1,924 research outputs found
Framework for estimation of nacelle drag on isolated aero-engines with separate jets
Typically, the evaluation of nacelle drag in preliminary design is required to find an overall optimum engine cycle and flight trajectory. This work focuses on the drag characteristics of aero-engine nacelles with separate jet exhausts. The main body of analysis comes from 3D numerical simulations. A new near-field method to compute the post-exit force of a nacelle is presented and evaluated. The effects of the engine size, Mach number, mass-flow capture ratio and angle of attack are assessed. The results obtained from the numerical assessments were used to evaluate conventional reduced order models for the estimation of nacelle drag. Within this context, the effect of the engine size is typically estimated by the scaling ratio between the maximum areas and Reynolds numbers. The effect of the angle of attack on nacelle drag is mostly a function of the nacelle geometry and angle of attack. In general, typical low order models based on skin friction and form factor can underestimate the friction drag by up to 15% at cruise operating point. Similarly, reduced order models based solely on Reynolds number and Mach number can underestimate the overall nacelle drag by up to 74% for free stream Mach number larger than the drag rise Mach number
Two Boundaries Separate Borrelia burgdorferi Populations in North America
Understanding the spread of infectious diseases is crucial for implementing effective control measures. For this, it is important to obtain information on the contemporary population structure of a disease agent and to infer the evolutionary processes that may have shaped it. Here, we investigate on a continental scale the population structure of Borrelia burgdorferi, the causative agent of Lyme borreliosis (LB), a tick-borne disease, in North America. We test the hypothesis that the observed d population structure is congruent with recent population expansions and that these were preceded by bottlenecks mostly likely caused by the near extirpation in the 1900s of hosts required for sustaining tick populations. Multilocus sequence typing and complementary population analytical tools were used to evaluate B. burgdorferi samples collected in the Northeastern, Upper Midwestern, and Far-Western United States and Canada. The spatial distribution of sequence types (STs) and inferred population boundaries suggest that the current populations are geographically separated. One major population boundary separated western B. burgdorferi populations transmitted by Ixodes pacificus in California from Eastern populations transmitted by I. scapularis; the other divided Midwestern and Northeastern populations. However, populations from all three regions were genetically closely related. Together, our findings suggest that although the contemporary populations of North American B. burgdorferi now com- prise three geographically separated subpopulations with no or limited gene flow among them, they arose from a common ancestral population. A comparative analysis of the B. burgdorferi outer surface protein C (ospC) gene revealed novel linkages and provides additional insights into the genetic characteristics of strains
Domain Walls Motion and Resistivity in a Fully-Frustrated Josephson Array
It is identified numerically that the resistivity of a fully-frustrated
Josephson-junction array is due to motion of domain walls in vortex lattice
rather than to motion of single vortices
Phase transitions in a frustrated XY model with zig-zag couplings
We study a new generalized version of the square-lattice frustrated XY model
where unequal ferromagnetic and antiferromagnetic couplings are arranged in a
zig-zag pattern. The ratio between the couplings can be used to tune the
system, continuously, from the isotropic square-lattice to the
triangular-lattice frustrated XY model. The model can be physically realized as
a Josephson-junction array with two different couplings, in a magnetic field
corresponding to half-flux quanta per plaquette. Mean-field approximation,
Ginzburg-Landau expansion and finite-size scaling of Monte Carlo simulations
are used to study the phase diagram and critical behavior. Depending on the
value of , two separate transitions or a transition line in the
universality class of the XY-Ising model, with combined and U(1)
symmetries, takes place. In particular, the phase transitions of the standard
square-lattice and triangular-lattice frustrated XY models correspond to two
different cuts through the same transition line. Estimates of the chiral
() critical exponents on this transition line deviate significantly from
the pure Ising values, consistent with that along the critical line of the
XY-Ising model. This suggests that a frustrated XY model or Josephson-junction
array with a zig-zag coupling modulation can provide a physical realization of
the XY-Ising model critical line.Comment: 11 pages, 9 figures, RevTex, to appear in Phys. Rev.
Dynamic Approach to the Fully Frustrated XY Model
Using Monte Carlo simulations, we systematically investigate the
non-equilibrium dynamics of the chiral degree of freedom in the two-dimensional
fully frustrated XY model. The critical initial increase of the staggered
chiral magnetization is observed. By means of the short-time dynamics approach,
we estimate the second order phase transition temperature and all the
dynamic and static critical exponents , z, and .Comment: 5 pages with 6 figures include
Chiral phase transitions: focus driven critical behavior in systems with planar and vector ordering
The fixed point that governs the critical behavior of magnets described by
the -vector chiral model under the physical values of () is
shown to be a stable focus both in two and three dimensions. Robust evidence in
favor of this conclusion is obtained within the five-loop and six-loop
renormalization-group analysis in fixed dimension. The spiral-like approach of
the chiral fixed point results in unusual crossover and near-critical regimes
that may imitate varying critical exponents seen in physical and computer
experiments.Comment: 4 pages, 5 figures. Discussion enlarge
Tricritical behavior of the frustrated XY antiferromagnet
Extensive histogram Monte-Carlo simulations of the XY antiferromagnet on a
stacked triangular lattice reveal exponent estimates which strongly favor a
scenario of mean-field tricritical behavior for the spin-order transition. The
corresponding chiral-order transition occurs at the same temperature but
appears to be decoupled from the spin-order. These results are relevant to a
wide class of frustrated systems with planar-type order and serve to resolve a
long-standing controversy regarding their criticality.Comment: J1K 2R1 4 pages (RevTex 3.0), 4 figures available upon request,
Report# CRPS-94-0
High Field ESR and Magnetization of the Triangular Lattice Antiferromagnet NiGa2S4
We report the experimental and the analytical results of electron spin
resonance (ESR) and magnetization in high magnetic fields up to about 68 T of
the quasi two-dimensional triangular lattice antiferromagnet NiGaS.
From the temperature evolution of the ESR absorption linewidth, we find a
distinct disturbing of the development of the spin correlation by
-vortices between 23 K and 8.5 K. Below K, spin-wave
calculations based on a 57 spiral spin order well explains the
frequency dependence of the ESR resonance fields and high field magnetization
processes for and , although the magnetization
for at high fields is different from the calculated one.
Furthermore, we explain the field independent specific heat with
-dependence by the same spin-wave calculation, but the magnitude of the
specific heat is much less than the observed one. Accordingly, these results
suggest the occurrence of a vortex-induced topological transition at
and may indicate quantum effects beyond the descriptions based on
the above classical spin models
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