1,711 research outputs found

### Monograph on prospective developments in oceanology

Excerpts from a chapter of a monograph, Oceanology in the Year 2000, which has been prepared for publication at the USSR Academy of Sciences' Institute of Oceanology, is presented. The author of this chapter is A. S. Morin, corresponding member of the USSR Academy of Sciences and director of the oceanology institute. The monograph is said to be the collective work of a group of specialists. Monin views prospective developments of oceanology and oceanology related research and development, technology and expedition research

### Evolution of Magnetic Fields in Freely Decaying Magnetohydrodynamic Turbulence

We study the evolution of magnetic fields in freely decaying
magnetohydrodynamic turbulence. By quasi-linearizing the Navier-Stokes
equation, we solve analytically the induction equation in quasi-normal
approximation. We find that, if the magnetic field is not helical, the magnetic
energy and correlation length evolve in time respectively as E_B \propto
t^{-2(1+p)/(3+p)} and \xi_B \propto t^{2/(3+p)}, where p is the index of
initial power-law spectrum. In the helical case, the magnetic helicity is an
almost conserved quantity and forces the magnetic energy and correlation length
to scale as E_B \propto (log t)^{1/3} t^{-2/3} and \xi_B \propto (log t)^{-1/3}
t^{2/3}.Comment: 4 pages, 2 figures; accepted for publication in PR

### Exact and Asymptotic Conditions on Traveling Wave Solutions of the Navier-Stokes Equations

We derive necessary conditions that traveling wave solutions of the
Navier-Stokes equations must satisfy in the pipe, Couette, and channel flow
geometries. Some conditions are exact and must hold for any traveling wave
solution irrespective of the Reynolds number ($Re$). Other conditions are
asymptotic in the limit $Re\to\infty$. The exact conditions are likely to be
useful tools in the study of transitional structures. For the pipe flow
geometry, we give computations up to $Re=100000$ showing the connection of our
asymptotic conditions to critical layers that accompany vortex structures at
high $Re$

### Anomalous scaling in two and three dimensions for a passive vector field advected by a turbulent flow

A model of the passive vector field advected by the uncorrelated in time
Gaussian velocity with power-like covariance is studied by means of the
renormalization group and the operator product expansion. The structure
functions of the admixture demonstrate essential power-like dependence on the
external scale in the inertial range (the case of an anomalous scaling). The
method of finding of independent tensor invariants in the cases of two and
three dimensions is proposed to eliminate linear dependencies between the
operators entering into the operator product expansions of the structure
functions. The constructed operator bases, which include the powers of the
dissipation operator and the enstrophy operator, provide the possibility to
calculate the exponents of the anomalous scaling.Comment: 9 pages, LaTeX2e(iopart.sty), submitted to J. Phys. A: Math. Ge

### Chaos from turbulence: stochastic-chaotic equilibrium in turbulent convection at high Rayleigh numbers

It is shown that correlation function of the mean wind velocity generated by
a turbulent thermal convection (Rayleigh number $Ra \sim 10^{11}$) exhibits
exponential decay with a very long correlation time, while corresponding
largest Lyapunov exponent is certainly positive. These results together with
the reconstructed phase portrait indicate presence of chaotic component in the
examined mean wind. Telegraph approximation is also used to study relative
contribution of the chaotic and stochastic components to the mean wind
fluctuations and an equilibrium between these components has been studied in
detail

### Fractal dimension crossovers in turbulent passive scalar signals

The fractal dimension $\delta_g^{(1)}$ of turbulent passive scalar signals is
calculated from the fluid dynamical equation. $\delta_g^{(1)}$ depends on the
scale. For small Prandtl (or Schmidt) number $Pr<10^{-2}$ one gets two ranges,
$\delta_g^{(1)}=1$ for small scale r and $\delta_g^{(1)}$=5/3 for large r, both
as expected. But for large $Pr> 1$ one gets a third, intermediate range in
which the signal is extremely wrinkled and has $\delta_g^{(1)}=2$. In that
range the passive scalar structure function $D_\theta(r)$ has a plateau. We
calculate the $Pr$-dependence of the crossovers. Comparison with a numerical
reduced wave vector set calculation gives good agreement with our predictions.Comment: 7 pages, Revtex, 3 figures (postscript file on request

### A search for strong, ordered magnetic fields in Herbig Ae/Be stars

The origin of magnetic fields in intermediate-mass and high-mass stars is
fundamentally a mystery. Clues toward solving this basic astrophysical problem
can likely be found at the pre-main sequence (PMS) evolutionary stage. With
this work, we perform the largest and most sensitive search for magnetic fields
in pre-main sequence Herbig Ae/Be (HAeBe) stars. Sixty-eight observations of 50
HAeBe stars have been obtained in circularly polarised light using the FORS1
spectropolarimeter at the ESO VLT. An analysis of both Balmer and metallic
lines reveals the possible presence of weak longitudinal magnetic fields in
photospheric lines of two HAeBe stars, HD 101412 and BF Ori. The intensity of
the longitudinal fields detected in HD 101412 and BF Ori suggest that they
correspond to globally-ordered magnetic fields with surface intensities of
order 1 kG. Monte Carlo simulations of the longitudinal field measurements of
the undetected stars allow us to place an upper limits of about 300 G on the
general presence of aligned magnetic dipole magnetic fields, and of about 500 G
on perpendicular dipole fields. We find that the observed bulk incidence of
magnetic HAeBe stars in our sample is 8-12%, in good agreement with that of
magnetic main sequence stars of similar masses. We also find that the rms
longitudinal field intensity of magnetically-detected HAeBe stars is similar to
that of Ap stars and consistent with magnetic flux conservation during stellar
evolution. These results are all in agreement with the hypothesis that the
magnetic fields of main sequence Ap/Bp stars are fossils, which already exist
within the stars at the pre-main sequence stage. Finally, we explore the
ability of our new magnetic data to constrain magnetospheric accretion in
Herbig Ae/Be stars.Comment: Accepted by Monthly Notices of the Royal Astronomical Society, 2007
January 11. Received 2007 January 11; in original form 2006 August 18. The
paper contains 18 pages, 11 figures and 2 table

### Universal dissipation scaling for non-equilibrium turbulence

It is experimentally shown that the non-classical high Reynolds number energy
dissipation behaviour, $C_{\epsilon} \equiv \epsilon L/u^3 = f(Re_M)/Re_L$,
observed during the decay of fractal square grid-generated turbulence is also
manifested in decaying turbulence originating from various regular grids. For
sufficiently high values of the global Reynolds numbers $Re_M$, $f(Re_M)\sim
Re_M$.Comment: 5 pages, 6 figure

### 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

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