2,113 research outputs found
Technique for Evaluating the Erosive Properties of Ablative Internal Insulation Materials
A technique for determining the average erosion rate versus Mach number of candidate internal insulation materials was developed for flight motor applications in 12 inch I.D. test firing hardware. The method involved the precision mounting of a mechanical measuring tool within a conical test cartridge fabricated from either a single insulation material or two non-identical materials each of which constituted one half of the test cartridge cone. Comparison of the internal radii measured at nine longitudinal locations and between eight to thirty two azimuths, depending on the regularity of the erosion pattern before and after test firing, permitted calculation of the average erosion rate and Mach number. Systematic criteria were established for identifying erosion anomalies such as the formation of localized ridges and for excluding such anomalies from the calculations. The method is discussed and results presented for several asbestos-free materials developed in-house for the internal motor case insulation in solid propellant rocket motors
Energy transfer and dissipation in forced isotropic turbulence
A model for the Reynolds number dependence of the dimensionless dissipation
rate was derived from the dimensionless
K\'{a}rm\'{a}n-Howarth equation, resulting in , where is the integral scale Reynolds
number. The coefficients and arise from asymptotic
expansions of the dimensionless second- and third-order structure functions.
This theoretical work was supplemented by direct numerical simulations (DNSs)
of forced isotropic turbulence for integral scale Reynolds numbers up to
(), which were used to establish that the decay of
dimensionless dissipation with increasing Reynolds number took the form of a
power law with exponent value , and that this
decay of was actually due to the increase in the Taylor
surrogate . The model equation was fitted to data from the DNS which
resulted in the value and in an asymptotic value for
in the infinite Reynolds number limit of
.Comment: 26 pages including references and 6 figures. arXiv admin note: text
overlap with arXiv:1307.457
Spectral analysis of structure functions and their scaling exponents in forced isotropic turbulence
The pseudospectral method, in conjunction with a new technique for obtaining
scaling exponents from the structure functions , is presented
as an alternative to the extended self-similarity (ESS) method and the use of
generalized structure functions. We propose plotting the ratio
against the separation in accordance with a standard
technique for analysing experimental data. This method differs from the ESS
technique, which plots against , with the assumption . Using our method for the particular case of we obtain the new
result that the exponent decreases as the Taylor-Reynolds number
increases, with as . This
supports the idea of finite-viscosity corrections to the K41 prediction for
, and is the opposite of the result obtained by ESS. The pseudospectral
method also permits the forcing to be taken into account exactly through the
calculation of the energy input in real space from the work spectrum of the
stirring forces.Comment: 31 pages including appendices, 10 figure
Texture, twinning and metastable "tetragonal" phase in ultrathin films of HfO<sub>2</sub> on a Si substrate
Thin HfO<sub>2</sub> films grown on the lightly oxidised surface of (100) Si wafers have been examined using dark-field transmission electron microscopy and selected area electron diffraction in plan view. The polycrystalline film has a grain size of the order of 100 nm and many of the grains show evidence of twinning on (110) and (001) planes. Diffraction studies showed that the film had a strong [110] out-of-plane texture, and that a tiny volume fraction of a metastable (possibly tetragonal) phase was retained. The reasons for the texture, twinning and the retention of the metastable phase are discussed
Dynamical Inequality in Growth Models
A recent exponent inequality is applied to a number of dynamical growth
models. Many of the known exponents for models such as the Kardar-Parisi-Zhang
(KPZ) equation are shown to be consistent with the inequality. In some cases,
such as the Molecular Beam Equation, the situation is more interesting, where
the exponents saturate the inequality. As the acid test for the relative
strength of four popular approximation schemes we apply the inequality to the
exponents obtained for two Non Local KPZ systems. We find that all methods but
one, the Self Consistent Expansion, violate the inequality in some regions of
parameter space. To further demonstrate the usefulness of the inequality, we
apply it to a specific model, which belongs to a family of models in which the
inequality becomes an equality. We thus show that the inequality can easily
yield results, which otherwise have to rely either on approximations or general
beliefs.Comment: 6 pages, 4 figure
Genetic diversity and phylogenetic relationships within Eucalyptus marginata (Myrtaceae)
The eucalypt species Eucalyptus marginata which is harvested for high quality timber comprises three subspecies recognized by morphological characters; E. marginata ssp. marginata, ssp. thalassica, and ssp. elegantella. Genetic diversity and phylogenetic relationships between the subspecies were examined using anonymous nuclear RFLP loci, with Eucalyptus staerei included as an outgroup in the phylogenetic analysis. The level of diversity within the nuclear genome was lower than that found in comparative studies with other eucalypts (A = 2. 7, H4 = 0.345). Most of the variation occurred with ill the populations (96. 9%, H4 = 0.334) The two populations sampled for each of ssp. thalassica and ssp. elegantella clustered together in the UPGMA analysis, however there was little differentiation between the three subspecies overall (D = 0.029). Eucalyptus marginata was clearly distinct from its closest relative E. staerei (0 = 0.16). There is little genetic support for the separation of the subspecies
Gauge symmetry and Slavnov-Taylor identities for randomly stirred fluids
The path integral for randomly forced incompressible fluids is shown to have
an underlying Becchi-Rouet-Stora (BRS) symmetry as a consequence of Galilean
invariance. This symmetry must be respected to have a consistent generating
functional, free from both an overall infinite factor and spurious relations
amongst correlation functions. We present a procedure for respecting this BRS
symmetry, akin to gauge fixing in quantum field theory. Relations are derived
between correlation functions of this gauge fixed, BRS symmetric theory,
analogous to the Slavnov-Taylor identities of quantum field theory.Comment: 5 pages, no figures, In Press Physical Review Letters, 200
Optical control of internal electric fields in band-gap graded InGaN nanowires
InGaN nanowires are suitable building blocks for many future optoelectronic
devices. We show that a linear grading of the indium content along the nanowire
axis from GaN to InN introduces an internal electric field evoking a
photocurrent. Consistent with quantitative band structure simulations we
observe a sign change in the measured photocurrent as a function of photon
flux. This negative differential photocurrent opens the path to a new type of
nanowire-based photodetector. We demonstrate that the photocurrent response of
the nanowires is as fast as 1.5 ps
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