8,220 research outputs found
A Navier Stokes Phase Field Crystal Model for Colloidal Suspensions
We develop a fully continuous model for colloidal suspensions with
hydrodynamic interactions. The Navier Stokes Phase Field Crystal (NS-PFC) model
combines ideas of dynamic density functional theory with particulate flow
approaches and is derived in detail and related to other dynamic density
functional theory approaches with hydrodynamic interactions. The derived system
is numerically solved using adaptive finite elements and used to analyse
colloidal crystallization in flowing environments demonstrating a strong
coupling in both directions between the crystal shape and the flow field. We
further validate the model against other computational approaches for
particulate flow systems for various colloidal sedimentation problems
Can planetesimals form by collisional fusion?
As a test bed for the growth of protoplanetary bodies in a turbulent
circumstellar disk we examine the fate of a boulder using direct numerical
simulations of particle seeded gas flowing around it. We provide an accurate
description of the flow by imposing no-slip and non-penetrating boundary
conditions on the boulder surface using the immersed boundary method pioneered
by Peskin (2002). Advected by the turbulent disk flow, the dust grains collide
with the boulder and we compute the probability density function (PDF) of the
normal component of the collisional velocity. Through this examination of the
statistics of collisional velocities we test the recently developed concept of
collisional fusion which provides a physical basis for a range of collisional
velocities exhibiting perfect sticking. A boulder can then grow sufficiently
rapidly to settle into a Keplerian orbit on disk evolution time scales.Comment: Astrophysical Journal, in pres
Rigid body mode identification of the PAH-2 helicopter using the eigensystem realization algorithm
The rigid body modes of the PAH-2 'Tiger' helicopter were identified using the Eigensystem Realization Algorithm (ERA). This work complements ground vibration tests performed using DLR's traditional phase resonance technique and the ISSPA (Identification of Structural System Parameters) method. Rigid body modal parameters are important for ground resonance prediction. Time-domain data for ERA were obtained by inverse Fourier transformation of frequency response functions measured with stepped-sine excitation. Mode purity (based on the Phase Resonance Criterion) was generally equal to or greater than corresponding results obtained in the ground vibration tests. All identified natural frequencies and mode shapes correlate well with corresponding ground vibration test results. The modal identification approach discussed in this report has become increasingly attractive in recent years due to the steadily declining cost and increased performance of scientific computers. As illustrated in this application, modern time-domain methods can be successfully applied to data acquired using DLR's existing test equipment. Some suggestions are made for future applications of time domain modal identification in this manner
CDM afforestation and reforestation baseline methodologies: An analysis of the submission and approval process
Afforestation and Reforestation (A/R), also widely termed LULUCF have been an important field of conflict in the Clean Development Mechanism (CDM) of the Kyoto Protocol. The first methodology for A/R projects has been submitted only by October 2004 and the first project was registered only in November 2006, two years after the first project in the energy sector. Like energy efficiency and transportation methodologies, A/R methodologies also suffer high rejection rate. 20 A/R CDM methodologies evaluated by the CDM Executive Board have been analyzed in this paper with respect to their approval history. On an average it took 4-5 months for approval of A/R methodologies in contrast to the long approval time taken in case of other methodologies (9-10 months). Most methodologies has been rejected because of not properly defining land eligibility, incomplete baseline scenario selection, lack of methods to prove additionality and insufficient treatment of uncertainties. --
ENSO suppression due to weakening of the North Atlantic thermohaline circulation
Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections
Eigensystem realization algorithm modal identification experiences with mini-mast
This paper summarizes work performed under a collaborative research effort between the National Aeronautics and Space Administration (NASA) and the German Aerospace Research Establishment (DLR, Deutsche Forschungsanstalt fur Luft- und Raumfahrt). The objective is to develop and demonstrate system identification technology for future large space structures. Recent experiences using the Eigensystem Realization Algorithm (ERA), for modal identification of Mini-Mast, are reported. Mini-Mast is a 20 m long deployable space truss used for structural dynamics and active vibration-control research at the Langley Research Center. A comprehensive analysis of 306 frequency response functions (3 excitation forces and 102 displacement responses) was performed. Emphasis is placed on two topics of current research: (1) gaining an improved understanding of ERA performance characteristics (theory vs. practice); and (2) developing reliable techniques to improve identification results for complex experimental data. Because of nonlinearities and numerous local modes, modal identification of Mini-Mast proved to be surprisingly difficult. Methods were available, ERA, for obtaining detailed, high-confidence results
The gauge structure of generalised diffeomorphisms
We investigate the generalised diffeomorphisms in M-theory, which are gauge
transformations unifying diffeomorphisms and tensor gauge transformations.
After giving an En(n)-covariant description of the gauge transformations and
their commutators, we show that the gauge algebra is infinitely reducible,
i.e., the tower of ghosts for ghosts is infinite. The Jacobiator of generalised
diffeomorphisms gives such a reducibility transformation. We give a concrete
description of the ghost structure, and demonstrate that the infinite sums give
the correct (regularised) number of degrees of freedom. The ghost towers belong
to the sequences of rep- resentations previously observed appearing in tensor
hierarchies and Borcherds algebras. All calculations rely on the section
condition, which we reformulate as a linear condition on the cotangent
directions. The analysis holds for n < 8. At n = 8, where the dual gravity
field becomes relevant, the natural guess for the gauge parameter and its
reducibility still yields the correct counting of gauge parameters.Comment: 24 pp., plain tex, 1 figure. v2: minor changes, including a few added
ref
Properties of - and -modes in hydromagnetic turbulence
With the ultimate aim of using the fundamental or -mode to study
helioseismic aspects of turbulence-generated magnetic flux concentrations, we
use randomly forced hydromagnetic simulations of a piecewise isothermal layer
in two dimensions with reflecting boundaries at top and bottom. We compute
numerically diagnostic wavenumber-frequency diagrams of the vertical velocity
at the interface between the denser gas below and the less dense gas above. For
an Alfv\'en-to-sound speed ratio of about 0.1, a 5% frequency increase of the
-mode can be measured when -, where is the
horizontal wavenumber and is the pressure scale height at the
surface. Since the solar radius is about 2000 times larger than ,
the corresponding spherical harmonic degree would be 6000-8000. For weaker
fields, a -dependent frequency decrease by the turbulent motions becomes
dominant. For vertical magnetic fields, the frequency is enhanced for
, but decreased relative to its nonmagnetic value for
.Comment: 17 pages, 22 figures, Version accepted in MNRA
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