33,253 research outputs found
Prediction of destabilizing blade tip forces for shrouded and unshrouded turbines
The effect of a nonuniform flow field on the Alford force calculation is investigated. The ideas used here are based on those developed by Horlock and Greitzer. It is shown that the nonuniformity of the flow field does contribute to the Alford force calculation. An attempt is also made to include the effect of whirl speed. The values predicted by the model are compared with those obtained experimentally by Urlicks and Wohlrab. The possibility of using existing turbine tip loss correlations to predict beta is also exploited. The nonuniform flow field induced by the tip clearnance variation tends to increase the resultant destabilizing force over and above what would be predicted on the basis of the local variation of efficiency. On the one hand, the pressure force due to the nonuniform inlet and exit pressure also plays a part even for unshrouded blades, and this counteracts the flow field effects, so that the simple Alford prediction remains a reasonable approximation. Once the efficiency variation with clearance is known, the presented model gives a slightly overpredicted, but reasonably accurate destabilizing force. In the absence of efficiency vs. clearance data, an empirical tip loss coefficient can be used to give a reasonable prediction of destabilizing force. To a first approximation, the whirl does have a damping effect, but only of small magnitude, and thus it can be ignored for some purposes
Projected Hartree Fock Theory as a Polynomial Similarity Transformation Theory of Single Excitations
Spin-projected Hartree-Fock is introduced as a particle-hole excitation
ansatz over a symmetry-adapted reference determinant. Remarkably, this
expansion has an analytic expression that we were able to decipher. While the
form of the polynomial expansion is universal, the excitation amplitudes need
to be optimized. This is equivalent to the optimization of orbitals in the
conventional projected Hartree-Fock framework of non-orthogonal determinants.
Using the inverse of the particle-hole expansion, we similarity transform the
Hamiltonian in a coupled-cluster style theory. The left eigenvector of the
non-hermitian Hamiltonian is constructed in a similar particle-hole expansion
fashion, and we show that to numerically reproduce variational projected
Hartree-Fock results, one needs as many pair excitations in the bra as the
number of strongly correlated entangled pairs in the system. This
single-excitation polynomial similarity transformation theory is an alternative
to our recently presented double excitation theory, but supports projected
Hartree-Fock and coupled cluster simultaneously rather than interpolating
between them
Single transverse-spin asymmetry in Drell-Yan lepton angular distribution
We calculate a single transverse-spin asymmetry for the Drell-Yan
lepton-pair's angular distribution in perturbative QCD. At leading order in the
strong coupling constant, the asymmetry is expressed in terms of a twist-3
quark-gluon correlation function T_F^{(V)}(x_1,x_2). In our calculation, the
same result was obtained in both light-cone and covariant gauge in QCD, while
keeping explicit electromagnetic current conservation for the virtual photon
that decays into the lepton pair. We also present a numerical estimate of the
asymmetry and compare the result to an existing other prediction.Comment: 15 pages, Revtex, 5 Postscript figures, uses aps.sty, epsfig.st
Educational videos for practitioners attending Baby Friendly Hospital Initiative workshops supporting breastfeeding positioning, attachment and hand expression skills: Effects on knowledge and confidence
UNICEF Baby Friendly Initiative (BFHI) is the global standard for maternity and community services requiring all practitioners to be trained to support mothers in the essential skills of supporting positioning and attachment, and hand expression. These studies aim to rigorously assess knowledge in nurses, midwives, and doctors in these skills, tested before and after watching short videos demonstrating these skills. Practitioners were attending BFHI education, and the video study was additional. In Phase 1 clinicians in England were randomised to one of two videos (practitioner role play or clinical demonstration). The results showed improvements in knowledge and confidence, and a preference for clinical demonstration by mothers and infants. The clinical demonstration video was evaluated in China in Phase 2 where expert trainers viewed the video after completing the BHFI workshop, and in Phase 3 practitioners viewed the video before the BHFI workshop. Phase 2 with expert trainers only showed improvement in knowledge of hand expression but not positioning and attachment. In Phase 3 clinicians showed improved knowledge for both skills. In all Phases there were statistically significant improvements in confidence in practice in both skills.
Viewing short videos increased knowledge, particularly about teaching hand expression, and confidence in both skills
Quantum phase transitions in a generalized compass chain with three-site interactions
We consider a class of one-dimensional compass models with XYZYZX-type of
three-site exchange interaction in an external magnetic field. We present the
exact solution derived by means of Jordan-Wigner transformation, and study the
excitation gap, spin correlations, and establish the phase diagram. Besides the
canted antiferromagnetic and polarized phases, the three-site interactions
induce two distinct chiral phases, corresponding to gapless spinless-fermion
systems having two or four Fermi points. We find that the component of
scalar chirality operator can act as an order parameter for these chiral
phases. We also find that the thermodynamic quantities including the Wilson
ratio can characterize the liquid phases. Finally, a nontrivial magnetoelectric
effect is explored, and we show that the polarization can be manipulated by the
magnetic field in the absence of electric field.Comment: 10 pages, 11 figure
Dilatation of Lateral Ventricles with Brain Volumes in Infants with 3D Transfontanelle US
Ultrasound (US) can be used to assess brain development in newborns, as MRI
is challenging due to immobilization issues, and may require sedation.
Dilatation of the lateral ventricles in the brain is a risk factor for poorer
neurodevelopment outcomes in infants. Hence, 3D US has the ability to assess
the volume of the lateral ventricles similar to clinically standard MRI, but
manual segmentation is time consuming. The objective of this study is to
develop an approach quantifying the ratio of lateral ventricular dilatation
with respect to total brain volume using 3D US, which can assess the severity
of macrocephaly. Automatic segmentation of the lateral ventricles is achieved
with a multi-atlas deformable registration approach using locally linear
correlation metrics for US-MRI fusion, followed by a refinement step using
deformable mesh models. Total brain volume is estimated using a 3D ellipsoid
modeling approach. Validation was performed on a cohort of 12 infants, ranging
from 2 to 8.5 months old, where 3D US and MRI were used to compare brain
volumes and segmented lateral ventricles. Automatically extracted volumes from
3D US show a high correlation and no statistically significant difference when
compared to ground truth measurements. Differences in volume ratios was 6.0 +/-
4.8% compared to MRI, while lateral ventricular segmentation yielded a mean
Dice coefficient of 70.8 +/- 3.6% and a mean absolute distance (MAD) of 0.88
+/- 0.2mm, demonstrating the clinical benefit of this tool in paediatric
ultrasound
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