Subsynchronous vibrations in a high pressure centrifugal compressor: A case history

Two distinct aerodynamically excited vibrations in a high pressure low flow centrifugal compressor are documented. A measured vibration near 21% of running speed was identified as a nonresonant forced vibration which results from rotating stall in the diffuser; a measured vibration near 50% of running speed was identified as a self excited vibration sustained by cross coupling forces acting at the compressor wheels. The dependence of these characteristics on speed, discharge pressure, and changes in bearing design are shown. The exciting mechanisms of diffuser stall and aerodynamic cross coupling are evidenced. It is shown how the rotor characteristics are expected to change as a result of modifications. The operation of the compressor after the modifications is described

Yang-Lee Theory for a Nonequilibrium Phase Transition

To analyze phase transitions in a nonequilibrium system we study its grand canonical partition function as a function of complex fugacity. Real and positive roots of the partition function mark phase transitions. This behavior, first found by Yang and Lee under general conditions for equilibrium systems, can also be applied to nonequilibrium phase transitions. We consider a one-dimensional diffusion model with periodic boundary conditions. Depending on the diffusion rates, we find real and positive roots and can distinguish two regions of analyticity, which can identified with two different phases. In a region of the parameter space both of these phases coexist. The condensation point can be computed with high accuracy.Comment: 4 pages, accepted for publication in Phys.Rev.Let

Symmetry breaking through a sequence of transitions in a driven diffusive system

In this work we study a two species driven diffusive system with open boundaries that exhibits spontaneous symmetry breaking in one dimension. In a symmetry broken state the currents of the two species are not equal, although the dynamics is symmetric. A mean field theory predicts a sequence of two transitions from a strongly symmetry broken state through an intermediate symmetry broken state to a symmetric state. However, a recent numerical study has questioned the existence of the intermediate state and instead suggested a single discontinuous transition. In this work we present an extensive numerical study that supports the existence of the intermediate phase but shows that this phase and the transition to the symmetric phase are qualitatively different from the mean-field predictions.Comment: 19 pages, 12 figure

Phase Transition in the ABC Model

Recent studies have shown that one-dimensional driven systems can exhibit phase separation even if the dynamics is governed by local rules. The ABC model, which comprises three particle species that diffuse asymmetrically around a ring, shows anomalous coarsening into a phase separated steady state. In the limiting case in which the dynamics is symmetric and the parameter $q$ describing the asymmetry tends to one, no phase separation occurs and the steady state of the system is disordered. In the present work we consider the weak asymmetry regime $q=\exp{(-\beta/N)}$ where $N$ is the system size and study how the disordered state is approached. In the case of equal densities, we find that the system exhibits a second order phase transition at some nonzero $\beta_c$. The value of $\beta_c = 2 \pi \sqrt{3}$ and the optimal profiles can be obtained by writing the exact large deviation functional. For nonequal densities, we write down mean field equations and analyze some of their predictions.Comment: 18 pages, 3 figure

Spontaneous Breaking of Translational Invariance in One-Dimensional Stationary States on a Ring

We consider a model in which positive and negative particles diffuse in an asymmetric, CP-invariant way on a ring. The positive particles hop clockwise, the negative counterclockwise and oppositely-charged adjacent particles may swap positions. Monte-Carlo simulations and analytic calculations suggest that the model has three phases; a "pure" phase in which one has three pinned blocks of only positive, negative particles and vacancies, and in which translational invariance is spontaneously broken, a "mixed" phase with a non-vanishing current in which the three blocks are positive, negative and neutral, and a disordered phase without blocks.Comment: 7 pages, LaTeX, needs epsf.st

Discontinuous Phase Transition in an Exactly Solvable One-Dimensional Creation-Annihilation System

An exactly solvable reaction-diffusion model consisting of first-class particles in the presence of a single second-class particle is introduced on a one-dimensional lattice with periodic boundary condition. The number of first-class particles can be changed due to creation and annihilation reactions. It is shown that the system undergoes a discontinuous phase transition in contrast to the case where the density of the second-class particles is finite and the phase transition is continuous.Comment: Revised, 8 pages, 1 EPS figure. Accepted for publication in Journal of Statistical Mechanics: theory and experimen

CANOZE measurements of the Arctic ozone hole

In CANOZE 1 (Canadian Ozone Experiment), a series of 20 ozone profile measurements were made in April, 1986 from Alert at 82.5 N. CANOZE is the Canadian program for study of the Arctic winter ozone layer. In CANOZE 2, ozone profile measurements were made at Saskatoon, Edmonton, Churchill and Resolute during February and March, 1987 with ECC ozonesondes. Ground based measurements of column ozone, nitrogen dioxide and hydrochloric acid were conducted at Saskatoon. Two STRATOPROBE balloon flights were conducted on February 26 and March 19, 1987. Two aerosol flights were conducted by the University of Wyoming. The overall results of this study will be reported and compared with the NOZE findings. The results from CANOZE 3 in 1988, are also discussed. In 1988, as part of CANOZE 3, STRATOPROBE balloon flights were conducted from Saskatchewan on January 27 and February 13. A new lightweight infrared instrument was developed and test flown. A science flight was successfully conducted from Alert (82.5 N) on March 9, 1988 when the vortex was close to Alert; a good measurement of the profile of nitric acid was obtained. Overall, the Arctic spring ozone layer exhibits many of the features of the Antarctic ozone phenomenon, although there is obviously not a hole present every year. The Arctic ozone field in March, 1986 demonstrated many similarities to the Antarctic ozone hole. The TOMS imagery showed a crater structure in the ozone field similar to the Antarctic crater in October. Depleted layers of ozone were found in the profiles around 15 km, very similar to those reported from McMurdo. Enhanced levels of nitric acid were measured in air which had earlier been in the vortex. The TOMS imagery for March 1987 did not show an ozone crater, but will be examined for an ozone crater in February and March, 1988, the target date for the CANOZE 3 project

The role of the Initial Teacher Training Coordinator in the school based element of partnership: to what extent does the Co-ordinator undertake supervision of aspects of quality assurance?

Over the last 14 years, Universities and colleges have organised school placements and award qualifications whilst teachers, acting as mentors take the lead in guiding, supervising and assessing students on lengthy teaching placements. Partnership developed quickly in response to legislation and Sheffield Hallam University was among the first of the Universities to 'sign up' to the new partnership model in 1992. The Partnership Survey 2006 reported on in this paper has focussed on the Senior Liaison Tutor with a view to finding out their views on their role, and perceived contributions to the quality assurance of the partnership. The survey focussed on some of the key issues identified in previous studies and meetings of the Partnership Management Board.</p