scholarly journals Organised large structure in the post-transition mixing layer. Part 2. Large-eddy simulation

2014 ◽  
Vol 762 ◽  
pp. 302-343 ◽  
Author(s):  
W. A. McMullan ◽  
S. Gao ◽  
C. M. Coats
Keyword(s):  
Linear Growth ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Continuous Linear ◽  
Large Structure ◽  
Transition Flow ◽  
Large Structures ◽  
Large Eddy ◽  
Self Similar ◽  
The Individual

AbstractThree-dimensional large-eddy simulations of two-stream mixing layers developing spatially from laminar boundary layers are presented, replicating wind-tunnel experiments carried out in Part 1 of this study. These simulations have been continued through the mixing transition and into the fully turbulent self-similar flow beyond. In agreement with the experiments, the simulations show that the familiar mechanism of growth by vortex amalgamation is replaced at the mixing transition by a previously unrecognised mechanism in which the spanwise-coherent large structures individually undergo continuous linear growth. In the post-transition flow it is this continuous linear growth of the individual structures that produces the self-similar growth of the mixing-layer thickness, the large-structure interactions occurring as a consequence of the growth, not its cause. New information is also presented on the topography of the organised post-transition flow and on its cyclical evolution through the lifetimes of the individual large structures. The dynamic and kinematic implications of these findings are discussed and shown to define quantitatively the growth rate of the homogeneous post-transition mixing layer in its organised state.

Organized large structure in the post-transition mixing layer. Part 1. Experimental evidence

2013 ◽  
Vol 737 ◽  
pp. 466-498 ◽  
Author(s):  
A. D’Ovidio ◽  
C. M. Coats
Keyword(s):  
Large Scale ◽  
Linear Growth ◽  
Initial Conditions ◽  
Mixing Layer ◽  
Mixing Layers ◽  
Continuous Linear ◽  
Transition Flow ◽  
Large Structures ◽  
The Individual ◽  
Density Ratios

AbstractNew flow-visualization experiments on mixing layers of various velocity and density ratios are reported. It is shown that, in mixing layers developing from laminar initial conditions, the familiar mechanism of growth by vortex amalgamation is replaced at the mixing transition by a previously unrecognized mechanism in which the spanwise-coherent large structures individually undergo continuous linear growth. In the organized post-transition flow it is this continuous linear growth of the individual structures that produces the self-similar growth of the mixing-layer thickness, with the occasional interactions between neighbouring structures occurring as a consequence of their growth, not its cause. It is also observed that periods during which the post-transition mixing layer comprises orderly processions of large structures alternate with periods during which no large-scale organization is apparent downstream of the transition location. These two fully turbulent flow states are characterized by different growth rates, entrainment ratios and orientations of the mixing layer relative to the free streams. The implications of these findings are discussed.

Effect of free-stream turbulence on large structure in turbulent mixing layers

1978 ◽  
Vol 85 (4) ◽  
pp. 693-704 ◽  
Author(s):  
C. Chandrsuda ◽  
R. D. Mehta ◽  
A. D. Weir ◽  
P. Bradshaw
Keyword(s):  
Turbulent Mixing ◽  
Bluff Body ◽  
Free Stream ◽  
Early Stage ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Large Structure ◽  
Free Stream Turbulence ◽  
Instability Modes ◽  
Slow Development

Flow-visualization investigations and correlation measurements show that the essentially two-dimensional structures which dominated the turbulent mixing layer of Brown & Roshko (1974) are formed only if the free-stream turbulence is low. If free-stream disturbances are significant, as is likely in most practical cases, including a mixing layer entraining ‘still air’ from the surroundings, three-dimensionality develops at an early stage in transition. Other recent experiments strongly suggest that the Brown-Roshko structure will not form if the initial mixing layer is turbulent or subject to instability modes other than spanwise vortices. Therefore the Brown-Roshko structure will be rare in practice. The alternative large structure in a mixing layer, found by several workers, is intense, but fully three-dimensional and thus less orderly than the Brown-Roshko structure.The balance of evidence suggests that if the Brown-Roshko structure does appear it will eventually relax into the alternative fully three-dimensional form: the Kármán vortex street behind a bluff body provides a precedent for slow development of three-dimensionality. However the Brown-Roshko structure, if formed, may well relax so slowly as to be identifiable for the full length of a practical flow.

scholarly journals Large-eddy simulation of the turbulent mixing layer

1997 ◽  
Vol 339 ◽  
pp. 357-390 ◽  
Author(s):  
BERT VREMAN ◽  
BERNARD GEURTS ◽  
HANS KUERTEN
Keyword(s):  
Reynolds Number ◽  
Eddy Viscosity ◽  
Dynamic Models ◽  
Mixing Layer ◽  
Viscosity Model ◽  
Computational Domain ◽  
Eddy Viscosity Model ◽  
Moderate Reynolds Number ◽  
Large Eddy ◽  
Self Similar

