18,266 research outputs found
Computer simulation of on-orbit manned maneuvering unit operations
Simulation of spacecraft on-orbit operations is discussed in reference to Martin Marietta's Space Operations Simulation laboratory's use of computer software models to drive a six-degree-of-freedom moving base carriage and two target gimbal systems. In particular, key simulation issues and related computer software models associated with providing real-time, man-in-the-loop simulations of the Manned Maneuvering Unit (MMU) are addressed with special attention given to how effectively these models and motion systems simulate the MMU's actual on-orbit operations. The weightless effects of the space environment require the development of entirely new devices for locomotion. Since the access to space is very limited, it is necessary to design, build, and test these new devices within the physical constraints of earth using simulators. The simulation method that is discussed here is the technique of using computer software models to drive a Moving Base Carriage (MBC) that is capable of providing simultaneous six-degree-of-freedom motions. This method, utilized at Martin Marietta's Space Operations Simulation (SOS) laboratory, provides the ability to simulate the operation of manned spacecraft, provides the pilot with proper three-dimensional visual cues, and allows training of on-orbit operations. The purpose here is to discuss significant MMU simulation issues, the related models that were developed in response to these issues and how effectively these models simulate the MMU's actual on-orbiter operations
Interactions of Satellite Galaxies in Cosmological Dark Matter Halos
We present a statistical analysis of the interactions between satellite
galaxies in cosmological dark matter halos taken from fully self-consistent
high-resolution simulations of galaxy clusters. We show that the number
distribution of satellite encounters has a tail that extends to as many as 3-4
encounters per orbit. On average 30% of the substructure population had at
least one encounter (per orbit) with another satellite galaxy. However, this
result depends on the age of the dark matter host halo with a clear trend for
more interactions in younger systems. We also report a correlation between the
number of encounters and the distance of the satellites to the centre of the
cluster: satellite galaxies closer to the centre experience more interactions.
However, this can be simply explained by the radial distribution of the
substructure population and merely reflects the fact that the density of
satellites is higher in those regions.
In order to find substructure galaxies we applied (and present) a new
technique based upon the N-body code MLAPM. This new halo finder MHF
(MLAPM's-Halo-Finder) acts with exactly the same accuracy as the N-body code
itself and is therefore free of any bias and spurious mismatch between
simulation data and halo finding precision related to numerical effects.Comment: 6 pages, 4 figures, accepted by PASA (refereed contribution to the
5th Galactic Chemodynamics workshop, July 2003
The plastic limit of clays
The plastic limit of soils was first described by Atterberg in 1911. The thread-rolling test was standardised at the US Public Roads Bureau in the 1920s and 1930s, and has subsequently become one of the standard tests of soil mechanics. This paper reviews the original definitions of plastic limit as proposed by Atterberg, and proposes that the brittle failure observed in the plastic limit test is caused by either air entry or cavitation in the clay. Critical state soil mechanics is used to show that the observed range of undrained shear strengths of soils at plastic limit is consistent with this hypothesis. The fallacy that strength at plastic limit is a constant is highlighted, and the implications for geotechnical practice are discussed. </jats:p
Well-Posedness And Accuracy Of The Ensemble Kalman Filter In Discrete And Continuous Time
The ensemble Kalman filter (EnKF) is a method for combining a dynamical model
with data in a sequential fashion. Despite its widespread use, there has been
little analysis of its theoretical properties. Many of the algorithmic
innovations associated with the filter, which are required to make a useable
algorithm in practice, are derived in an ad hoc fashion. The aim of this paper
is to initiate the development of a systematic analysis of the EnKF, in
particular to do so in the small ensemble size limit. The perspective is to
view the method as a state estimator, and not as an algorithm which
approximates the true filtering distribution. The perturbed observation version
of the algorithm is studied, without and with variance inflation. Without
variance inflation well-posedness of the filter is established; with variance
inflation accuracy of the filter, with resepct to the true signal underlying
the data, is established. The algorithm is considered in discrete time, and
also for a continuous time limit arising when observations are frequent and
subject to large noise. The underlying dynamical model, and assumptions about
it, is sufficiently general to include the Lorenz '63 and '96 models, together
with the incompressible Navier-Stokes equation on a two-dimensional torus. The
analysis is limited to the case of complete observation of the signal with
additive white noise. Numerical results are presented for the Navier-Stokes
equation on a two-dimensional torus for both complete and partial observations
of the signal with additive white noise
Mapping Substructures in Dark Matter Halos
We present a detailed study of the real and integrals-of-motion space
distributions of a satellite obtained from a self-consistent high-resolution
simulation of a galaxy cluster and re-simulated using various analytical halo
potentials. We found that the disrupted satellite appears as a coherent
structure in integrals-of-motion space in all models (``live'' and analytical
potential) although the distribution is significantly smeared for the live host
halo. Further the primary mechanism for this smearing is the mass growth of the
host, which changes both the energy and angular momentum of the satellite
debris. Hence, this must be considered when searching for (stellar) streams
with future observational experiments such as RAVE and GAIA.Comment: 5 pages, 6 figures, MNRAS accepted - minor editing without changing
the conclusions, a high-resolution version of the paper is available from
http://astronomy.swin.edu.au/~sgill/downloads/downloads.htm
Stability of Filters for the Navier-Stokes Equation
Data assimilation methodologies are designed to incorporate noisy
observations of a physical system into an underlying model in order to infer
the properties of the state of the system. Filters refer to a class of data
assimilation algorithms designed to update the estimation of the state in a
on-line fashion, as data is acquired sequentially. For linear problems subject
to Gaussian noise filtering can be performed exactly using the Kalman filter.
