1,505 research outputs found
Computer simulation of liquid crystals
A review is presented of molecular and mesoscopic computer simulations of liquid crystalline systems. Molecular simulation approaches applied to such systems are described and the key findings for bulk phase behaviour are reported. Following this, recently developed lattice Boltzmann (LB) approaches to the mesoscale modelling of nemato-dynamics are reviewed. The article concludes with a discussion of possible areas for future development in this field.</p
A lattice spring model of heterogeneous materials with plasticity
A three-dimensional lattice spring model of a heterogeneous material is presented. For small deformations, the model is shown to recover the governing equations for an isotropic elastic medium. The model gives reasonable agreement with theoretical predictions for the elastic fields generated by a spherical inclusion, although for small particle sizes the discretization of the underlying lattice causes some departures from the predicted values. Plasticity is introduced by decreasing the elastic moduli locally whilst maintaining stress continuity. Results are presented for a spherical inclusion in a plastic matrix and are found to be in good agreement with the predictions of Wilner (1988 J. Mech. Phys. Solids 36 141-65).</p
Seeding of the nematic-isotropic phase transition by an electric field
In this paper, we use a relatively simple continuum model to investigate the effects of dielectric inhomogeneity within confined liquid crystal cells. Specifically, we consider, in planar, cylindrical and spherical geometries, the stability of a nematic-isotropic interface subject to an applied voltage. Depending on the magnitude of this voltage, the temperature and the geometry of the cell, the nematic region may shrink until the material is completely isotropic within the cell, grow until the nematic phase cells the cell or, in certain geometries, coexist with the isotropic phase. For planar geometry, no coexistence is found, but we are able to give analytical expressions for the critical voltage for an electric-field-induced phase transition as well as the critical wetting layer thickness for arbitrary applied voltage. In cells with cylindrical and spherical geometries, however, stable nematic-isotropic coexistence is predicted, the thickness of the nematic region being controllable by alteration of the applied voltage.</p
Classification of the chiral Z2XZ2 fermionic models in the heterotic superstring
The first particle physics observable whose origin may be sought in string
theory is the triple replication of the matter generations. The class of Z2XZ2
orbifolds of six dimensional compactified tori, that have been most widely
studied in the free fermionic formulation, correlate the family triplication
with the existence of three twisted sectors in this class. In this work we seek
an improved understanding of the geometrical origin of the three generation
free fermionic models. Using fermionic and orbifold techniques we classify the
Z2XZ2 orbifold with symmetric shifts on six dimensional compactified internal
manifolds. We show that perturbative three generation models are not obtained
in the case of Z2XZ2 orbifolds with symmetric shifts on complex tori, and that
the perturbative three generation models in this class necessarily employ an
asymmetric shift. We present a class of three generation models in which the
SO(10) gauge symmetry cannot be broken perturbatively, while preserving the
Standard Model matter content. We discuss the potential implications of the
asymmetric shift for strong-weak coupling duality and moduli stabilization. We
show that the freedom in the modular invariant phases in the N=1 vacua that
control the chiral content, can be interpreted as vacuum expectation values of
background fields of the underlying N=4 theory, whose dynamical components are
projected out by the Z2-fermionic projections. In this class of vacua the
chiral content of the models is determined by the underlying N=4 mother theory.Comment: 36 pages. Standard LaTe
Phenomenology of A Three-Family Standard-like String Model
We discuss the phenomenology of a three-family supersymmetric Standard-like
Model derived from the orientifold construction, in which the ordinary chiral
states are localized at the intersection of branes at angles. In addition to
the Standard Model group, there are two additional U(1)' symmetries, one of
which has family non-universal and therefore flavor changing couplings, and a
quasi-hidden non-abelian sector which becomes strongly coupled above the
electroweak scale. The perturbative spectrum contains a fourth family of exotic
(SU(2)- singlet) quarks and leptons, in which, however, the left-chiral states
have unphysical electric charges. It is argued that these decouple from the low
energy spectrum due to hidden sector charge confinement, and that anomaly
matching requires the physical left-chiral states to be composites. The model
has multiple Higgs doublets and additional exotic states. The moduli-dependent
predictions for the gauge couplings are discussed. The strong coupling agrees
with experiment for reasonable moduli, but the electroweak couplings are too
small.Comment: 22 pages, 4 figure
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Incremental profile ring rolling with axial and circumferential constraints
