179 research outputs found
Small scale aspects of flows in proximity of the turbulent/non-turbulent interface
The work reported below is a first of its kind study of the properties of
turbulent flow without strong mean shear in a Newtonian fluid in proximity of
the turbulent/non-turbulent interface, with emphasis on the small scale
aspects. The main tools used are a three-dimensional particle tracking system
(3D-PTV) allowing to measure and follow in a Lagrangian manner the field of
velocity derivatives and direct numerical simulations (DNS). The comparison of
flow properties in the turbulent (A), intermediate (B) and non-turbulent (C)
regions in the proximity of the interface allows for direct observation of the
key physical processes underlying the entrainment phenomenon. The differences
between small scale strain and enstrophy are striking and point to the definite
scenario of turbulent entrainment via the viscous forces originating in strain.Comment: 4 pages, 4 figures, Phys. Fluid
On an alternative explanation of anomalous scaling and how well-defined is the concept of inertial range
The main point of this communication is that there is a small non-negligible
amount of eddies-outliers/very strong events (comprising a significant subset
of the tails of the PDF of velocity increments in the nominally-defined
inertial range) for which viscosity/dissipation is of utmost importance at
whatever high Reynolds number. These events contribute significantly to the
values of higher-order structure functions and their anomalous scaling. Thus
the anomalous scaling is not an attribute of the conventionally-defined
inertial range, and the latter is not a well-defined concept. The claim above
is supported by an analysis of high-Reynolds-number flows in which among other
things it was possible to evaluate the instantaneous rate of energy
dissipation.Comment: 7 pages, 6 figure
On the evolution of flow topology in turbulent Rayleigh-Bénard convection
Copyright 2016 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.Small-scale dynamics is the spirit of turbulence physics. It implicates many attributes of flow topology evolution, coherent structures, hairpin vorticity dynamics, and mechanism of the kinetic energy cascade. In this work, several dynamical aspects of the small-scale motions have been numerically studied in a framework of Rayleigh-Benard convection (RBC). To do so, direct numerical simulations have been carried out at two Rayleigh numbers Ra = 10(8) and 10(10), inside an air-filled rectangular cell of aspect ratio unity and pi span-wise open-ended distance. As a main feature, the average rate of the invariants of the velocity gradient tensor (Q(G), R-G) has displayed the so-calledPeer ReviewedPostprint (author's final draft
Viscous tilting and production of vorticity in homogeneous turbulence
Viscous depletion of vorticity is an essential and well known property of turbulent flows, balancing, in the mean, the net vorticity production associated with the vortex stretching mechanism. In this letter, we, however, demonstrate that viscous effects are not restricted to a mere destruction process, but play a more complex role in vorticity dynamics that is as important as vortex stretching. Based on the results from three dimensional particle tracking velocimetry experiments and direct numerical simulation of homogeneous and quasi-isotropic turbulence, we show that the viscous term in the vorticity equation can also locally induce production of vorticity and changes of the orientation of the vorticity vector (viscous tilting)
Large scale flow effects, energy transfer, and self-similarity on turbulence
The effect of large scales on the statistics and dynamics of turbulent
fluctuations is studied using data from high resolution direct numerical
simulations. Three different kinds of forcing, and spatial resolutions ranging
from 256^3 to 1024^3, are being used. The study is carried out by investigating
the nonlinear triadic interactions in Fourier space, transfer functions,
structure functions, and probability density functions. Our results show that
the large scale flow plays an important role in the development and the
statistical properties of the small scale turbulence. The role of helicity is
also investigated. We discuss the link between these findings and
intermittency, deviations from universality, and possible origins of the
bottleneck effect. Finally, we briefly describe the consequences of our results
for the subgrid modeling of turbulent flows
Log-Poisson Cascade Description of Turbulent Velocity Gradient Statistics
The Log-Poisson phenomenological description of the turbulent energy cascade
is evoked to discuss high-order statistics of velocity derivatives and the
mapping between their probability distribution functions at different Reynolds
numbers. The striking confirmation of theoretical predictions suggests that
numerical solutions of the flow, obtained at low/moderate Reynolds numbers can
play an important quantitative role in the analysis of experimental high
Reynolds number phenomena, where small scales fluctuations are in general
inaccessible from direct numerical simulations
Acceleration, pressure and related quantities in the proximity of the turbulent/non-turbulent interface
