scholarly journals Large-scale instability of a generalized turbulent Kolmogorov flow

2009 ◽  
Vol 16 (4) ◽  
pp. 569-577 ◽  
Author(s):  
B. Legras ◽  
B. Villone
Keyword(s):  
Turbulent Flows ◽  
Mirror Symmetry ◽  
Large Scale ◽  
Necessary Condition ◽  
Scale Separation ◽  
Kolmogorov Flow ◽  
Nonlinear Contribution ◽  
Basic Scale ◽  
Negative Viscosity ◽  
Molecular Viscosity

Abstract. We present an analytical study of the large scale instability of a generalized turbulent Kolmogorov flow, i.e. a periodic shear flow where the molecular viscosity has been substituted by an eddy viscosity parameterized with the Clark-Smagorinsky model and where the external forcing is adapted to maintain the flow against this dissipation. We employ multiscaling technique assuming a scale separation between the basic scale of such a generalized turbulent Kolmogorov flow and the largest scales of the flow. The main result is that an amplitude equation for the large-scale secondary flow is obtained which exhibits, like for the standard Kolmogorov flow, an instability of the negative viscosity type. We find that the presence of mirror symmetry in the basic flow is a necessary condition and that further propagative and nonlinear contribution are produced otherwise. The result is encouraging for the generic existence of large-scale instabilities of the negative viscosity type in fully turbulent flows.

scholarly journals Dissipation-scale fluctuations and mixing transition in turbulent flows

2008 ◽  
Vol 606 ◽  
pp. 325-337 ◽  
Author(s):  
VICTOR YAKHOT
Keyword(s):  
Probability Density ◽  
Turbulent Flows ◽  
Large Scale ◽  
Molecular Diffusion ◽  
Numerical Data ◽  
Reaction Rates ◽  
Diffusion Time ◽  
Necessary Condition ◽  
Scalar Dissipation ◽  
Dissipation Scale

A small separation between reactants, not exceeding 10−8 − 10−7 cm, is the necessary condition for various chemical reactions. It is shown that random advection and stretching by turbulence leads to the formation of scalar-enriched sheets of strongly fluctuating thickness ηc. The molecular-level mixing is achieved by diffusion across these sheets (interfaces) separating the reactigants. Since the diffusion time scale is $\tau_{d}\,{\propto}\,\eta_{c}^{2}$, knowledge of the probability density Q(ηc, Re) is crucial for evaluation of mixing times and chemical reaction rates. According to Kolmogorov–Batchelor phenomenology, the stretching time τeddy ≈ L/urms = O(1) is independent of large-scale Reynolds number Re = urmsL/ν and the diffusion time $\tau_{d}\,{\approx}\,\tau_{\it eddy}/\sqrt{{\it Re}}\,{\ll}\, \tau_{\it eddy}$ is very small. Therefore, in previous studies, molecular diffusion was frequently neglected as being too fast to contribute substantially to the reaction rates. In this paper, taking into account strong intermittent fluctuations of the scalar dissipation scales, this conclusion is re-examined. We derive the probability density Q(ηc, Re, Sc), calculate the mean scalar dissipation scale and predict transition in the reaction rate behaviour from ${\cal R}\,{\propto}\,\sqrt{Re}$ ($Re\,{\leq}\, 10^{3}-10^{4})$ to the high-Re asymptotics ${\cal R}\,{\propto}\, {\it Re}^{0}$. These conclusions are compared with known experimental and numerical data.

scholarly journals Potential-vorticity inversion and the wave-turbulence jigsaw: some recent clarifications

2008 ◽  
Vol 15 ◽  
pp. 47-56 ◽  
Author(s):  
M. E. McIntyre
Keyword(s):  
Potential Vorticity ◽  
Large Scale ◽  
Ocean Dynamics ◽  
Turbulence Theory ◽  
Wave Turbulence ◽  
Scale Separation ◽  
Potential Vorticity Inversion ◽  
Rossby Wave Propagation ◽  
Negative Viscosity ◽  
The Individual

Abstract. Two key ideas stand out as crucial to understanding atmosphere-ocean dynamics, and the dynamics of other planets including the gas giants. The first key idea is the invertibility principle for potential vorticity (PV). Without it, one can hardly give a coherent account of even so important and elementary a process as Rossby-wave propagation, going beyond the simplest textbook cases. Still less can one fully understand nonlinear processes like the self-sharpening or narrowing of jets – the once-mysterious "negative viscosity" phenomenon. The second key idea, also crucial to understanding jets, might be summarized in the phrase "there is no such thing as turbulence without waves", meaning Rossby waves especially. Without this idea one cannot begin to make sense of, for instance, momentum budgets and eddy momentum transports in complex large-scale flows. Like the invertibility principle the idea has long been recognized, or at least adumbrated. However, it is worth articulating explicitly if only because it can be forgotten when, in the usual way, we speak of "turbulence" and "turbulence theory" as if they were autonomous concepts. In many cases of interest, such as the well-studied terrestrial stratosphere, reality is more accurately described as a highly inhomogeneous "wave-turbulence jigsaw puzzle" in which wavelike and turbulent regions fit together and crucially affect each other's evolution. This modifies, for instance, formulae for the Rhines scale interpreted as indicating the comparable importance of wavelike and turbulent dynamics. Also, weakly inhomogeneous turbulence theory is altogether inapplicable. For instance there is no scale separation. Eddy scales are not much smaller than the sizes of the individual turbulent regions in the jigsaw. Here I review some recent progress in clarifying these ideas and their implications.

