1,856 research outputs found
Spectra of turbulence in dilute polymer solutions
We investigate turbulence in dilute polymer solutions when polymers are
strongly stretched by the flow. We establish power-law spectrum of velocity,
which is not associated with a flux of a conserved quantity, in two cases. The
first case is the elastic waves range of high Reynolds number turbulence of
polymer solutions above the coil-stretch transition. The second case is the
elastic turbulence, where chaotic flow is excited due to elastic instabilities
at small Reynolds numbers.Comment: 14 pages, RevTe
The Influence of Ambiguity and Noise on the Measurement of Turbulent Spectra by Doppler Scattering
A fundamental uncertainty in velocity measurement by Doppler scattering is caused by the finite residence time of the scattering particles in the observation volume; the arrival of scattering particles at arbitrary times gives rise to fluctuations in phase (and hence frequency) of the observed Doppler frequency. An estimate is obtained for the spectrum of these frequency fluctuations (called ambiguity noise). The frequency at which the spectral levels of a turbulent signal and the ambiguity noise are equal, provides a limit to the temporal resolution of an instantaneous velocity measurement; this limit is obtained, and shown to be quite restrictive. The influence of electronic noise is also analyzed and found to be negligible.
An experimental installation is described in which instantaneous fluctuating turbulent velocities may be measured by Doppler scattering using coherent radiation from a laser. Measurements are presented of the spectra of ambiguity noise end electronic noise. The agreement with theory is excellent
Second-order closures for compressible turbulence
This viewgraph presentation discusses project description, turbulence models, and computational engine and results for second-order closures for compressible turbulence
Toward a structural understanding of turbulent drag reduction: nonlinear coherent states in viscoelastic shear flows
Nontrivial steady flows have recently been found that capture the main
structures of the turbulent buffer layer. We study the effects of polymer
addition on these "exact coherent states" (ECS) in plane Couette flow. Despite
the simplicity of the ECS flows, these effects closely mirror those observed
experimentally: Structures shift to larger length scales, wall-normal
fluctuations are suppressed while streamwise ones are enhanced, and drag is
reduced. The mechanism underlying these effects is elucidated. These results
suggest that the ECS are closely related to buffer layer turbulence.Comment: 5 pages, 3 figures, published version, Phys. Rev. Lett. 89, 208301
(2002
Small scale statistics of viscoelastic turbulence
The small scale statistics of homogeneous isotropic turbulence of dilute
polymer solutions is investigated by means of direct numerical simulations of a
simplified viscoelastic fluid model. It is found that polymers only partially
suppress the turbulent cascade below the Lumley scale, leaving a remnant energy
flux even for large elasticity. As a consequence, fluid acceleration in
viscoelastic flows is reduced with respect to Newtonian turbulence, whereas its
rescaled probability density is left unchanged. At large scales the velocity
field is found to be unaffected by the presence of polymers.Comment: 7 pages, 4 figure
Dynamics of threads and polymers in turbulence: power-law distributions and synchronization
We study the behavior of threads and polymers in a turbulent flow. These
objects have finite spatial extension, so the flow along them differs slightly.