Six subgrid models for the turbulent stress tensor are tested by conducting large-eddy simulations (LES) of the weakly compressible temporal mixing layer: the Smagorinsky, similarity, gradient, dynamic eddy-viscosity, dynamic mixed and dynamic Clark models. The last three models are variations of the first three models using the dynamic approach. Two sets of simulations are performed in order to assess the quality of the six models. The LES results corresponding to the first set are compared with filtered results obtained from a direct numerical simulation (DNS). It appears that the dynamic models lead to more accurate results than the non-dynamic models tested. An adequate mechanism to dissipate energy from resolved to subgrid scales is essential. The dynamic models have this property, but the Smagorinsky model is too dissipative during transition, whereas the similarity and gradient models are not sufficiently dissipative for the smallest resolved scales. In this set of simulations, at moderate Reynolds number, the dynamic mixed and Clark models are found to be slightly more accurate than the dynamic eddy-viscosity model. The second set of LES concerns the mixing layer at a considerably higher Reynolds number and in a larger computational domain. An accurate DNS for this mixing layer can currently not be performed, thus in this case the LES are tested by investigating whether they resemble a self-similar turbulent flow. It is found that the dynamic models generate better results than the non-dynamic models. The closest approximation to a self-similar state was obtained using the dynamic eddy-viscosity model.

Reconstructing a lost world: Ediacaran rangeomorphs from Spaniard's Bay, Newfoundland

2009 ◽  
Vol 83 (4) ◽  
pp. 503-523 ◽  
Author(s):  
Guy M. Narbonne ◽  
Marc Laflamme ◽  
Carolyn Greentree ◽  
Peter Trusler
Keyword(s):  
Three Dimensional ◽  
Higher Order ◽  
Morphological Features ◽  
Attachment Point ◽  
Architectural Elements ◽  
Order Branch ◽  
Central Stalk ◽  
Self Similar ◽  
The Individual ◽  
Avalon Peninsula

Ediacaran fronds at Spaniard's Bay on the Avalon Peninsula of Newfoundland exhibit exquisite, three-dimensional preservation with morphological features less than 0.05 mm in width visible on the best preserved specimens. Most of the nearly 100 specimens are juvenile rangeomorphs, an extinct Ediacaran clade that numerically dominated the early evolution of complex multicellular life. Spaniard's Bay rangeomorphs are characterized by cm-scale architectural elements exhibiting self-similar branching over several fractal scales that were used as modules in construction of larger structures. Four taxa of rangeomorph fronds are present – Avalofractus abaculus n. gen. et sp., Beothukis mistakensis Brasier and Antcliffe, Trepassia wardae (Narbonne and Gehling), and Charnia cf. C. masoni Ford. All of these taxa exhibit an alternate array of primary rangeomorph branches that pass off a central stalk or furrow that marks the midline of the petalodium. Avalofractus is remarkably self similar over at least four fractal scales, with each scale represented by double-sided rangeomorph elements that were constrained only at their attachment point with the higher-order branch and thus were free to rotate and pivot relative to other branches. Beothukis is similar in organization, but its primary branches show only one side of a typical rangeomorph element, probably due to longitudinal branch folding, and the position of the individual branches was moderately constrained. Trepassia shows only single-sided branches with both primary and secondary branches emanating from a central stalk or furrow; primary branches were capable of minor pivoting as reflected in bundles of secondary branches. Charnia shows only single-sided primary branches that branch from a zigzag central furrow and that were firmly constrained relative to each. This sequence provides a developmental linkage between Rangea-type and Charnia-type rangeomorphs. Avalonian assemblages show a wide array of rangeomorph constructions, but later Ediacaran assemblages contain a lower diversity of rangeomorphs represented mainly by well-constrained forms.

scholarly journals On density effects and large structure in turbulent mixing layers

1974 ◽  
Vol 64 (4) ◽  
pp. 775-816 ◽  
Author(s):  
Garry L. Brown ◽  
Anatol Roshko
Keyword(s):  
Turbulent Mixing ◽  
High Speed ◽  
Density Ratio ◽  
Mixing Layer ◽  
Density Fluctuations ◽  
Large Structure ◽  
Density Effects ◽  
Large Structures ◽  
Different Gases ◽  
Compressibility Effects

Plane turbulent mixing between two streams of different gases (especially nitrogen and helium) was studied in a novel apparatus. Spark shadow pictures showed that, for all ratios of densities in the two streams, the mixing layer is dominated by large coherent structures. High-speed movies showed that these convect at nearly constant speed, and increase their size and spacing discontinuously by amalgamation with neighbouring ones. The pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainment rates. Large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle; it is concluded that the strong effects, which are observed when one stream is supersonic, are due to compressibility effects, not density effects, as has been generally supposed.