This paper presents an analysis of flow properties in the proximity of the turbulent/non-turbulent interface (TNTI), with particular focus on the acceleration of fluid particles, pressure and related small scale quantities such as enstrophy, ω2 = ωiωi, and strain, s2 = sijsij. The emphasis is on the qualitative differences between turbulent, intermediate and non-turbulent flow regions, emanating from the solenoidal nature of the turbulent region, the irrotational character of the non-turbulent region and the mixed nature of the intermediate region in between. The results are obtained from a particle tracking experiment and direct numerical simulations (DNS) of a temporally developing flow without mean shear. The analysis reveals that turbulence influences its neighbouring ambient flow in three different ways depending on the distance to the TNTI: (i) pressure has the longest range of influence into the ambient region and in the far region non-local effects dominate. This is felt on the level of velocity as irrotational fluctuations, on the level of acceleration as local change of velocity due to pressure gradients, Du/Dt ∂u/∂t − p/ρ, and, finally, on the level of strain due to pressure-Hessian/strain interaction, (D/Dt)(s2/2) (∂/∂t)(s2/2) −sijp,ij > 0; (ii) at intermediate distances convective terms (both for acceleration and strain) as well as strain production −sijsjkski > 0 start to set in. Comparison of the results at Taylor-based Reynolds numbers Reλ = 50 and Reλ = 110 suggests that the distances to the far or intermediate regions scale with the Taylor microscale λ or the Kolmogorov length scale η of the flow, rather than with an integral length scale; (iii) in the close proximity of the TNTI the velocity field loses its purely irrotational character as viscous effects lead to a sharp increase of enstrophy and enstrophy-related terms. Convective terms show a positive peak reflecting previous findings that in the laboratory frame of reference the interface moves locally with a velocity comparable to the fluid velocity fluctuation
New subgrid-scale models for large-eddy simulation of Rayleigh-Bénard convection
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.At the crossroad between flow topology analysis and the theory of turbulence, a new eddy-viscosity model for Large-eddy simulation has been recently proposed by Trias et al.[PoF, 27, 065103 (2015)]. The S3PQR-model has the proper cubic near-wall behaviour and no intrinsic limitations for statistically inhomogeneous flows. In this work, the new model has been tested for an air turbulent Rayleigh-Benard convection in a rectangular cell of aspect ratio unity and n span-wise open-ended distance. To do so, direct numerical simulation has been carried out at two Rayleigh numbers Ra = 108 and 1010, to assess the model performance and investigate a priori the effect of the turbulent Prandtl number. Using an approximate formula based on the Taylor series expansion, the turbulent Prandtl number has been calculated and revealed a constant and Ra-independent value across the bulk region equals to 0.55. It is found that the turbulent components of eddy-viscosity and eddy-diffusivity are positively prevalent to maintain a turbulent wind essentially driven by the mean buoyant force at the sidewalls. On the other hand, the new eddy-viscosity model is preliminary tested for the case of Ra = 108 and showed overestimation of heat flux within the boundary layer but fairly good prediction of turbulent kinetics at this moderate turbulent flow.Peer ReviewedPostprint (published version
Local and nonlocal pressure Hessian effects in real and synthetic fluid turbulence
The Lagrangian dynamics of the velocity gradient tensor A in isotropic and
homogeneous turbulence depend on the joint action of the self-streching term
and the pressure Hessian. Existing closures for pressure effects in terms of A
are unable to reproduce one important statistical role played by the
anisotropic part of the pressure Hessian, namely the redistribution of the
probabilities towards enstrophy production dominated regions. As a step towards
elucidating the required properties of closures, we study several synthetic
velocity fields and how well they reproduce anisotropic pressure effects. It is
found that synthetic (i) Gaussian, (ii) Multifractal and (iii) Minimal Turnover
Lagrangian Map (MTLM) incompressible velocity fields reproduce many features of
real pressure fields that are obtained from numerical simulations of the Navier
Stokes equations, including the redistribution towards enstrophy-production
regions. The synthetic fields include both spatially local, and nonlocal,
anisotropic pressure effects. However, we show that the local effects appear to
be the most important ones: by assuming that the pressure Hessian is local in
space, an expression in terms of the Hessian of the second invariant Q of the
velocity gradient tensor can be obtained. This term is found to be well
correlated with the true pressure Hessian both in terms of eigenvalue
magnitudes and eigenvector alignments.Comment: 10 pages, 4 figures, minor changes, final version, published in Phys.
Fluid
Slip-velocity of large neutrally-buoyant particles in turbulent flows
We discuss possible definitions for a stochastic slip velocity that describes
the relative motion between large particles and a turbulent flow. This
definition is necessary because the slip velocity used in the standard drag
model fails when particle size falls within the inertial subrange of ambient
turbulence. We propose two definitions, selected in part due to their
simplicity: they do not require filtration of the fluid phase velocity field,
nor do they require the construction of conditional averages on particle
locations. A key benefit of this simplicity is that the stochastic slip
velocity proposed here can be calculated equally well for laboratory, field,
and numerical experiments. The stochastic slip velocity allows the definition
of a Reynolds number that should indicate whether large particles in turbulent
flow behave (a) as passive tracers; (b) as a linear filter of the velocity
field; or (c) as a nonlinear filter to the velocity field. We calculate the
value of stochastic slip for ellipsoidal and spherical particles (the size of
the Taylor microscale) measured in laboratory homogeneous isotropic turbulence.
The resulting Reynolds number is significantly higher than 1 for both particle
shapes, and velocity statistics show that particle motion is a complex
non-linear function of the fluid velocity. We further investigate the nonlinear
relationship by comparing the probability distribution of fluctuating
velocities for particle and fluid phases
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