scholarly journals New insights into the fine-scale structure of turbulence

2015 ◽  
Vol 784 ◽  
pp. 1-4 ◽  
Author(s):  
Michael Wilczek
Keyword(s):  
Velocity Gradient ◽  
Extensive Study ◽  
Scale Structure ◽  
Small Scale ◽  
Fine Scale ◽  
Image Velocimetry ◽  
Novel Variant ◽  
Mean Structure ◽  
Scale Turbulence ◽  
Low Dimensional

In a recent study, Lawson & Dawson (J. Fluid Mech., vol. 780, 2015, pp. 60–98) present experimental results on the fine-scale structure of turbulence, which are obtained with a novel variant of particle image velocimetry, to elucidate the relation between the small-scale structure, dynamics and statistics of turbulence. The results are carefully validated against direct numerical simulation data. Their extensive study focuses on the mean structure of the velocity gradient and the pressure Hessian fields for various small-scale flow topologies. It thereby reveals the dynamical impact of turbulent strain and vorticity structures on the velocity gradient statistics through non-local interactions, and points out ways to improve low-dimensional closure models for the dynamics of small-scale turbulence.

An experimental study of burner-stabilized turbulent flames in premixed reactants

1962 ◽  
Vol 268 (1333) ◽  
pp. 222-239 ◽  
Keyword(s):  
Burning Velocity ◽  
Short Circuit ◽  
Turbulent Flames ◽  
Laminar Flames ◽  
Small Scale ◽  
Light Sources ◽  
Gas Streams ◽  
Slow Flows ◽  
Mean Structure ◽  
Scale Turbulence

Current concepts of flame propagation in premixed, turbulent gas streams are examined. This leads to the conclusion that the link between theory and experiment is entirely inadequate and incapable of improvement by existing methods. A series of new methods is implemented in an attempt to short-circuit the chain of hypothesis and experiment which has hampered the identification of dubious steps. Methods of introducing uniform turbulence at relatively slow flows and improvements in light sources allow analysis of the approach flow by photographing particles illuminated by an interrupted Tyndall beam. Three new optical deflexion methods are used to give a measure of the randomness of flame-front orientation, of the time-mean structure of the flame and of the instantaneous shape of the corrugated front. It is found that this corrugated surface propagates at a velocity considerably in excess of the normal laminar burning velocity. Quantitative analysis of the frequency of ‘peaks’ and ‘valleys’ on the surface, together with comparative data from the apex of laminar flames, suggests an explanation in terms of the effects of curvature and, secondarily, of the influence of small-scale turbulence.

scholarly journals On the small-scale structure of turbulence and its impact on the pressure field

2018 ◽  
Vol 861 ◽  
pp. 422-446 ◽  
Author(s):  
Dimitar G. Vlaykov ◽  
Michael Wilczek
Keyword(s):  
Velocity Gradient ◽  
Pressure Field ◽  
Reynolds Numbers ◽  
Scale Structure ◽  
Small Scale ◽  
Local Pressure ◽  
Small Scale Structure ◽  
Kolmogorov Scale ◽  
Non Local ◽  
The Impact

Understanding the small-scale structure of incompressible turbulence and its implications for the non-local pressure field is one of the fundamental challenges in fluid mechanics. Intense velocity gradient structures tend to cluster on a range of scales which affects the pressure through a Poisson equation. Here we present a quantitative investigation of the spatial distribution of these structures conditional on their intensity for Taylor-based Reynolds numbers in the range [160, 380]. We find that the correlation length of the second invariant of the velocity gradient is proportional to the Kolmogorov scale. It is also a good indicator for the spatial localization of intense enstrophy and strain-dominated regions, as well as the separation between them. We describe and quantify the differences in the two-point statistics of these regions and the impact they have on the non-locality of the pressure field as a function of the intensity of the regions. Specifically, across the examined range of Reynolds numbers, the pressure in strong rotation-dominated regions is governed by a dissipation-scale neighbourhood. In strong strain-dominated regions, on the other hand, it is determined primarily by a larger neighbourhood reaching inertial scales.

scholarly journals A Free-Floating PIV System: Measurements of Small-Scale Turbulence under the Wind Wave Surface

