18,075 research outputs found
The economic impact of drag in general aviation
General aviation aircraft fuel consumption and operating costs are closely linked to drag reduction methods. Improvements in airplane drag are envisioned for new models; their effects will be in the 5 to 10% range. Major improvements in fuel consumption over existing turbofan airplanes will be the combined results of improved aerodynamics plus additional effects from advanced turbofan engine designs
Overview of external Nacelle drag and interference drag
A historical view of multi-jet engine installations is given that emphasizes integration of the powerplant and the airframe in aircraft design for improved reduction in external nacelle drag and interference drag characteristics
The Bar--Halo Interaction--I. From Fundamental Dynamics to Revised N-body Requirements
Only through resonances can non-axisymmetric features such as spiral arms and
bars exert torques over large scales and change the overall structure of a
near-equilibrium galaxy. We describe the resonant interaction mechanism in
detail and derive explicit criteria for the particle number required to
simulate these dynamical processes accurately using N-body simulations and
illustrate them with numerical experiments. To do this, we perform direct
numerical solution of perturbation theory and make detailed comparisons with
N-body simulations. The criteria include: sufficient particle coverage in phase
space near the resonance and enough particles to minimize gravitational
potential fluctuations that will change the dynamics of the resonant encounter.
Some of our more surprising findings are as follows. First, the
Inner-Lindblad-like resonance (ILR), responsible for coupling the bar to the
central halo cusp, requires almost 10^9 equal mass particles within the virial
radius for a Milky-Way-like bar in an NFW profile. Second, orbits that linger
near the resonance receive more angular momentum than orbits that move through
the resonance quickly. Small-scale fluctuations present in state-of-the-art
particle-particle simulations can knock orbits out of resonance, preventing
them from lingering and, thereby, decrease the torque. The required particle
numbers are sufficiently high for scenarios of interest that apparent
convergence in particle number is misleading: the convergence is in the
noise-dominated regime. State-of-the-art simulations are not adequate to follow
all aspects of secular evolution driven by the bar-halo interaction. We present
a procedure to test the requirements for individual N-body codes for the actual
problem of interest. [abridged]Comment: 30 pages, 19 figures, submitted to Monthly Notices. For paper with
figures at full resolution:
http://www.astro.umass.edu/~weinberg/weinberg_katz_1.ps.g
The Bar-Halo Interaction - II. Secular evolution and the religion of N-body simulations
This paper explores resonance-driven secular evolution between a bar and
dark-matter halo using N-body simulations. We make direct comparisons to our
analytic theory (Weinberg & Katz 2005) to demonstrate the great difficulty that
an N-body simulation has representing these dynamics for realistic astronomical
interactions. In a dark-matter halo, the bar's angular momentum is coupled to
the central density cusp (if present) by the Inner Lindblad Resonance. Owing to
this angular momentum transfer and self-consistent re-equilibration, strong
realistic bars WILL modify the cusp profile, lowering the central densities
within about 30% of the bar radius in a few bar orbits. Past results to the
contrary (Sellwood 2006, McMillan & Dehnen 2005) may be the result of weak bars
or numerical artifacts. The magnitude depends on many factors and we illustrate
the sensitivity of the response to the dark-matter profile, the bar shape and
mass, and the galaxy's evolutionary history. For example, if the bar length is
comparable to the size of a central dark-matter core, the bar may exchange
angular momentum without changing its pattern speed significantly. We emphasise
that this apparently simple example of secular evolution is remarkably subtle
in detail and conclude that an N-body exploration of any astronomical scenario
requires a deep investigation into the underlying dynamical mechanisms for that
particular problem to set the necessary requirements for the simulation
parameters and method (e.g. particle number and Poisson solver). Simply put,
N-body simulations do not divinely reveal truth and hence their results are not
infallible. They are unlikely to provide useful insight on their own,
particularly for the study of even more complex secular processes such as the
production of pseudo-bulges and disk heating.Comment: 23 pages, 18 figures, submitted to Monthly Notices. For paper with
figures at full resolution:
http://www.astro.umass.edu/~weinberg/weinberg_katz_2.ps.g
Crises in The Global Economy from Tulips to Today: Contagion and Consequences
We examine the historical record of the financial crises that have often accompanied surges of globalization in the past. The issue of contagion, the spread of financial turbulence from the crisis center to its trading partners, is confronted with historical and statistical evidence on the causes and consequences of well-known crises. Special attention is given to the gold standard period of 1880-1913, which we find useful to divide into the initial period of deflation, 1880-1896, and the following period of mild inflation, 1897-1913. We find evidence of changes in the pattern of "contagion" from core to periphery countries between the two periods, finding that apparent contagions can more readily be interpreted as responses to common shocks. Lessons for the present period can only be tentative, but the similarities in learning experiences are striking.contagion; gold standard
Learjet model 25 drag analysis
Procedures and data for estimating drag were used to calculate the drag characteristics of the Model 25 airplane. Based on cruise flight test data obtained on the Model 25, these methods generally predicted the total drag characteristics within current acceptable and reasonable engineering accuracy
A remarkably simple and accurate method for computing the Bayes Factor from a Markov chain Monte Carlo Simulation of the Posterior Distribution in high dimension
Weinberg (2012) described a constructive algorithm for computing the marginal
likelihood, Z, from a Markov chain simulation of the posterior distribution.
Its key point is: the choice of an integration subdomain that eliminates
subvolumes with poor sampling owing to low tail-values of posterior
probability. Conversely, this same idea may be used to choose the subdomain
that optimizes the accuracy of Z. Here, we explore using the simulated
distribution to define a small region of high posterior probability, followed
by a numerical integration of the sample in the selected region using the
volume tessellation algorithm described in Weinberg (2012). Even more promising
is the resampling of this small region followed by a naive Monte Carlo
integration. The new enhanced algorithm is computationally trivial and leads to
a dramatic improvement in accuracy. For example, this application of the new
algorithm to a four-component mixture with random locations in 16 dimensions
yields accurate evaluation of Z with 5% errors. This enables Bayes-factor model
selection for real-world problems that have been infeasible with previous
methods.Comment: 14 pages, 3 figures, submitted to Bayesian Analysi
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