A study of the chain extending effect of elongational flow in polymer solutions

Summary The effect of elongational flow on the orientation of dissolved macromolecules has been studied by observation of localised birefringence within appropriate regions of the flow field, with the objective of linking up experimentally observed chain alignment with basic theory. Benefitting from previous experience the highest available molecular weight fractions were used, i. e. polystyrene ofM _w = 2 · 10⁶, and two opposed suck jets were employed to provide the high strain rates required. Birefringence set in above a critical strain rate and rose rapidly to a maximum value, confirming expectations from theory. Both the maximum birefringence and critical strain rate were independent of concentration indicating that the chains are independent of each other. The value of the birefringence was consistent with complete chain extension, while the critical strain rate yielded a relaxation time of 3 · 10^–5 s in accord with the value calculated fromZimm's non-free draining model. Other observations yielded an estimate of the critical entanglement concentration. The prospects and limitations of the present kind of experimental approach are discussed.With 5 figures and 2 tables     

A study of high transverse momentum η and π0 mesons at the CERN ISR

The production of high transverse momentum η mesons from proton-proton collisions has been studied at the CERN ISR. The decay η→γγ was identified in an array of lead-glass ?erenkov counters. The preliminary results, obtained at a total centre-of-mass energy √s=52.7 GeV, show an η signal, which amounts to approximately one half of the π⁰ cross-section for centre-of-mass transverse momenta between 3 and 5 GeV/c. The production of high transverse momentum π⁰'s was also measured and compared with previous results.     

Yang-Mills instantons and the S-matrix

We present a scheme for calculating gauge-invariant S-matrix elements in the presence of instantons. We exploit the conformal invariance of the zero-mass field equations. The asymptotic in and out states are defined by their values on null infinity $\text{J}$. We use this method to calculate to lowest-order S-matrix elements for scalar particles and fermions in a dilute gas of SU(2) instantons and anti-instantons. The scalar particles acquire an effective mass and an effective interaction of the form $\text{exp}\text{(?(?}{}^2\text{/16π) ?}\text{?}\text{)}$, where ? is the scale of the instanton, plus other interactions which cannot be presented by a local effective lagrangian. The fermions acquire the effective lagrangian obtained by 't Hooft. In the case of a single flavour of fermions, this corresponds to a mass term.     

The measurement of damping in large panels

In this paper we consider the problem of measuring the loss factors of large panels, particularly when their damping is high. A method of measurement which may be used over a wide range of damping values is described. It is shown that it is important to measure the damping of panels and walls in situ since the damping can depend critically on the mounting conditions used and is often greater than the internal damping of the wall material itself. It is suggested that a measurement of damping should be made whenever the sound insulation of a panel or wall is measured in the laboratory.     

M-theory conifolds

Seven manifolds of G2 holonomy provide a bridge between M-theory and string theory, via Kaluza-Klein reduction to Calabi-Yau six manifolds. We find first-order equations for a new family of G2 metrics D7, with S3 x S3 principal orbits. These are related at weak string coupling to the resolved conifold, paralleling earlier examples B7 that are related to the deformed conifold, allowing a deeper study of topology change and mirror symmetry in M-theory. The D7 metrics' nontrivial parameter characterizes the squashing of an S3 bolt, which limits to S2 at weak coupling. In general the D7 metrics are asymptotically locally conical, with a nowhere-singular circle action.     

Turbulent piloted partially-premixed flames with varying levels of O2/N2: stability limits and PDF calculations

This paper reports measured stability limits and PDF calculations of piloted, turbulent flames of compressed natural gas (CNG) partially-premixed with either pure oxygen, or with varying levels of O₂/N₂. Stability limits are presented for flames of CNG fuel premixed with up to 20% oxygen as well as CNG–O₂–N₂ fuel where the O₂ content is varied from 8 to 22% by volume. Calculations are presented for (i) Sydney flame B [Masri et al. 1988] which uses pure CNG as well as flames B15 to B25 where the CNG is partially-premixed with 15–25% oxygen by volume, respectively and (ii) Sandia methane–air (1:3 by volume) flame E [Barlow et al. 2005] as well as new flames E15 and E25 that are partially-premixed with ‘reconstituted air’ where the O₂ content in nitrogen is 15 and 25% by volume, respectively. The calculations solve a transported PDF of composition using a particle-based Monte Carlo method and employ the EMST mixing model as well as detailed chemical kinetics. The addition of oxygen to the fuel increases stability, shortens the flames, broadens the reaction zone, and shifts the stoichiometric mixture fraction towards the inner side of the jet. It is found that for pure CNG flames where the reaction zone is narrow (～0.1 in mixture fraction space), the PDF calculations fail to reproduce the correct level of local extinction on approach to blow-off. A broadening in the reaction zone up to about 0.25 in mixture fraction space is needed for the PDF/EMST approach to be able to capture these finite-rate chemistry effects. It is also found that for the same level of partial premixing, increasing the O₂/N₂ ratio increases the maximum levels of CO and NO but shifts the peak to richer mixture fractions. Over the range of oxygenation investigated here, stability limits have shown to improve almost linearly with increasing oxygen levels in the fuel and with increasing the contribution of release rate from the pilot.     

Computational study of lean premixed turbulent flames using RANSPDF and LESPDF methods

A computational study is performed on a series of four piloted, lean, premixed turbulent jet flames. These flames use the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM150, PM1100, PM1150 and PM1200, respectively. Calculations are performed using the RANSPDF and LESPDF methodologies, with different treatments of molecular diffusion, with detailed chemistry and flamelet-based chemistry modelling, and using different imposed boundary conditions. The sensitivities of the calculations to these different aspects of the modelling are compared and discussed. Comparisons are made to experimental data and to previously-performed calculations. It is found that, given suitable boundary conditions and treatment of molecular diffusion, excellent agreement between the calculations and experimental measurements of the mean and variance fields can be achieved for PM150 and PM1100. The application of a recently developed implementation of molecular diffusion results in a large improvement in the computed variance fields in the LESPDF calculations. The inclusion of differential diffusion in the LESPDF calculations provides insight on the behaviour in the near-field region of the jet, but its effects are found to be confined to this region and to the species CO, OH and H₂. A major discrepancy observed in many previous calculations of these flames is an overprediction of reaction progress in PM1150 and PM1200, and this discrepancy is also observed in the LESPDF calculations; however, a parametric study of the LESPDF mixing model reveals that, with a sufficiently large mixing frequency, calculations of these two flames are capable of yielding improved reaction progress in good qualitative agreement with the mean and RMS scalar measurements up to an x/D of 30. Lastly, the merits of each computational methodology are discussed in light of their computational costs.