9,911 research outputs found
Permanent and separable aerospace tubing/ fitting evaluation program, volume 1
The tube fitting evaluation program was conducted to develop a standard overall test program including methods, procedures, and equipment as well as recommended test sequences for qualifying fitting/tubing assemblies. The program consisted of testing the MS flareless (separable) fitting and utilizing the results as baseline data from which all other fittings will be evaluated. Five separable designs and five permanent designs were tested in three sizes with two types of tubing materials. The basic design requirements were 4,000 psi operating pressure within the temperature range of minus 65 F to plus 450 F while also being compatible with 21-6-9 and titanium 3Al-2.5V tubing alloys
Effects of Cavitation on Rotordynamic Force Matrices
When designing a turbomachine, particularly one which is to operate at high speed, it is important to be able to predict the fluid-induced forces, both steady and unsteady, acting on the various components of the machine. This paper concentrates on the fluid-induced rotordynamic forces acting upon the impeller and therefore on the bearings. Self-excited whirl, where the rotor moves away from and whirls along a trajectory eccentric to its undeflected position, can result from these fluid-induced forces. The purpose of the present work is to study the full range of these forces so that they can be included in any rotordynamic analysis at the design stage.
To study the fluid-induced rotordynamic force on an impeller vibrating around its machine axis of rotation, an experiment in forced vibration was conducted. The prescribed whirl trajectory of the rotor is a circular orbit of a fixed radius. A rotating dynamometer mounted behind the rotor measures the force on the impeller. The force measured is a combination of a steady radial force due to volute asymmetries and an unsteady force due to the eccentric motion of the rotor. These measurements have been conducted over a full range of whirl/impeller speed ratios at different flow coefficients for various turbomachines including both centrifugal impellers aand axial inducers. A destabilizing force was observed over a region of positive whirl ratio. The range of flow conditions includes an examination of the effects of cavitation on the observed rotordynamic forces
On the Effect of Cavitation on the Radial Forces and Hydrodynamic Stiffness of a Centrifugal Pump
The asymmetric flow within a volute exerts a radial force on a centrifugal impeller. The present paper presents experimental measurements of the radial forces on the impeller in the presence of cavitation
Rotordynamic Forces on Centrifugal Pump Impellers
The asymmetric flow around an impeller in a volute exerts a force upon the impeller. To study the rotordynamic force on an impeller which is vibrating around its machine axis of rotation, the impeller, mounted on a dynamometer, is made to whirl in a circular orbit within the volute. The measured force is expressed as the sum of a steady radial force and an unsteady force due to the eccentric motion of the impeller. These forces were measured in separate tests on a centrifugal pump with radically increased shroud clearance, a two-dimensional impeller, and an impeller with an inducer, the impeller of the HPOTP (High Pressure Oxygen Turbopump) of the SSME (Space Shuttle Main Enginer). In each case, a destabilizing force was observed over a region of positive whirl
Patch-repetition correlation length in glassy systems
We obtain the patch-repetition entropy Sigma within the Random First Order
Transition theory (RFOT) and for the square plaquette system, a model related
to the dynamical facilitation theory of glassy dynamics. We find that in both
cases the entropy of patches of linear size l, Sigma(l), scales as s_c l^d+A
l^{d-1} down to length-scales of the order of one, where A is a positive
constant, s_c is the configurational entropy density and d the spatial
dimension. In consequence, the only meaningful length that can be defined from
patch-repetition is the cross-over length xi=A/s_c. We relate xi to the typical
length-scales already discussed in the literature and show that it is always of
the order of the largest static length. Our results provide new insights, which
are particularly relevant for RFOT theory, on the possible real space structure
of super-cooled liquids. They suggest that this structure differs from a mosaic
of different patches having roughly the same size.Comment: 6 page
Subdiffusive motion in kinetically constrained models
We discuss a kinetically constrained model in which real-valued local
densities fluctuate in time, as introduced recently by Bertin, Bouchaud and
Lequeux. We show how the phenomenology of this model can be reproduced by an
effective theory of mobility excitations propagating in a disordered
environment. Both excitations and probe particles have subdiffusive motion,
characterised by different exponents and operating on different time scales. We
derive these exponents, showing that they depend continuously on one of the
parameters of the model.Comment: 12 pages, 5 figure
Modeling microevolution in a changing environment: The evolving quasispecies and the Diluted Champion Process
Several pathogens use evolvability as a survival strategy against acquired
immunity of the host. Despite their high variability in time, some of them
exhibit quite low variability within the population at any given time, a
somehow paradoxical behavior often called the evolving quasispecies. In this
paper we introduce a simplified model of an evolving viral population in which
the effects of the acquired immunity of the host are represented by the
decrease of the fitness of the corresponding viral strains, depending on the
frequency of the strain in the viral population. The model exhibits evolving
quasispecies behavior in a certain range of its parameters, ans suggests how
punctuated evolution can be induced by a simple feedback mechanism.Comment: 15 pages, 12 figures. Figures redrawn, some additional clarifications
in the text. To appear in Journal of Statistical Mechanics: Theory and
Experimen
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