4,429 research outputs found
Hypersonic Buckshot: Astrophysical Jets as Heterogeneous Collimated Plasmoids
Herbig-Haro (HH) jets are commonly thought of as homogeneous beams of plasma
traveling at hypersonic velocities. Structure within jet beams is often
attributed to periodic or ``pulsed'' variations of conditions at the jet
source. Simulations based on this scenario result in knots extending across the
jet diameter. Observations and recent high energy density laboratory
experiments shed new light on structures below this scale and indicate they may
be important for understanding the fundamentals of jet dynamics. In this paper
we offer an alternative to ``pulsed'' models of protostellar jets. Using direct
numerical simulations we explore the possibility that jets are chains of
sub-radial clumps propagating through a moving inter-clump medium. Our models
explore an idealization of this scenario by injecting small (),
dense () spheres embedded in an otherwise smooth inter-clump
jet flow. The spheres are initialized with velocities differing from the jet
velocity by %. We find the consequences of shifting from homogeneous to
heterogeneous flows are significant as clumps interact with each other and with
the inter-clump medium in a variety of ways. Structures which mimic what is
expected from pulsed-jet models can form, as can previously unseen
``sub-radial'' behaviors including backward facing bow shocks and off-axis
working surfaces. While these small-scale structures have not been seen before
in simulation studies, they are found in high resolution jet observations. We
discuss implications of our simulations for the interpretation of protostellar
jets with regard to characterization of knots by a ``lifetime'' or ``velocity
history'' approach as well as linking observed structures with central engines
which produce the jets.Comment: 15 pages, 3 figures (1 color), submitted to Ap
MHD simulations of the formation and propagation of protostellar jets to observational length scales
We present 2.5-D global, ideal MHD simulations of magnetically and
rotationally driven protostellar jets from Keplerian accretion discs, wherein
only the initial magnetic field strength at the inner radius of the disc,
, is varied. Using the AMR-MHD code AZEUS, we self-consistently
follow the jet evolution into the observational regime ()
with a spatial dynamic range of . The simulations reveal a
three-component outflow: 1) A hot, dense, super-fast and highly magnetised 'jet
core'; 2) a cold, rarefied, trans-fast and highly magnetised 'sheath'
surrounding the jet core and extending to a tangential discontinuity; and 3) a
warm, dense, trans-slow and weakly magnetised shocked ambient medium entrained
by the advancing bow shock. The simulations reveal power-law relationships
between and the jet advance speed, , the average jet
rotation speed, , as well as fluxes of mass,
momentum, and kinetic energy. Quantities that do not depend on
include the plasma- of the transported material which, in all cases,
seems to asymptote to order unity. Jets are launched by a combination of the
'magnetic tower' and 'bead-on-a-wire' mechanisms, with the former accounting
for most of the jet acceleration---even for strong fields---and continuing well
beyond the fast magnetosonic point. At no time does the leading bow shock leave
the domain and, as such, these simulations generate large-scale jets that
reproduce many of the observed properties of protostellar jets including their
characteristic speeds and transported fluxes.Comment: 26 pages, 16 figures. Accepted for publication in MNRA
IGA-based Multi-Index Stochastic Collocation for random PDEs on arbitrary domains
This paper proposes an extension of the Multi-Index Stochastic Collocation
(MISC) method for forward uncertainty quantification (UQ) problems in
computational domains of shape other than a square or cube, by exploiting
isogeometric analysis (IGA) techniques. Introducing IGA solvers to the MISC
algorithm is very natural since they are tensor-based PDE solvers, which are
precisely what is required by the MISC machinery. Moreover, the
combination-technique formulation of MISC allows the straight-forward reuse of
existing implementations of IGA solvers. We present numerical results to
showcase the effectiveness of the proposed approach.Comment: version 3, version after revisio
The Evolution of Hetergeneous "Clumpy Jets": A Parameter Study
We investigate the role discrete clumps embedded in an astrophysical jet play
on the jet's morphology and line emission characteristics. By varying clumps'
size, density, position, and velocity, we cover a range of parameter space
motivated by observations of objects such as the Herbig Haro object HH~34. We
here extend the results presented in Yirak et al. 2009, including how analysis
of individual observations may lead to spurious sinusoidal variation whose
parameters vary widely over time, owing chiefly to interacts between clumps.