scholarly journals Fluctuation of Mass Flux in a Cloud-Resolving Simulation with Interactive Radiation

2010 ◽  
Vol 67 (2) ◽  
pp. 400-418 ◽  
Author(s):  
J. Davoudi ◽  
N. A. McFarlane ◽  
T. Birner
Keyword(s):  
Mass Flux ◽  
Time Scale ◽  
Large Scale ◽  
Canonical Ensemble ◽  
Cloud Base ◽  
Necessary Condition ◽  
Quasi Equilibrium ◽  
Tropical Ocean ◽  
Scale Separation ◽  
Time Scale Separation

Abstract It was shown by Craig and Cohen that fluctuations of cumulus clouds under homogeneous large-scale forcing satisfy the Gibbs canonical ensemble in a strict radiative–convective equilibrium (RCE). In the limit of random noninteracting convective cells, an analytical expression for the distribution function of total mass flux over a region of given size was derived. The authors examine the consistency of the Gibbs canonical ensemble as a representation for the mass flux fluctuations when the large-scale forcing is time dependent. A cloud-resolving simulation (CRM) with interactive radiation, fixed imposed surface temperature, and diurnally varying solar forcing to mimic the diurnal cycle over the tropical ocean is used. As a necessary condition for the existence of a state of quasi-equilibrium, the time-scale separation between convective processes and forcing is studied. Detailed evaluation of time scales of convective adjustment and memory in a three-month run confirms the hypothesis of time-scale separation. The Craig and Cohen theory, in a varying range of heights between the cloud base up to the level of neutral buoyancy (LNB), is tested. It is shown that, although the theory is capable of reproducing the qualitative features of the variability, systematic deviations are detected. By quantifying the spatial distribution of the clouds, the authors suggest that deviations are associated with clustering effects.

Effects of Squish Flow on Tangential Flow and Turbulence in a Diesel Engine

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Yanzhe Sun ◽  
Kai Sun ◽  
Tianyou Wang ◽  
Yufeng Li ◽  
Zhen Lu
Keyword(s):  
Diesel Engine ◽  
Turbulent Flows ◽  
Large Scale ◽  
Radial Flow ◽  
Tangential Component ◽  
Tangential Flow ◽  
Image Velocimetry ◽  
Turbulence Production ◽  
Large Scale Flow ◽  
Main Effects

Emission and fuel consumption in swirl-supported diesel engines strongly depend on the in-cylinder turbulent flows. But the physical effects of squish flow on the tangential flow and turbulence production are still far from well understood. To identify the effects of squish flow, Particle image velocimetry (PIV) experiments are performed in a motored optical diesel engine equipped with different bowls. By comparing and associating the large-scale flow and turbulent kinetic energy (k), the main effects of the squish flow are clarified. The effect of squish flow on the turbulence production in the r−θ plane lies in the axial-asymmetry of the annular distribution of radial flow and the deviation between the ensemble-averaged swirl field and rigid body swirl field. Larger squish flow could promote the swirl center to move to the cylinder axis and reduce the deformation of swirl center, which could decrease the axial-asymmetry of annular distribution of radial flow, further, that results in a lower turbulence production of the shear stress. Moreover, larger squish flow increases the radial fluctuation velocity which makes a similar contribution to k with the tangential component. The understanding of the squish flow and its correlations with tangential flow and turbulence obtained in this study is beneficial to design and optimize the in-cylinder turbulent flow.

Large-scale separation of alkaloids from Corydalis decumbens by pH-zone-refining counter-current chromatography

2006 ◽  
Vol 1115 (1-2) ◽  
pp. 267-270 ◽  
Author(s):  
Xiao Wang ◽  
Yanling Geng ◽  
Fuwei Li ◽  
Xingang Shi ◽  
Jianhua Liu
Keyword(s):  
Large Scale ◽  
Zone Refining ◽  
Scale Separation ◽  
Counter Current

scholarly journals Structural dissimilarity of large-scale structures in turbulent flows over wavy walls

2012 ◽  
Vol 24 (5) ◽  
pp. 055112 ◽  
Author(s):  
Adrian Zenklusen ◽  
Simon Kuhn ◽  
Philipp Rudolf von Rohr
Keyword(s):  
Turbulent Flows ◽  
Large Scale ◽  
Large Scale Structures ◽  
Scale Structures

SPECIAL REVIEWS

PEDIATRICS ◽  
1948 ◽  
Vol 2 (4) ◽  
pp. 489-497
Author(s):  
CHARLES A. JANEWAY
Keyword(s):  
Plasma Proteins ◽  
Chemical Structure ◽  
Large Scale ◽  
Blood Donors ◽  
Red Cross ◽  
American Red Cross ◽  
Scale Separation ◽  
Medical Progress ◽  
Scientific Curiosity ◽  
Free People

This brief review of some of the recent accessions to our knowledge of the chemical structure, physiologic functions, and therapeutic applications of the plasma proteins serves to emphasize three important elements in medical progress—scientific curiosity, the humanitarian impulse, and effective social organization. We have had the privilege of summarizing the work of hundreds of investigators, whose studies are giving us new tools for the investigation and treatment of disease. Their work has only been possible because the magnificent response of a free people to the call for blood donors by a voluntary philanthropic agency, the American Red Cross, was coupled with a technical triumph, the development of practical methods for the large-scale separation of the plasma proteins, itself the culmination of highly theoretical and seemingly impractical investigations by protein chemists in various countries for many years.

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