6,714 research outputs found
Hamiltonian formalism and the Garrett-Munk spectrum of internal waves in the ocean
Wave turbulence formalism for long internal waves in a stratified fluid is
developed, based on a natural Hamiltonian description. A kinetic equation
appropriate for the description of spectral energy transfer is derived, and its
self-similar stationary solution corresponding to a direct cascade of energy
toward the short scales is found. This solution is very close to the high
wavenumber limit of the Garrett-Munk spectrum of long internal waves in the
ocean. In fact, a small modification of the Garrett-Munk formalism includes a
spectrum consistent with the one predicted by wave turbulence.Comment: 4 pages latex fil
Design guidelines for assessing and controlling spacecraft charging effects
The need for uniform criteria, or guidelines, to be used in all phases of spacecraft design is discussed. Guidelines were developed for the control of absolute and differential charging of spacecraft surfaces by the lower energy space charged particle environment. Interior charging due to higher energy particles is not considered. A guide to good design practices for assessing and controlling charging effects is presented. Uniform design practices for all space vehicles are outlined
Thinking territory historically.
BACKGROUND:
While the randomised controlled trial (RCT) is generally regarded as the design of
choice for assessing the effects of health care, within the social sciences there is
considerable debate about the relative suitability of RCTs and non-randomised
studies (NRSs) for evaluating public policy interventions.
// OBJECTIVES:
To determine whether RCTs lead to the same effect size and variance as NRSs of
similar policy interventions; and whether these findings can be explained by other
factors associated with the interventions or their evaluation.
// METHODS:
Analyses of methodological studies, empirical reviews, and individual health and
social services studies investigated the relationship between randomisation and
effect size of policy interventions by:
1) Comparing controlled trials that are identical in all respects other than the use of
randomisation by 'breaking' the randomisation in a trial to create non-randomised
trials (re-sampling studies).
2) Comparing randomised and non-randomised arms of controlled trials mounted
simultaneously in the field (replication studies).
3) Comparing similar controlled trials drawn from systematic reviews that include
both randomised and non-randomised studies (structured narrative reviews and
sensitivity analyses within meta-analyses).
4) Investigating associations between randomisation and effect size using a pool of
more diverse RCTs and NRSs within broadly similar areas (meta-epidemiology).
// RESULTS:
Prior methodological reviews and meta-analyses of existing reviews comparing
effects from RCTs and nRCTs suggested that effect sizes from RCTs and nRCTs
may indeed differ in some circumstances and that these differences may well be
associated with factors confounded with design.
Re-sampling studies offer no evidence that the absence of randomisation directly
influences the effect size of policy interventions in a systematic way. No consistent
explanations were found for randomisation being associated with changes in effect
sizes of policy interventions in field trials
Elastic energy of proteins and the stages of protein folding
We propose a universal elastic energy for proteins, which depends only on the
radius of gyration and the residue number . It is constructed using
physical arguments based on the hydrophobic effect and hydrogen bonding.
Adjustable parameters are fitted to data from the computer simulation of the
folding of a set of proteins using the CSAW (conditioned self-avoiding walk)
model. The elastic energy gives rise to scaling relations of the form
in different regions. It shows three folding stages
characterized by the progression with exponents , which we
identify as the unfolded stage, pre-globule, and molten globule, respectively.
The pre-globule goes over to the molten globule via a break in behavior akin to
a first-order phase transition, which is initiated by a sudden acceleration of
hydrogen bonding
Strict limit on in-plane ordered magnetic dipole moment in URu2Si2
Neutron diffraction is used to examine the polarization of weak static
antiferromagnetism in high quality single crystalline URu2Si2. As previously
documented, elastic Bragg-like diffraction develops for temperature T<T_{HO}=
17.5 K at q=(100) but not at wave vector transfer q=(001). The peak width
indicates correlation lengths \xi_c=230(12) \AA \ and \xi_a=240(15) \AA. The
integrated intensity of the T-dependent peaks corresponds to a sample averaged
c-oriented staggered moment of \mu_{c}=0.022(1) \mu_B at T=1.7 K. The absence
of T-dependent diffraction at q=(001) places a limit \mu_{\perp}<0.0011 \mu_B
on an f- or d-orbital based in-plane staggered magnetic dipole moment, which is
associated with multipolar orders proposed for URu_2Si_2.Comment: 9 pages, 7 figure
Fall speed measurement and high-resolution multi-angle photography of hydrometeors in free fall
We describe here a new instrument for imaging hydrometeors in free fall. The Multi-Angle Snowflake Camera (MASC) captures high-resolution photographs of hydrometeors from three angles while simultaneously measuring their fall speed. Based on the stereoscopic photographs captured over the two months of continuous measurements obtained at a high altitude location within the Wasatch Front in Utah, we derive statistics for fall speed, hydrometeor size, shape, orientation and aspect ratio. From a selection of the photographed hydrometeors, an illustration is provided for how the instrument might be used for making improved microwave scattering calculations. Complex, aggregated snowflake shapes appear to be more strongly forward scattering, at the expense of reduced back-scatter, than heavily rimed graupel particles of similar size
- …