The goodness of the fits, while poor in all simulations, are best when
clump-clump collisions are minimal. Our results indicate that a large velocity
dispersion leads to a clump-clump collision-dominated flow which disrupts the
jet beam. Finally, we present synthetic emission images of H- and [SII]
and note an excess of [SII] emission along the jet length as compared to
observations. This suggests that observed beams undergo earlier processing, if
they are present at all.Comment: 18 pages, 7 figures. Submitted to the Astrophysical Journa
Fixed gain and adaptive techniques for rotorcraft vibration control
The results of an analysis effort performed to demonstrate the feasibility of employing approximate dynamical models and frequency shaped cost functional control law desgin techniques for helicopter vibration suppression are presented. Both fixed gain and adaptive control designs based on linear second order dynamical models were implemented in a detailed Rotor Systems Research Aircraft (RSRA) simulation to validate these active vibration suppression control laws. Approximate models of fuselage flexibility were included in the RSRA simulation in order to more accurately characterize the structural dynamics. The results for both the fixed gain and adaptive approaches are promising and provide a foundation for pursuing further validation in more extensive simulation studies and in wind tunnel and/or flight tests
Restructurable Controls
Restructurable control system theory, robust reconfiguration for high reliability and survivability for advanced aircraft, restructurable controls problem definition and research, experimentation, system identification methods applied to aircraft, a self-repairing digital flight control system, and state-of-the-art theory application are addressed
T-spline based unifying registration procedure for free-form surface workpieces in intelligent CMM
With the development of the modern manufacturing industry, the free-form surface is widely used in various fields, and the automatic detection of a free-form surface is an important function of future intelligent three-coordinate measuring machines (CMMs). To improve the intelligence of CMMs, a new visual system is designed based on the characteristics of CMMs. A unified model of the free-form surface is proposed based on T-splines. A discretization method of the T-spline surface formula model is proposed. Under this discretization, the position and orientation of the workpiece would be recognized by point cloud registration. A high accuracy evaluation method is proposed between the measured point cloud and the T-spline surface formula. The experimental results demonstrate that the proposed method has the potential to realize the automatic detection of different free-form surfaces and improve the intelligence of CMMs
A segmentation-free isogeometric extended mortar contact method
This paper presents a new isogeometric mortar contact formulation based on an
extended finite element interpolation to capture physical pressure
discontinuities at the contact boundary. The so called two-half-pass algorithm
is employed, which leads to an unbiased formulation and, when applied to the
mortar setting, has the additional advantage that the mortar coupling term is
no longer present in the contact forces. As a result, the computationally
expensive segmentation at overlapping master-slave element boundaries, usually
required in mortar methods (although often simplified with loss of accuracy),
is not needed from the outset. For the numerical integration of general contact
problems, the so-called refined boundary quadrature is employed, which is based
on adaptive partitioning of contact elements along the contact boundary. The
contact patch test shows that the proposed formulation passes the test without
using either segmentation or refined boundary quadrature. Several numerical
examples are presented to demonstrate the robustness and accuracy of the
proposed formulation.Comment: In this version, we have removed the patch test comparison with the
classical mortar method and removed corresponding statements. They will be
studied in further detail in future work, so that the focus is now entirely
on the new IGA mortar formulatio
Adaptive feedforward control design for gust loads alleviation and LCO suppression
An adaptive feedforward controller is designed for gust loads alleviation and limit cycle oscillations suppression. Two sets of basis functions, based on the finite impulse response and modified finite impulse response approaches, are investigated to design the controller for gust loads alleviation. Limit cycle oscillations suppression is shown by using the modified finite impulse response controller. Worst case gust search is performed by using a nonlinear technique of model reduction to speed up the costs of calculations. Both the “one–minus–cosine” and Von Kármán continuous turbulence gusts of different intensities were generated to examine the performance of controllers. The responses of these two types of gust can be reduced effectively by finite impulse response controller in the whole process, while the modified finite impulse response controller is found to increase the loads during the initial transient response. The above two types of gust induced limit cycle oscillations were used to test the modified finite impulse response controller. Results show that it can suppress limit cycle oscillations to some exten
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