The corresponding drag forces produce a finite average stretching and the
thread is stretched most of the time. Nevertheless, the probability of
shrinking fluctuations is significant and is known to decay only as a
power-law. We show that the exponent of the power law is a universal number
independent of the statistics of the flow. For polymers the coil-stretch
transition exists: the flow must have a sufficiently large Lyapunov exponent to
overcome the elastic resistance and stretch the polymer from the coiled state
it takes otherwise. The probability of shrinking from the stretched state above
the transition again obeys a power law but with a non-universal exponent. We
show that well above the transition the exponent becomes universal and derive
the corresponding expression. Furthermore, we demonstrate synchronization: the
end-to-end distances of threads or polymers above the transition are
synchronized by the flow and become identical. Thus, the transition from
Newtonian to non-Newtonian behavior in dilute polymer solutions can be seen as
an ordering transition.Comment: 13 pages, version accepted to Journal of Statistical Mechanic
Linear Stochastic Models of Nonlinear Dynamical Systems
We investigate in this work the validity of linear stochastic models for
nonlinear dynamical systems. We exploit as our basic tool a previously proposed
Rayleigh-Ritz approximation for the effective action of nonlinear dynamical
systems started from random initial conditions. The present paper discusses
only the case where the PDF-Ansatz employed in the variational calculation is
``Markovian'', i.e. is determined completely by the present values of the
moment-averages. In this case we show that the Rayleigh-Ritz effective action
of the complete set of moment-functions that are employed in the closure has a
quadratic part which is always formally an Onsager-Machlup action. Thus,
subject to satisfaction of the requisite realizability conditions on the noise
covariance, a linear Langevin model will exist which reproduces exactly the
joint 2-time correlations of the moment-functions. We compare our method with
the closely related formalism of principal oscillation patterns (POP), which,
in the approach of C. Penland, is a method to derive such a linear Langevin
model empirically from time-series data for the moment-functions. The
predictive capability of the POP analysis, compared with the Rayleigh-Ritz
result, is limited to the regime of small fluctuations around the most probable
future pattern. Finally, we shall discuss a thermodynamics of statistical
moments which should hold for all dynamical systems with stable invariant
probability measures and which follows within the Rayleigh-Ritz formalism.Comment: 36 pages, 5 figures, seceq.sty for sequential numbering of equations
by sectio
The Polymer Stress Tensor in Turbulent Shear Flows
The interaction of polymers with turbulent shear flows is examined. We focus
on the structure of the elastic stress tensor, which is proportional to the
polymer conformation tensor. We examine this object in turbulent flows of
increasing complexity. First is isotropic turbulence, then anisotropic (but
homogenous) shear turbulence and finally wall bounded turbulence. The main
result of this paper is that for all these flows the polymer stress tensor
attains a universal structure in the limit of large Deborah number \De\gg 1.
We present analytic results for the suppression of the coil-stretch transition
at large Deborah numbers. Above the transition the turbulent velocity
fluctuations are strongly correlated with the polymer's elongation: there
appear high-quality "hydro-elastic" waves in which turbulent kinetic energy
turns into polymer potential energy and vice versa. These waves determine the
trace of the elastic stress tensor but practically do not modify its universal
structure. We demonstrate that the influence of the polymers on the balance of
energy and momentum can be accurately described by an effective polymer
viscosity that is proportional to to the cross-stream component of the elastic
stress tensor. This component is smaller than the stream-wise component by a
factor proportional to \De ^2 . Finally we tie our results to wall bounded
turbulence and clarify some puzzling facts observed in the problem of drag
reduction by polymers.Comment: 11 p., 1 Fig., included, Phys. Rev. E., submitte
Experimental evolution reveals that sperm competition intensity selects for longer, more costly sperm
It is the differences between sperm and eggs that fundamentally underpin the differences between the sexes within reproduction. For males, it is theorized that widespread sperm competition leads to selection for investment in sperm numbers, achieved by minimizing sperm size within limited resources for spermatogenesis in the testis. Here, we empirically examine how sperm competition shapes sperm size, after more than 77 generations of experimental selection of replicate lines under either high or low sperm competition intensities in the promiscuous flour beetle Tribolium castaneum. After this experimental evolution, populations had diverged significantly in their sperm competitiveness, with sperm in ejaculates from males evolving under high sperm competition intensities gaining 20% greater paternity than sperm in ejaculates from males that had evolved under low sperm competition intensity. Males did not change their relative investment into sperm production following this experimental evolution, showing no difference in testis sizes between high and low intensity regimes. However, the more competitive males from high sperm competition intensity regimes had evolved significantly longer sperm and, across six independently selected lines, there was a significant association between the degree of divergence in sperm length and average sperm competitiveness. To determine whether such sperm elongation is costly, we used dietary restriction experiments, and revealed that protein-restricted males produced significantly shorter sperm. Our findings therefore demonstrate that sperm competition intensity can exert positive directional selection on sperm size, despite this being a costly reproductive trait
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