5,028 research outputs found
Experimental study of the effects of secondary air on the emissions and stability of a lean premixed combustor
Tests were run using a perforated plate flameholder with a relatively short attached recirculation zone and a vee gutter flameholder with a relatively long attached recirculation zone. Combustor streamlines were traced in cold flow tests at ambient pressure. The amount of secondary air entrainment in the recirculation zones of the flameholders was determined by tracer gas testing at cold flow ambient pressure conditions. Combustion tests were caried out at entrance conditions of 0.5 MPa/630K and emission of NOx, CO and unburned hydrocarbons were measured along with lean stability and flashback limits. The degree of entrainment increases as dilution air injection decreases. Flashback appears to be a function of overall equivalence ratio and resistance to flashback increases with increasing combustor entrance velocity. Lean stability limit appears to be a function of both primary zone and flameholder recirculation zone equivalence ratios and resistance to lean blowout increases with increasing combustor entrance velocity
Tropical rainforest bird community structure in relation to altitude, tree species composition, and null models in the Western Ghats, India
Studies of species distributions on elevational gradients are essential to
understand principles of community organisation as well as to conserve species
in montane regions. This study examined the patterns of species richness,
abundance, composition, range sizes, and distribution of rainforest birds at 14
sites along an elevational gradient (500-1400 m) in the Kalakad-Mundanthurai
Tiger Reserve (KMTR) of the Western Ghats, India. In contrast to theoretical
expectation, resident bird species richness did not change significantly with
elevation although the species composition changed substantially (<10%
similarity) between the lowest and highest elevation sites. Constancy in
species richness was possibly due to relative constancy in productivity and
lack of elevational trends in vegetation structure. Elevational range size of
birds, expected to increase with elevation according to Rapoport's rule, was
found to show a contrasting inverse U-shaped pattern because species with
narrow elevational distributions, including endemics, occurred at both ends of
the gradient (below 800 m and above 1,200 m). Bird species composition also did
not vary randomly along the gradient as assessed using a hierarchy of null
models of community assembly, from completely unconstrained models to ones with
species richness and range-size distribution restrictions. Instead, bird
community composition was significantly correlated with elevation and tree
species composition of sites, indicating the influence of deterministic factors
on bird community structure. Conservation of low- and high-elevation areas and
maintenance of tree species composition against habitat alteration are
important for bird conservation in the southern Western Ghats rainforests.Comment: 36 pages, 5 figures, two tables (including one in the appendix)
Submitted to the Journal of the Bombay Natural History Society (JBNHS
Succinct Indexable Dictionaries with Applications to Encoding -ary Trees, Prefix Sums and Multisets
We consider the {\it indexable dictionary} problem, which consists of storing
a set for some integer , while supporting the
operations of \Rank(x), which returns the number of elements in that are
less than if , and -1 otherwise; and \Select(i) which returns
the -th smallest element in . We give a data structure that supports both
operations in O(1) time on the RAM model and requires bits to store a set of size , where {\cal B}(n,m) = \ceil{\lg
{m \choose n}} is the minimum number of bits required to store any -element
subset from a universe of size . Previous dictionaries taking this space
only supported (yes/no) membership queries in O(1) time. In the cell probe
model we can remove the additive term in the space bound,
answering a question raised by Fich and Miltersen, and Pagh.
We present extensions and applications of our indexable dictionary data
structure, including:
An information-theoretically optimal representation of a -ary cardinal
tree that supports standard operations in constant time,
A representation of a multiset of size from in bits that supports (appropriate generalizations of) \Rank
and \Select operations in constant time, and
A representation of a sequence of non-negative integers summing up to
in bits that supports prefix sum queries in constant
time.Comment: Final version of SODA 2002 paper; supersedes Leicester Tech report
2002/1
Quantum and Classical Dynamics of a BEC in a Large-Period Optical Lattice
We experimentally investigate diffraction of a Rb-87 Bose-Einstein condensate
from a 1D optical lattice. We use a range of lattice periods and timescales,
including those beyond the Raman-Nath limit. We compare the results to quantum
mechanical and classical simulations, with quantitative and qualitative
agreement, respectively. The classical simulation predicts that the envelope of
the time-evolving diffraction pattern is shaped by caustics: singularities in
the phase space density of classical trajectories. This behavior becomes
increasingly clear as the lattice period grows.Comment: 7 pages, 6 figure
Ultrasonic Velocity and other Parameters in Sulphuric Acid-Sodium Hydroxide Solutions
Ultrasonic velocity, adiabatic compressibility and refractive index have been determined in aqueous sulphuric acid=sodium hydroxide mixtures at different concentrations. Variation of these parameters with composition of the mixture was studied with a view of fix the end points of neutralization
The eigenspectra of Indian musical drums
In a family of drums used in the Indian subcontinent, the circular drum head
is made of material of non-uniform density. Remarkably, and in contrast to a
circular membrane of uniform density, the low eigenmodes of the non-uniform
membrane are harmonic. In this work we model the drum head by a non-uniform
membrane whose density varies smoothly between two prescribed values. Using a
Fourier-Chebyshev spectral collocation method we obtain the eigenmodes and
eigenvalues of the drum head. For a suitable choice of parameters, which we
find by optimising a cost function, the eigenspectra obtained from our model
are in excellent agreement with experimental values. Our model and the
numerical method should find application in numerical sound synthesis
A multienvironment conditional probability density function model for turbulent reacting flows
The multienvironment conditional probability density function (MECPDF) model was first proposed by Fox [Computational Models for Turbulent Reacting Flows (Cambridge University Press, Cambridge, 2003)] as a simple extension of multienvironment probability density function models for turbulent reacting flows. Like the conditional moment closure (CMC) and the laminar flamelet model (LFM), the MECPDF model describes the reacting scalars conditioned on the value of the mixture fraction. However, unlike CMC and LFM, the new model provides a consistent description of conditional fluctuations in both the scalar dissipation rate and the reacting scalars, and hence can be used to model partial extinction and reignition in homogeneous turbulent reacting flows. In this work, a general derivation of the MECPDF model is presented for a single reaction-progress variable using the direct quadrature method of moments. Extensions of the model to multiple reaction-progress variables and conditioning on the mixture-fraction vector are also discussed. After deriving the model, the closure assumptions are validated using direct simulations for pure diffusion of two randomly distributed, initially correlated scalar fields. Two homogeneous applications are then considered: nonreactive mixing starting from nontrivial initial conditions, and reactive mixing with partial extinction and reignition
Quantum thermodynamics at critical points during melting and solidification processes
We systematically explore and show the existence of finite-temperature
continuous quantum phase transition (CTQPT) at a critical point, namely, during
solidification or melting such that the first-order thermal phase transition is
a special case within CTQPT. Infact, CTQPT is related to chemical reaction
where quantum fluctuation (due to wavefunction transformation) is caused by
thermal energy and it can occur maximally for temperatures much higher than
zero Kelvin. To extract the quantity related to CTQPT, we use the ionization
energy theory and the energy-level spacing renormalization group method to
derive the energy-level spacing entropy, renormalized Bose-Einstein
distribution and the time-dependent specific heat capacity. This work
unambiguously shows that the quantum phase transition applies for any finite
temperatures.Comment: To be published in Indian Journal of Physics (Kolkata
- …