284 research outputs found
Saturation of Turbulent Drag Reduction in Dilute Polymer Solutions
Drag reduction by polymers in turbulent wall-bounded flows exhibits universal
and non-universal aspects. The universal maximal mean velocity profile was
explained in a recent theory. The saturation of this profile and the crossover
back to the Newtonian plug are non-universal, depending on Reynolds number Re,
concentration of polymer and the degree of polymerization . We
explain the mechanism of saturation stemming from the finiteness of
extensibility of the polymers, predict its dependence on and in the
limit of small and large Re, and present the excellent comparison of our
predictions to experiments on drag reduction by DNA.Comment: 4 pages, 4 figs., included, PRL, submitte
Exact Resummations in the Theory of Hydrodynamic Turbulence: II A Ladder to Anomalous Scaling
In paper I of this series on fluid turbulence we showed that exact
resummations of the perturbative theory of the structure functions of velocity
differences result in a finite (order by order) theory. These findings exclude
any known perturbative mechanism for anomalous scaling of the velocity
structure functions. In this paper we continue to build the theory of
turbulence and commence the analysis of nonperturbative effects that form the
analytic basis of anomalous scaling. Starting from the Navier-Stokes equations
(at high Reynolds number Re) we discuss the simplest examples of the appearance
of anomalous exponents in fluid mechanics. These examples are the nonlinear
(four-point) Green's function and related quantities. We show that the
renormalized perturbation theory for these functions contains ``ladder``
diagrams with (convergent!) logarithmic terms that sum up to anomalous
exponents. Using a new sum rule which is derived here we calculate the leading
anomalous exponent and show that it is critical in a sense made precise below.
This result opens up the possibility of multiscaling of the structure functions
with the outer scale of turbulence as the renormalization length. This
possibility will be discussed in detail in the concluding paper III of this
series.Comment: PRE in press, 15 pages + 21 figures, REVTeX, The Eps files of figures
will be FTPed by request to [email protected]
Fusion Rules in Turbulent Systems with Flux Equilibrium
Fusion rules in turbulence specify the analytic structure of many-point
correlation functions of the turbulent field when a group of coordinates
coalesce. We show that the existence of flux equilibrium in fully developed
turbulent systems combined with a direct cascade induces universal fusion
rules. In certain examples these fusion rules suffice to compute the
multiscaling exponents exactly, and in other examples they give rise to an
infinite number of scaling relations that constrain enormously the structure of
the allowed theory.Comment: Submitted to PRL on July 95, 4 pages, REVTe
Parametric Generation of Second Sound by First Sound in Superfluid Helium
We report the first experimental observation of parametric generation of
second sound (SS) by first sound (FS) in superfluid helium in a narrow
temperature range in the vicinity of . The temperature dependence
of the threshold FS amplitude is found to be in a good quantitative agreement
with the theory suggested long time ago and corrected for a finite geometry.
Strong amplitude fluctuations and two types of the SS spectra are observed
above the bifurcation. The latter effect is quantitatively explained by the
discreteness of the wave vector space and the strong temperature dependence of
the SS dissipation length.Comment: 4 pages, 4 postscript figures, REVTE
Cooper pair turbulence in atomic Fermi gases
We investigate the stability of spatially uniform solutions for the
collisionless dynamics of a fermionic superfluid. We demonstrate that, if the
system size is larger than the superfluid coherence length, the solution
characterized by a periodic in time order parameter is unstable with respect to
spatial fluctuations. The instability is due to the parametric excitations of
pairing modes with opposite momenta. The growth of spatial modulations is
suppressed by nonlinear effects resulting in a state characterized by a random
superposition of wave packets of the superfluid order parameter. We suggest
that this state can be probed by spectroscopic noise measurements.Comment: 4 pages, 2 figure
Inertial- and Dissipation-Range Asymptotics in Fluid Turbulence
We propose and verify a wave-vector-space version of generalized extended
self similarity and broaden its applicability to uncover intriguing, universal
scaling in the far dissipation range by computing high-order (\leq 20\/)
structure functions numerically for: (1) the three-dimensional, incompressible
Navier Stokes equation (with and without hyperviscosity); and (2) the GOY shell
model for turbulence. Also, in case (2), with Taylor-microscale Reynolds
numbers 4 \times 10^{4} \leq Re_{\lambda} \leq 3 \times 10^{6}\/, we find
that the inertial-range exponents (\zeta_{p}\/) of the order - p\/
structure functions do not approach their Kolmogorov value p/3\/ as
Re_{\lambda}\/ increases.Comment: RevTeX file, with six postscript figures. epsf.tex macro is used for
figure insertion. Packaged using the 'uufiles' utilit
Measurement of the Electric and Magnetic Polarizabilities of the Proton
The Compton scattering cross section on the proton has been measured at
laboratory angles of 90 and 135 using tagged photons in the
energy range 70--100 MeV and simultaneously using untagged photons in the range
100--148~MeV. With the aid of dispersion relations, these cross sections were
used to extract the electric and magnetic polarizabilities, and
respectively, of the proton. We find
in agreement with a model-independent dispersion sum rule, and
where the errors shown are statistical, systematic, and model-dependent,
respectively. A comparison with previous experiments is given and global values
for the polarizabilities are extracted.Comment: 35 pages, 11 PostScript figures, uses RevTex 3.
Anomalous Scaling in the N-Point Functions of Passive Scalar
A recent analysis of the 4-point correlation function of the passive scalar
advected by a time-decorrelated random flow is extended to the N-point case. It
is shown that all stationary-state inertial-range correlations are dominated by
homogeneous zero modes of singular operators describing their evolution. We
compute analytically the zero modes governing the N-point structure functions
and the anomalous dimensions corresponding to them to the linear order in the
scaling exponent of the 2-point function of the advecting velocity field. The
implications of these calculations for the dissipation correlations are
discussed.Comment: 16 pages, latex fil
Comment on ``Evidence for Narrow Baryon Resonances in Inelastic pp Scattering''
Compton scattering data are sensitive to the existence of low-mass resonances
reported by Tatischeff et al. We show that such states, with their reported
properties, are excluded by previous Compton scattering experiments.Comment: One page, submitted to PR
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