Improved Navier–Stokes Boundary Conditions Based on Generalized Characteristics

2015 ◽  
Vol 23 (02) ◽  
pp. 1550006
Author(s):  
Ramin Kaviani ◽  
Mansour Nikkhah-Bahrami
Keyword(s):  
Boundary Conditions ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Curvilinear Coordinates ◽  
Navier Stokes ◽  
Test Problems ◽  
Characteristic Boundary ◽  
Large Eddy ◽  
Compressible Viscous Flows ◽  
New Formulation

In this paper, the three-dimensional Navier–Stokes characteristic boundary conditions for large-eddy and aeroacoustic simulations are extended to curvilinear coordinates formulations. A robust way of treating the transverse and gradient terms on boundary planes is presented which is different from previous generalized characteristic boundary conditions. The performance of the new formulation is examined via four test problems: an inviscid convective vortex, a two-dimensional mixing layer, a Mach 0.75 round jet, and a Mach 0.51 nozzle/jet. For each test problem, the numerical schemes used to implement the boundary conditions, the numerical parameters employed, and the predicted three-dimensional flow fields are presented. Based on the numerical experiments conducted, the new boundary conditions show promise for high-fidelity simulations of compressible viscous flows.

Semi-automated methods for 3-D reconstruction of macromolecular complexes

1995 ◽  
Vol 53 ◽  
pp. 42-43
Author(s):  
B. Carragher ◽  
M. Whittaker
Keyword(s):  
Three Dimensional ◽  
Time Saving ◽  
Macromolecular Complexes ◽  
Symmetry Properties ◽  
Dimensional Reconstruction ◽  
Experienced Operator ◽  
Projection Images ◽  
The Individual ◽  
Natural Symmetry

Techniques for three-dimensional reconstruction of macromolecular complexes from electron micrographs have been successfully used for many years. These include methods which take advantage of the natural symmetry properties of the structure (for example helical or icosahedral) as well as those that use single axis or other tilting geometries to reconstruct from a set of projection images. These techniques have traditionally relied on a very experienced operator to manually perform the often numerous and time consuming steps required to obtain the final reconstruction. While the guidance and oversight of an experienced and critical operator will always be an essential component of these techniques, recent advances in computer technology, microprocessor controlled microscopes and the availability of high quality CCD cameras have provided the means to automate many of the individual steps.During the acquisition of data automation provides benefits not only in terms of convenience and time saving but also in circumstances where manual procedures limit the quality of the final reconstruction.

Phonetic iconicity in the evaluative morphology of a sample of Indo-European, Niger-Congo and Austronesian languages

2010 ◽  
Vol 3 (2) ◽  
pp. 156-180 ◽  
Author(s):  
Renáta Gregová ◽  
Lívia Körtvélyessy ◽  
Július Zimmermann
Keyword(s):  
Three Dimensional ◽  
Sound Symbolism ◽  
Three Dimensional Model ◽  
Austronesian Languages ◽  
Place Of Articulation ◽  
European Languages ◽  
Phonetic Symbolism ◽  
The Individual ◽  
Core Vocabulary

Universals Archive (Universal #1926) indicates a universal tendency for sound symbolism in reference to the expression of diminutives and augmentatives. The research ( Štekauer et al. 2009 ) carried out on European languages has not proved the tendency at all. Therefore, our research was extended to cover three language families – Indo-European, Niger-Congo and Austronesian. A three-step analysis examining different aspects of phonetic symbolism was carried out on a core vocabulary of 35 lexical items. A research sample was selected out of 60 languages. The evaluative markers were analyzed according to both phonetic classification of vowels and consonants and Ultan's and Niewenhuis' conclusions on the dominance of palatal and post-alveolar consonants in diminutive markers. Finally, the data obtained in our sample languages was evaluated by means of a three-dimensional model illustrating the place of articulation of the individual segments.

Proteins and direct methods

1991 ◽  
Vol 197 (3-4) ◽  
Author(s):  
Fan Hai-fu ◽  
Hao Quan ◽  
M. M. Woolfson
Keyword(s):  
First Principles ◽  
Figure Of Merit ◽  
Direct Methods ◽  
Large Structure ◽  
Low Resolution ◽  
Figures Of Merit ◽  
Large Structures ◽  
Resolution Data

AbstractConventional direct methods, which work so well for small structures, are less successful for macromolecules. Where it has been demonstrated that a solution might be found using direct methods it is then found that the usual figures of merit are unable to distinguish the few good sets of phases from the large number of sets generated. The reasons for the difficulties with very large structures are considered from a first-principles approach taking into account both the factors of having a large number of atoms and low resolution data. A proposal is made for trying to recognize good phase sets by taking a large structure as a sum of a number of smaller structures for each of which a conventional figure of merit can be applied.

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