6,651 research outputs found
Statistically Preserved Structures and Anomalous Scaling in Turbulent Active Scalar Advection
The anomalous scaling of correlation functions in the turbulent statistics of
active scalars (like temperature in turbulent convection) is understood in
terms of an auxiliary passive scalar which is advected by the same turbulent
velocity field. While the odd-order correlation functions of the active and
passive fields differ, we propose that the even-order correlation functions are
the same to leading order (up to a trivial multiplicative factor). The leading
correlation functions are statistically preserved structures of the passive
scalar decaying problem, and therefore universality of the scaling exponents of
the even-order correlations of the active scalar is demonstrated.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Polymers in Fluid Flows
The interaction of flexible polymers with fluid flows leads to a number of
intriguing phenomena observed in laboratory experiments, namely drag reduction,
elastic turbulence and heat transport modification in natural convection, and
is one of the most challenging subjects in soft matter physics. In this paper
we review our present knowledge on the subject. Our present knowledge is mostly
based on direct numerical simulations performed in the last twenty years, which
have successfully explained, at least qualitatively, most of the experimental
results. Our goal is to disentangle as much as possible the basic mechanisms
acting in the system in order to capture the basic features underlying
different theoretical approaches and explanations
Dependence of heat transport on the strength and shear rate of prescribed circulating flows
We study numerically the dependence of heat transport on the maximum velocity
and shear rate of physical circulating flows, which are prescribed to have the
key characteristics of the large-scale mean flow observed in turbulent
convection. When the side-boundary thermal layer is thinner than the viscous
boundary layer, the Nusselt number (Nu), which measures the heat transport,
scales with the normalized shear rate to an exponent 1/3. On the other hand,
when the side-boundary thermal layer is thicker, the dependence of Nu on the
Peclet number, which measures the maximum velocity, or the normalized shear
rate when the viscous boundary layer thickness is fixed, is generally not a
power law. Scaling behavior is obtained only in an asymptotic regime. The
relevance of our results to the problem of heat transport in turbulent
convection is also discussed.Comment: 7 pages, 7 figures, submitted to European Physical Journal
On Conditional Statistics in Scalar Turbulence: Theory vs. Experiment
We consider turbulent advection of a scalar field T(\B.r), passive or
active, and focus on the statistics of gradient fields conditioned on scalar
differences across a scale . In particular we focus on two
conditional averages and
. We find exact relations between
these averages, and with the help of the fusion rules we propose a general
representation for these objects in terms of the probability density function
of . These results offer a new way to analyze
experimental data that is presented in this paper. The main question that we
ask is whether the conditional average is linear in . We show that there exists a dimensionless
parameter which governs the deviation from linearity. The data analysis
indicates that this parameter is very small for passive scalar advection, and
is generally a decreasing function of the Rayleigh number for the convection
data.Comment: Phys. Rev. E, Submitted. REVTeX, 10 pages, 5 figs. (not included) PS
Source of the paper with figure available at
http://lvov.weizmann.ac.il/onlinelist.html#unpub
Mean Temperature Profiles in Turbulent Thermal Convection
To predict the mean temperature profiles in turbulent thermal convection, the
thermal boundary layer (BL) equation including the effects of fluctuations has
to be solved. In Shishkina et al., Phys. Rev. Lett. 114 (2015), the thermal BL
equation with the fluctuations taken into account as an eddy thermal
diffusivity has been solved for large Prandtl-number fluids for which the eddy
thermal diffusivity and the velocity field can be approximated respectively as
a cubic and a linear function of the distance from the plate. In the present
work we make use of the idea of Prandtl's mixing length model and relate the
eddy thermal diffusivity to the stream function. With this proposed relation,
we can solve the thermal BL equation and obtain a closed-form expression for
the dimensionless mean temperature profile in terms of two independent
parameters for fluids with a general Prandtl number. With a proper choice of
the parameters, our predictions of the temperature profiles are in excellent
agreement with the results of our direct numerical simulations for a wide range
of Prandtl numbers from 0.01 to 2547.9 and Rayleigh numbers from 10^7 to 10^9.Comment: 8 pages, 4 figure
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