For nonlinear systems it can be approximated in a systematic way by particle
filters. However in high dimensions these particle filtering methods can break
down. Hence, for the large nonlinear systems arising in applications such as
weather forecasting, various ad hoc filters are used, mostly based on making
Gaussian approximations. The purpose of this work is to study the properties of
these ad hoc filters, working in the context of the 2D incompressible
Navier-Stokes equation. By working in this infinite dimensional setting we
provide an analysis which is useful for understanding high dimensional
filtering, and is robust to mesh-refinement. We describe theoretical results
showing that, in the small observational noise limit, the filters can be tuned
to accurately track the signal itself (filter stability), provided the system
is observed in a sufficiently large low dimensional space; roughly speaking
this space should be large enough to contain the unstable modes of the
linearized dynamics. Numerical results are given which illustrate the theory.
In a simplified scenario we also derive, and study numerically, a stochastic
PDE which determines filter stability in the limit of frequent observations,
subject to large observational noise. The positive results herein concerning
filter stability complement recent numerical studies which demonstrate that the
ad hoc filters perform poorly in reproducing statistical variation about the
true signal
R-Mode Oscillations in Rotating Magnetic Neutron Stars
We show that r-mode oscillations distort the magnetic fields of neutron stars
and that their occurrence is likely to be limited by this interaction. If the
field is gtrsim 10^{16} (Omega/Omega_B) G, where Omega and Omega_B are the
angular velocities of the star and at which mass shedding occurs, r-mode
oscillations cannot occur. Much weaker fields will prevent gravitational
radiation from exciting r-mode oscillations or damp them on a relatively short
timescale by extracting energy from the modes faster than gravitational wave
emission can pump energy into them. For example, a 10^{10} G poloidal magnetic
field that threads the star's superconducting core is likely to prevent the
ell=2 mode from being excited unless Omega exceeds 0.35 Omega_B. If Omega is
larger than 0.35 Omega_B initially, the ell=2 mode may be excited but is likely
to decay rapidly once Omega falls below 0.35 Omega_B, which happens in lesssim
15^d if the saturation amplitude is gtrsim 0.1. The r-mode oscillations may
play an important role in determining the structure of neutron star magnetic
fields.Comment: 4 pages, 1 postscript figure, uses emulateapj; submitted to ApJ
Letters 1999 Nov 8; accepted 2000 Jan 25; this version is essentially
identical to the original version except that Figure 2 was deleted in order
to fit within the ApJ Letters page limi
Cosmology on a Mesh
An adaptive multi grid approach to simulating the formation of structure from
collisionless dark matter is described. MLAPM (Multi-Level Adaptive Particle
Mesh) is one of the most efficient serial codes available on the cosmological
'market' today. As part of Swinburne University's role in the development of
the Square Kilometer Array, we are implementing hydrodynamics, feedback, and
radiative transfer within the MLAPM adaptive mesh, in order to simulate
baryonic processes relevant to the interstellar and intergalactic media at high
redshift. We will outline our progress to date in applying the existing MLAPM
to a study of the decay of satellite galaxies within massive host potentials.Comment: 3 pages, 2 figures, to appear in the proceedings of "The IGM/Galaxy
Connection - The Distribution of Baryons at z=0", ed. M. Putman & J.
Rosenber
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