848 research outputs found
Eulerian spectral closures for isotropic turbulence using a time-ordered fluctuation-dissipation relation
Procedures for time-ordering the covariance function, as given in a previous
paper (K. Kiyani and W.D. McComb Phys. Rev. E 70, 066303 (2004)), are extended
and used to show that the response function associated at second order with the
Kraichnan-Wyld perturbation series can be determined by a local (in wavenumber)
energy balance. These time-ordering procedures also allow the two-time
formulation to be reduced to time-independent form by means of exponential
approximations and it is verified that the response equation does not have an
infra-red divergence at infinite Reynolds number. Lastly, single-time
Markovianised closure equations (stated in the previous paper above) are
derived and shown to be compatible with the Kolmogorov distribution without the
need to introduce an ad hoc constant.Comment: 12 page
Re-examination of the infra-red properties of randomly stirred hydrodynamics
Dynamic renormalization group (RG) methods were originally used by Forster,
Nelson and Stephen (FNS) to study the large-scale behaviour of
randomly-stirred, incompressible fluids governed by the Navier-Stokes
equations. Similar calculations using a variety of methods have been performed
since, but have led to a discrepancy in results. In this paper, we carefully
re-examine in -dimensions the approaches used to calculate the renormalized
viscosity increment and, by including an additional constraint which is
neglected in many procedures, conclude that the original result of FNS is
correct. By explicitly using step functions to control the domain of
integration, we calculate a non-zero correction caused by boundary terms which
cannot be ignored. We then go on to analyze how the noise renormalization,
absent in many approaches, contributes an correction to the
force autocorrelation and show conditions for this to be taken as a
renormalization of the noise coefficient. Following this, we discuss the
applicability of this RG procedure to the calculation of the inertial range
properties of fluid turbulence.Comment: 16 pages, 6 figure
Recent Advances of the Forest in Iowa
At the time of settlement Iowa was definitely a prairie state with less than 20 per cent of its area covered by timber. Such a distribution of vegetation, coupled with short time observations, has led to the general assumption that prairie is the climax vegetation of the state. The writers have recently (7) assembled evidence that the present climate of Iowa is capable of supporting a forest climax. Prairie is therefore a subclimax associes maintained by its early establishment and by the marginal nature of the climate, but particularly by a combination of soil factors favoring prairie over woodland. The establishment of our classification depends upon evidence that forest is not only able to survive in Iowa, but that it has been able to invade the prairie at an appreciable rate under undisturbed conditions. Additional evidence of the spread of forest in Iowa, both after and before settlement, is presented here
Effects of light and soil moisture on forest tree seedling establishment
The present studies were designed to aid in the solution of forest tree seedling establishment problems common to stand conversion practices in Iowa. The primary objectives were: (1) to determine the minimum treatment needed to insure successful survival and growth; (2) to study the relationships of light and soil moisture in plant competition resulting from stand conversion; and (3) to evaluate five species of conifers - European larch, Scotch pine, eastern white pine, Norway spruce and red pine - for adaptability to region, site and underplanting.
Field studies were made at the Brayton Forest in northeastern Iowa and consisted of ( 1 ) practical understory treatments to increase survival and growth of five underplanted conifers and (2) controlled experiments to evaluate overstory and understory competition in such plantings. Studies at the State Forest Nursery near Ames were planned to determine the relative growth and photosynthetic characteristics of three of the underplanted conifers (European larch, eastern white pine and Norway spruce) and two shrubby hardwood species ( dogwood and hazel) which offer serious understory competition in the forest
Coal Spoil-Bank Materials as a Medium for Plant Growth
The use of draglines and mechanical shovels to uncover and remove coal from surface mines has greatly increased in importance in the last 20 years. Strip mining, as this is called, has resulted in a greater per man day coal production and a greater percentage recovery of coal. Along with this increased production and increased efficiency in the use of one resource there occurs a change in the use capability of land mined for coal
The 1000 GeV gamma rays from ms pulsars
The detection of 1000 GeV gamma-rays with the characteristic 6.1 ms periodicity of the radio pulsar PSR 1953 +29 is reported. This result, significant at the 5.4 beta level, provides the first direct evidence for the association of the 6 ms radio pulsar PSR1953+29 with the gamma-ray source 2CG065+0. Extensive observations of the 1.5 ms pulsar PSR 1937 are also reported
1000 GeV gamma rays from Cygnus X-3: An update
Measurements of 1000 GeV gamma-rays from Cygnus X-3 made with the University of Durham facility at Dugway, Utah in 1981/82 are reviewed. The light curve of the 4.8 hour modulated emission is updated and shows evidence significant at the 4.4 sigma level for strong emission (9% of the cosmic ray rate) at phase 0.625 and less significant (1.4 sigma level) indications of weaker emission (3% of the cosmic ray rate) at phase 0.125. The effect constituting the excess on the few nights showing the strongest emission appears to arise from the smallest Cerenkov light signals suggesting a steep gamma-ray spectrum. The 1982 data have been searched unsuccessfully for evidence of emission at phase 0.2, in coincidence with the results from the ultra-high energy (extensive Air Showers (EAS) measurements in 1979-1982. A systematic investigation of a long term variation in the strength of the peak of the 4.8 hr modulated 1000 GeV gamma-ray emission has been made. We find that in addition to the approximately 34 d variation reported by us previously, a stronger effect exists at around 19d
The 1000 GeV gamma ray emission from radio pulsars
Radio pulsars have concentrated on long observations of the Crab pulsar and showed that it emits short intense bursts and a persistent weak periodic flux at gamma-ray energies 1000 GeV. It was shown that the light curve of the persistent emission was dominated by a single peak, coincident with the position of the radio and low energy gamma-ray main pulse. The results of a more detailed analysis of the structure of this main pulse are reported following an appraisal of the timing system. It is shown that at energies 1000 GeV the duration of the main pulse is not greater than 0.4 ms, which is less than that seen at all frequencies other than radio. Flux limits for the emission of 1000 GeV gamma-rays by seven other radio pulsars are reporte
Fluctuating hydrodynamics and turbulence in a rotating fluid: Universal properties
We analyze the statistical properties of three-dimensional () turbulence
in a rotating fluid. To this end we introduce a generating functional to study
the statistical properties of the velocity field . We obtain the master
equation from the Navier-Stokes equation in a rotating frame and thence a set
of exact hierarchical equations for the velocity structure functions for
arbitrary angular velocity . In particular we obtain the {\em
differential forms} for the analogs of the well-known von Karman-Howarth
relation for fluid turbulence. We examine their behavior in the limit of
large rotation. Our results clearly suggest dissimilar statistical behavior and
scaling along directions parallel and perpendicular to . The
hierarchical relations yield strong evidence that the nature of the flows for
large rotation is not identical to pure two-dimensional flows. To complement
these results, by using an effective model in the small- limit, within
a one-loop approximation, we show that the equal-time correlation of the
velocity components parallel to displays Kolmogorov scaling
, where as for all other components, the equal-time correlators scale
as in the inertial range where is a wavevector in . Our
results are generally testable in experiments and/or direct numerical
simulations of the Navier-Stokes equation in a rotating frame.Comment: 24 pages in preprint format; accepted for publication in Phys. Rev. E
(2011
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