39 research outputs found
Effects of forcing in three dimensional turbulent flows
We present the results of a numerical investigation of three-dimensional
homogeneous and isotropic turbulence, stirred by a random forcing with a power
law spectrum, . Numerical simulations are performed at
different resolutions up to . We show that at varying the spectrum slope
, small-scale turbulent fluctuations change from a {\it forcing independent}
to a {\it forcing dominated} statistics. We argue that the critical value
separating the two behaviours, in three dimensions, is . When the
statistics is forcing dominated, for , we find dimensional scaling, i.e.
intermittency is vanishingly small. On the other hand, for , we find the
same anomalous scaling measured in flows forced only at large scales. We
connect these results with the issue of {\it universality} in turbulent flows.Comment: 4 pages, 4 figure
Isotropy vs anisotropy in small-scale turbulence
The decay of large-scale anisotropies in small-scale turbulent flow is
investigated. By introducing two different kinds of estimators we discuss the
relation between the presence of a hierarchy for the isotropic and the
anisotropic scaling exponents and the persistence of anisotropies. Direct
measurements from a channel flow numerical simulation are presented.Comment: 7 pages, 2 figure
Multiscale properties of Large Eddy Simulations: correlations between resolved-scale velocity-field increments and subgrid-scale quantities
We provide analytical and numerical results concerning multi-scale
correlations between the resolved velocity field and the subgrid-scale (SGS)
stress-tensor in large eddy simulations (LES). Following previous studies for
Navier-Stokes equations (NSE), we derive the exact hierarchy of LES equations
governing the spatio-temporal evolution of velocity structure functions of any
order. The aim is to assess the influence of the sub-grid model on the inertial
range intermittency. We provide a series of predictions, within the
multifractal theory, for the scaling of correlation involving the SGS stress
and we compare them against numerical results from high-resolution Smagorinsky
LES and from a-priori filtered data generated from direct numerical simulations
(DNS). We find that LES data generally agree very well with filtered DNS
results and with the multifractal prediction for all leading terms in the
balance equations. Discrepancies are measured for some of the subleading terms
involving cross-correlation between resolved velocity increments and the SGS
tensor or the SGS energy transfer, suggesting that there must be room to
improve the SGS modelisation to further extend the inertial range properties
for any fixed LES resolution.Comment: Journal of Turbulence (2018
Inverse velocity statistics in two dimensional turbulence
We present a numerical study of two-dimensional turbulent flows in the
enstrophy cascade regime, with different large-scale forcings and energy sinks.
In particular, we study the statistics of more-than-differentiable velocity
fluctuations by means of two recently introduced sets of statistical
estimators, namely {\it inverse statistics} and {\it second order differences}.
We show that the 2D turbulent velocity field, , cannot be simply
characterized by its spectrum behavior, . There
exists a whole set of exponents associated to the non-trivial smooth
fluctuations of the velocity field at all scales. We also present a numerical
investigation of the temporal properties of measured in different
spatial locations.Comment: 9 pages, 12 figure
Intermittency in Turbulence: computing the scaling exponents in shell models
We discuss a stochastic closure for the equation of motion satisfied by
multi-scale correlation functions in the framework of shell models of
turbulence. We give a systematic procedure to calculate the anomalous scaling
exponents of structure functions by using the exact constraints imposed by the
equation of motion. We present an explicit calculation for fifth order scaling
exponent at varying the free parameter entering in the non-linear term of the
model. The same method applied to the case of shell models for Kraichnan
passive scalar provides a connection between the concept of zero-modes and
time-dependent cascade processes.Comment: 12 pages, 5 eps figure
Lyapunov exponents of heavy particles in turbulence
Lyapunov exponents of heavy particles and tracers advected by homogeneous and
isotropic turbulent flows are investigated by means of direct numerical
simulations. For large values of the Stokes number, the main effect of inertia
is to reduce the chaoticity with respect to fluid tracers. Conversely, for
small inertia, a counter-intuitive increase of the first Lyapunov exponent is
observed. The flow intermittency is found to induce a Reynolds number
dependency for the statistics of the finite time Lyapunov exponents of tracers.
Such intermittency effects are found to persist at increasing inertia.Comment: 4 pages, 4 figure
Fully developed turbulence and the multifractal conjecture
We review the Parisi-Frisch MultiFractal formalism for
Navier--Stokes turbulence with particular emphasis on the issue of
statistical fluctuations of the dissipative scale. We do it for both Eulerian
and Lagrangian Turbulence. We also show new results concerning the application
of the formalism to the case of Shell Models for turbulence. The latter case
will allow us to discuss the issue of Reynolds number dependence and the role
played by vorticity and vortex filaments in real turbulent flows.Comment: Special Issue dedicated to E. Brezin and G. Paris
New Exact Betchov-like Relation for the Helicity Flux in Homogeneous Turbulence
In homogeneous and isotropic turbulence, the relative contributions of
different physical mechanisms to the energy cascade can be quantified by an
exact decomposition of the energy flux (P. Johnson, Phys. Rev. Lett., 124,
104501 (2020), J. Fluid Mech. 922, A3(2021)). We extend the formalism to the
transfer of kinetic helicity across scales, important in the presence of
large-scale mirror breaking mechanisms, to identify physical processes
resulting in helicity transfer and quantify their contributions to the mean
flux in the inertial range. All subfluxes transfer helicity from large to small
scales. About 50% of the mean flux is due to the scale-local vortex flattening
and vortex twisting. We derive a new exact relation between these effects,
similar to the Betchov relation for the energy flux, revealing that the mean
contribution of the former is three times larger than that of the latter.
Multi-scale effects account for the remaining 50% of the mean flux, with
approximate equipartition between multi-scale vortex flattening, twisting and
entangling
Anisotropy in large eddy simulations determined from SO(3) symmetry group
The issue of small-scale anisotropy in the context of eddy-viscosity-type subgrid-scale models is discussed in the present work. Recent developments in turbulence research suggest that anisotropy from large spatial scales is felt far into the inertial subrange of turbulence. This is of particular importance for subgrid-scale models of large eddy simulations. To address this issue, we present solutions of the random-phase Kolmogorov flow at moderate Reynolds numbers using direct numerical simulations and large eddy simulations with four subgrid-scale models: the Smagorinsky, the dynamic, the dynamic Clark, and the tensor-diffusivity models. The degree of anisotropy at different scales is analysed by decomposing the structure function into their irreducible representation of the SO(3) symmetry group. The results suggest that the dynamic model and the dynamic Clark model reproduce the statistical behaviour reasonably well, even in the anisotropic sectors at small length scales
Evidences of Bolgiano scaling in 3D Rayleigh-Benard convection
We present new results from high-resolution high-statistics direct numerical
simulations of a tri-dimensional convective cell. We test the fundamental
physical picture of the presence of both a Bolgiano-like and a Kolmogorov-like
regime. We find that the dimensional predictions for these two distinct regimes
(characterized respectively by an active and passive role of the temperature
field) are consistent with our measurements.Comment: 4 pages, 3 figure