9,672 research outputs found
Ring-cusp ion thruster with shell anode
An improved ion thruster for low specific impulse operation in the 1500 sec to 6000 sec range has a multicusp boundary field provided by high strength magnets on an iron anode shell which lengthens the paths of electrons from a hollow cathode assembly. A downstream anode pole piece in the form of an iron ring supports a ring of magnets to provide a more uniform beam profile. A cylindrical cathode magnet can be moved selectively in an axial direction along a feed tube to produce the desired magnetic field at the cathode tip
Orthotropic rotation-free thin shell elements
A method to simulate orthotropic behaviour in thin shell finite elements is
proposed. The approach is based on the transformation of shape function
derivatives, resulting in a new orthogonal basis aligned to a specified
preferred direction for all elements. This transformation is carried out solely
in the undeformed state leaving minimal additional impact on the computational
effort expended to simulate orthotropic materials compared to isotropic,
resulting in a straightforward and highly efficient implementation. This method
is implemented for rotation-free triangular shells using the finite element
framework built on the Kirchhoff--Love theory employing subdivision surfaces.
The accuracy of this approach is demonstrated using the deformation of a
pinched hemispherical shell (with a 18{\deg} hole) standard benchmark. To
showcase the efficiency of this implementation, the wrinkling of orthotropic
sheets under shear displacement is analyzed. It is found that orthotropic
subdivision shells are able to capture the wrinkling behavior of sheets
accurately for coarse meshes without the use of an additional wrinkling model.Comment: 10 pages, 8 figure
Collapse of orthotropic spherical shells
We report on the buckling and subsequent collapse of orthotropic elastic
spherical shells under volume and pressure control. Going far beyond what is
known for isotropic shells, a rich morphological phase space with three
distinct regimes emerges upon variation of shell slenderness and degree of
orthotropy. Our extensive numerical simulations are in agreement with
experiments using fabricated polymer shells. The shell buckling pathways and
corresponding strain energy evolution are shown to depend strongly on material
orthotropy. We find surprisingly robust orthotropic structures with strong
similarities to stomatocytes and tricolpate pollen grains, suggesting that the
shape of several of Nature's collapsed shells could be understood from the
viewpoint of material orthotropy.Comment: 7 pages, 5 figure
Subdivision Shell Elements with Anisotropic Growth
A thin shell finite element approach based on Loop's subdivision surfaces is
proposed, capable of dealing with large deformations and anisotropic growth. To
this end, the Kirchhoff-Love theory of thin shells is derived and extended to
allow for arbitrary in-plane growth. The simplicity and computational
efficiency of the subdivision thin shell elements is outstanding, which is
demonstrated on a few standard loading benchmarks. With this powerful tool at
hand, we demonstrate the broad range of possible applications by numerical
solution of several growth scenarios, ranging from the uniform growth of a
sphere, to boundary instabilities induced by large anisotropic growth. Finally,
it is shown that the problem of a slowly and uniformly growing sheet confined
in a fixed hollow sphere is equivalent to the inverse process where a sheet of
fixed size is slowly crumpled in a shrinking hollow sphere in the frictionless,
quasi-static, elastic limit.Comment: 20 pages, 12 figures, 1 tabl
Aging to Equilibrium Dynamics of SiO2
Molecular dynamics computer simulations are used to study the aging dynamics
of SiO2 (modeled by the BKS model). Starting from fully equilibrated
configurations at high temperatures T_i =5000K/3760K the system is quenched to
lower temperatures T_f=2500K, 2750K, 3000K, 3250K and observed after a waiting
time t_w. Since the simulation runs are long enough to reach equilibrium at
T_f, we are able to study the transition from out-of-equilibrium to equilibrium
dynamics. We present results for the partial structure factors, for the
generalized incoherent intermediate scattering function C_q(t_w, t_w+t), and
for the mean square displacement msd(t_w,t_w+t). We conclude that there are
three different t_w regions: (I) At very short waiting times, C_q(t_w, t_w+t)
decays very fast without forming a plateau. Similarly msd(t_w,t_w+t) increases
without forming a plateau. (II) With increasing t_w a plateau develops in
C_q(t_w, t_w+t) and msd(t_w,t_w+t). For intermediate waiting times the plateau
height is independent of t_w and T_i. Time superposition applies, i.e.
C_q=C_q(t/t_r) where t_r=t_r(t_w) is a waiting time dependent decay time.
Furthermore C_q=C(q,t_w,t_w+t) scales as C_q=C(q,z(t_w,t) where z is a function
of t_w and t only, i.e. independent of q. (III) At large t_w the system reaches
equilibrium, i.e. C_q(t_w,t_w+t) and msd(t_w,t_w+t) are independent of t_w and
T_i. For C_q(t_w,t_w+t) we find that the time superposition of intermediate
waiting times (II) includes the equilibrium curve (III).Comment: 9 pages, 11 figures, submission to PR
Vibrational energy transfer in ultracold molecule - molecule collisions
We present a rigorous study of vibrational relaxation in p-H2 + p-H2
collisions at cold and ultracold temperatures and identify an efficient
mechanism of ro-vibrational energy transfer. If the colliding molecules are in
different rotational and vibrational levels, the internal energy may be
transferred between the molecules through an extremely state-selective process
involving simultaneous conservation of internal energy and total rotational
angular momentum. The same transition in collisions of distinguishable
molecules corresponds to the rotational energy transfer from one vibrational
state of the colliding molecules to another.Comment: 4 pages, 4 figure
Weighted Radon transforms for which the Chang approximate inversion formula is precise
We describe all weighted Radon transforms on the plane for which the Chang
approximate inversion formula is precise. Some subsequent results, including
the Cormack type inversion for these transforms, are also given
The global electroweak fit at NNLO and prospects for the LHC and ILC
For a long time, global fits of the electroweak sector of the Standard Model
(SM) have been used to exploit measurements of electroweak precision
observables at lepton colliders (LEP, SLC), together with measurements at
hadron colliders (Tevatron, LHC), and accurate theoretical predictions at
multi-loop level, to constrain free parameters of the SM, such as the Higgs and
top masses. Today, all fundamental SM parameters entering these fits are
experimentally determined, including information on the Higgs couplings, and
the global fits are used as powerful tools to assess the validity of the theory
and to constrain scenarios for new physics. Future measurements at the Large
Hadron Collider (LHC) and the International Linear Collider (ILC) promise to
improve the experimental precision of key observables used in the fits. This
paper presents updated electroweak fit results using newest NNLO theoretical
predictions, and prospects for the LHC and ILC. The impact of experimental and
theoretical uncertainties is analysed in detail. We compare constraints from
the electroweak fit on the Higgs couplings with direct LHC measurements, and
examine present and future prospects of these constraints using a model with
modified couplings of the Higgs boson to fermions and bosons.Comment: 26 pages, 9 figure
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