Unemi
Keyword(s):  
User Interface ◽  
Three Dimensional ◽  
Interactive System ◽  
Small Computer ◽  
Two Dimensional ◽  
The Creation ◽  
Color Value ◽  
Two Dimensional Images

In this article, the author gives an overview of SBART 2.4, an interactive system used to create abstract two-dimensional images, collages and movies. The system, one of the successors of Karl Sims's system, runs on a small computer that uses a function to calculate the color value of each pixel as a genotype. All of the ranges and domains are three-dimensional vectors. The system utilizes a multi-field user interface to enhance the diversity of production and has optional facilities that allow the creation of collages of external images or short movies.

The anatomy of the mixing transition in homogeneous and stratified free shear layers

2000 ◽  
Vol 413 ◽  
pp. 1-47 ◽  
Author(s):  
C. P. CAULFIELD ◽  
W. R. PELTIER
Keyword(s):  
Shear Layer ◽  
Three Dimensional ◽  
Streamwise Vortex ◽  
Shear Layers ◽  
Finite Amplitude ◽  
Small Scale ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Free Shear Layers ◽  
Nonlinear Amplification

We investigate the detailed nature of the ‘mixing transition’ through which turbulence may develop in both homogeneous and stratified free shear layers. Our focus is upon the fundamental role in transition, and in particular the associated ‘mixing’ (i.e. small-scale motions which lead to an irreversible increase in the total potential energy of the flow) that is played by streamwise vortex streaks, which develop once the primary and typically two-dimensional Kelvin–Helmholtz (KH) billow saturates at finite amplitude.Saturated KH billows are susceptible to a family of three-dimensional secondary instabilities. In homogeneous fluid, secondary stability analyses predict that the stream-wise vortex streaks originate through a ‘hyperbolic’ instability that is localized in the vorticity braids that develop between billow cores. In sufficiently strongly stratified fluid, the secondary instability mechanism is fundamentally different, and is associated with convective destabilization of the statically unstable sublayers that are created as the KH billows roll up.We test the validity of these theoretical predictions by performing a sequence of three-dimensional direct numerical simulations of shear layer evolution, with the flow Reynolds number (defined on the basis of shear layer half-depth and half the velocity difference) Re = 750, the Prandtl number of the fluid Pr = 1, and the minimum gradient Richardson number Ri(0) varying between 0 and 0.1. These simulations quantitatively verify the predictions of our stability analysis, both as to the spanwise wavelength and the spatial localization of the streamwise vortex streaks. We track the nonlinear amplification of these secondary coherent structures, and investigate the nature of the process which actually triggers mixing. Both in stratified and unstratified shear layers, the subsequent nonlinear amplification of the initially localized streamwise vortex streaks is driven by the vertical shear in the evolving mean flow. The two-dimensional flow associated with the primary KH billow plays an essentially catalytic role. Vortex stretching causes the streamwise vortices to extend beyond their initially localized regions, and leads eventually to a streamwise-aligned collision between the streamwise vortices that are initially associated with adjacent cores.It is through this collision of neighbouring streamwise vortex streaks that a final and violent finite-amplitude subcritical transition occurs in both stratified and unstratified shear layers, which drives the mixing process. In a stratified flow with appropriate initial characteristics, the irreversible small-scale mixing of the density which is triggered by this transition leads to the development of a third layer within the flow of relatively well-mixed fluid that is of an intermediate density, bounded by narrow regions of strong density gradient.

scholarly journals Making Anaglyphs in Photoshop

2005 ◽  
Vol 13 (3) ◽  
pp. 36-39 ◽  
Author(s):  
Jerry Sedgewick
Keyword(s):  
Imaging System ◽  
Three Dimensional ◽  
Ct Scans ◽  
Two Dimensional ◽  
The Right ◽  
Two Dimensional Images

In order to achieve a three dimensional appearance to a pair of two dimensional images, two off-axis images can be produced and colorized. These can be overlayed slightly apart and then viewed through glasses with two differently colored sides, one color for the left eye and another for the right eye in combinations containing red, green or blue colors. These off-axis and colorized images are referred to as anaglyphs.Off-axis images can be achieved through the use of a tilting stage on a microscope, by physically changing the position of a camera in relation to a still object, or through changing the axis of an optical stack of sections, such as what is created by confocal/CT scans. Some images lend themselves more to a 3D look both by virtue of inherent three dimensionality limited by the resolution of the imaging system.

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