© 2017 If profile ring rolling could be achieved without part-specific tooling, significant savings in material, energy and downstream processing could be realised. One approach, âincremental ring rollingâ previously suffered difficulties controlling material flow, resulting in multiple form errors. Inspired by studying an expert using a potter's wheel, two additions to this process are proposed; the use of axial and circumferential constraints. A 12-axis ring rolling machine has been built to demonstrate these process enhancements, producing metal rings up to 1 m in diameter. The production of both rectangular and L-shape rings is examined, showing significant improvements in ring cross-sectional form and circularity.The first author was supported by an EPSRC I-Case Studentship (12220703), with Primetals Technologies Ltd. Prof. Allwood was supported by EPSRC grant EP/K018108/1
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Implementing material efficiency in practice: A case study to improve the material utilisation of automotive sheet metal components
© 2019 There is an opportunity to reduce the amount of sheet metal currently used to manufacture automotive components, despite the available cost and CO2 savings, the automotive industry has not realised the full potential of these saving opportunities. To understand why, a practical case study was set up with an automotive manufacturer. A cross-functional team was established with the scope to make changes to five components using a structured design process to improve material efficiency. The trial identified realistic opportunities to improve material utilisation by 20%pts, and save £9million and 5 kilotonnes of CO2 annually. The greatest saving opportunities were found early in the product development cycle, before the production method is determined by component geometry. Of these, 3%pts were actually implemented on the production vehicle, saving £1.8million and 1.5 kilotonnes of CO2 annually. The case study identified significant barriers to implementing material efficiency strategies in an industrial setting. To overcome these barriers material utilisation should be considered early in the product design process and high in the vehicle platform hierarchy. As a result of this investigation, new business processes are being generated to support design for material utilisation at the automotive manufacturer. This case study approach should be considered to increase implementation for other aspects of material demand reduction.EP/N02351X/
Minimal Standard Heterotic String Models
Three generation heterotic-string vacua in the free fermionic formulation
gave rise to models with solely the MSSM states in the observable Standard
Model charged sector. The relation of these models to Z_2 x Z_2 orbifold
compactifications dictates that they produce three pairs of untwisted Higgs
multiplets. The reduction to one pair relies on the analysis of supersymmetric
flat directions, that give superheavy mass to the dispensable Higgs states. We
explore the removal of the extra Higgs representations by using the free
fermion boundary conditions and hence directly at the string level, rather than
in the effective low energy field theory. We present a general mechanism that
achieves this reduction by using asymmetric boundary conditions between the
left- and right-moving internal fermions. We incorporate this mechanism in
explicit string models containing three twisted generations and a single
untwisted Higgs doublet pair. We further demonstrate that an additional effect
of the asymmetric boundary conditions is to substantially reduce the
supersymmetric moduli space.Comment: 20 pages, LaTeX; added reference
Stringent Phenomenological Investigation into Heterotic String Optical Unification
For the weakly coupled heterotic string (WCHS) there is a well-known factor
of twenty conflict between the minimum string coupling unification scale,
Lambda_H ~5x10^(17) GeV, and the projected MSSM unification scale, Lambda_U ~
2.5x10^(16) GeV, assuming an intermediate scale desert (ISD). Renormalization
effects of intermediate scale MSSM-charged exotics (ISME) (endemic to
quasi-realistic string models) can resolve this issue, pushing the MSSM scale
up to the string scale. However, for a generic string model, this implies that
the projected Lambda_U unification under ISD is accidental. If the true
unification scale is 5.0x10^(17) GeV, is it possible that illusionary
unification at 2.5x10^(17) GeV in the ISD scenario is not accidental? If it is
not, then under what conditions would the assumption of ISME in a WCHS model
imply apparent unification at Lambda_U when ISD is falsely assumed? Geidt's
"optical unification" suggests that Lambda_U is not accidental, by offering a
mechanism whereby a generic MSSM scale Lambda_U < Lambda_H is guaranteed. A
WCHS model was constructed that offers the possibility of optical unification,
depending on the availability of anomaly-cancelling flat directions meeting
certain requirements. This paper reports on the systematic investigation of the
optical unification properties of the set of stringent flat directions of this
model. Stringent flat directions can be guaranteed to be F-flat to all finite
order (or to at least a given finite order consistent with electroweak scale
supersymmetry breaking) and can be viewed as the likely roots of more general
flat directions. Analysis of the phenomenology of stringent flat directions
gives an indication of the remaining optical unification phenomenology that
must be garnered by flat directions developed from them.Comment: standard latex, 18 pages of tex
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