2013 ◽  
Vol 30 (7) ◽  
pp. 1494-1510 ◽  
Author(s):  
Binbin Wang ◽  
Qian Liao ◽  
Jianen Xiao ◽  
Harvey A. Bootsma
Keyword(s):  
Dissipation Rate ◽  
Particle Image ◽  
Wind Wave ◽  
Open Water ◽  
Small Scale ◽  
Wind Condition ◽  
Wave Surface ◽  
Image Velocimetry ◽  
Air Water Interface ◽  
Scale Turbulence

Abstract An in situ free-floating underwater miniature particle image velocimetry (UWMPIV) system is developed and applied to measure the structure of turbulence in the aqueous side of the wind wave surface boundary layer. The UWMPIV system provides a direct way to measure the aqueous side turbulence dissipation rate and vortex structures immediately below the air–water interface, which are important parameters that determine the gas exchange rate across the air–water interface subjected to a low-to-moderate wind shear. The impact of platform motion on the measurement of small-scale turbulence is discussed and found to be insignificant. A series of field experiments under a near “zero-fetch” wind wave condition and one open water experiment under a low wind condition were conducted on Lake Michigan to demonstrate the capabilities of the free-floating particle image velocimetry (PIV) system. The dissipation rate estimated with a “direct method” and with a “spectra fitting” method are compared. Vertical profiles of the turbulence dissipation rate suggest a power-law dependency with depth below the water surface. Surface shear velocities estimated through the aqueous side Reynolds stress distribution agreed well with wind stresses estimated by the classic drag law for zero-fetch wind wave conditions, where the primary source of turbulence was wind shear. For the open water experiment under a very low wind condition, a high dissipation rate was observed near the water surface, suggesting a high turbulence production rate by surface waves, and the profile of dissipation rate showed a slower decay rate with depth in the presence of waves.

Predicting viscous-range velocity gradient dynamics in large-eddy simulations of turbulence

2017 ◽  
Vol 837 ◽  
pp. 80-114 ◽  
Author(s):  
Perry L. Johnson ◽  
Charles Meneveau
Keyword(s):  
Velocity Gradient ◽  
Volume Fraction ◽  
Large Eddy Simulations ◽  
Coarse Grained ◽  
Small Scale ◽  
Turbulence Structure ◽  
Gradient Tensor ◽  
Velocity Gradient Tensor ◽  
Large Eddy ◽  
Scale Turbulence

The detailed dynamics of small-scale turbulence are not directly accessible in large-eddy simulations (LES), posing a modelling challenge, because many micro-physical processes such as deformation of aggregates, drops, bubbles and polymers dynamics depend strongly on the velocity gradient tensor, which is dominated by the turbulence structure in the viscous range. In this paper, we introduce a method for coupling existing stochastic models for the Lagrangian evolution of the velocity gradient tensor with coarse-grained fluid simulations to recover small-scale physics without resorting to direct numerical simulations (DNS). The proposed approach is implemented in LES of turbulent channel flow and detailed comparisons with DNS are carried out. An application to modelling the fate of deformable, small (sub-Kolmogorov) droplets at negligible Stokes number and low volume fraction with one-way coupling is carried out and results are again compared to DNS results. Results illustrate the ability of the proposed model to predict the influence of small-scale turbulence on droplet micro-physics in the context of LES.

Noise from fine-scale turbulence of nonaxisymmetric jets

AIAA Journal ◽  
2002 ◽  
Vol 40 ◽  
pp. 456-464 ◽  
Author(s):  
C. K. W. Tam ◽  
N. Pastouchenko
Keyword(s):  
Fine Scale ◽  
Scale Turbulence

Effects of forward flight on jet mixing noise from fine-scale turbulence

AIAA Journal ◽  
2001 ◽  
Vol 39 ◽  
pp. 1261-1269 ◽  
Author(s):  
Christopher K. W. Tam ◽  
Nikolai Pastouchenko ◽  
Laurent Auriault
Keyword(s):  
Fine Scale ◽  
Forward Flight ◽  
Jet Mixing ◽  
Mixing Noise ◽  
Scale Turbulence
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