Unsteady momentum fluxes in two-phase flow and the vibration of nuclear reactor components

The steady and unsteady components of the momentum flux in a twophase flow have been measured at the exit of a vertical pipe. Measured momentum flux data has been machine processed by standard random vibration techniques to obtain the power spectral density curves. From these curves, the predominant frequency and the rms value of the unsteady momentum flux have been obtained. The effects of the average flow velocity, volumetric quality, system pressure, flow channel size and geometry on the unsteady momentum fluxes have been observed. It has been found that the fluctuation of momentum fluxes is important only in the low frequency range. The maximum values of unsteady momentum fluxes appeared in either the high void slug flow or the low void annular flow regime. The experimental results have been correlated and suggestions have been made for constructing the power spectral density curve of momentum fluxes under untested conditions. In the sample problems, using the experimental results, the effect of the unsteady momentum fluxes on a steam generator U-tube and a reactor fuel rod has been studied. The amplitudes of the structural vibrations resulting from the two-phase excitation have been found. In addition, it has also been found that there is a possibility of unstable vibrations owing to a nonlinear restoring force on the mechanical system. This nonlinearity is due to the unsteady component of the momentum flux in the flow past the system. In both examples, the major vibrations occurred in a narrow frequency band around the natural frequency of the mechanical system.Sponsored by the U. S. Atomic Energy Commissio

The Measurement, interpretation and use of unsteady momentum fluxes in two-phase flow.

The steady and unsteady components of the momentum flux in a two-phase flow have been measured at the exit of a vertical pipe by means of an impulse technique using a turning tee and beam. Different electrical filters have been tried in the recording circuit for eliminating the signals around the natural frequency of the beam system. A special filter set has been designed to approximate the inverse of the transfer function of the beam system. Thus the signals recorded after passing through the beam-filter combination can be considered as the excitation times a constant. Two different analog methods have been used to analyze the random signals for obtaining some statistical quantities such as the predominant frequency and the rms value of the unsteady momentum flux. These quanities are useful in some applications involving two-phase flow. In the preliminary measurements of the unsteady momentum flux for the adiabatic up flow of an air-water mixture in a 5/8 inch pipe, the greatest unsteadiness of momentum flux appeared in the quality range of one per cent to six per cent. Above ten per cent quality no appreciable fluctuation has been detected. In an example problem, using the preliminary results, the effect of the unsteady momentum flux on a fuel rod has been studied. The amplitude of the vibrations resulting from the two-phase excitation has been found. In addition it has also been found that there is a possibility of unstable vibrations due to a nonlinear restoring force on the fuel element. This nonlinearity is due to the unsteady component of the momentum flux in the flow past the rod

Clustering Memes in Social Media

The increasing pervasiveness of social media creates new opportunities to study human social behavior, while challenging our capability to analyze their massive data streams. One of the emerging tasks is to distinguish between different kinds of activities, for example engineered misinformation campaigns versus spontaneous communication. Such detection problems require a formal definition of meme, or unit of information that can spread from person to person through the social network. Once a meme is identified, supervised learning methods can be applied to classify different types of communication. The appropriate granularity of a meme, however, is hardly captured from existing entities such as tags and keywords. Here we present a framework for the novel task of detecting memes by clustering messages from large streams of social data. We evaluate various similarity measures that leverage content, metadata, network features, and their combinations. We also explore the idea of pre-clustering on the basis of existing entities. A systematic evaluation is carried out using a manually curated dataset as ground truth. Our analysis shows that pre-clustering and a combination of heterogeneous features yield the best trade-off between number of clusters and their quality, demonstrating that a simple combination based on pairwise maximization of similarity is as effective as a non-trivial optimization of parameters. Our approach is fully automatic, unsupervised, and scalable for real-time detection of memes in streaming data.Comment: Proceedings of the 2013 IEEE/ACM International Conference on Advances in Social Networks Analysis and Mining (ASONAM'13), 201

Large-wavelength instabilities in free-surface Hartmann flow at low magnetic Prandtl numbers

We study the linear stability of the flow of a viscous electrically conducting capillary fluid on a planar fixed plate in the presence of gravity and a uniform magnetic field. We first confirm that the Squire transformation for MHD is compatible with the stress and insulating boundary conditions at the free surface, but argue that unless the flow is driven at fixed Galilei and capillary numbers, the critical mode is not necessarily two-dimensional. We then investigate numerically how a flow-normal magnetic field, and the associated Hartmann steady state, affect the soft and hard instability modes of free surface flow, working in the low magnetic Prandtl number regime of laboratory fluids. Because it is a critical layer instability, the hard mode is found to exhibit similar behaviour to the even unstable mode in channel Hartmann flow, in terms of both the weak influence of Pm on its neutral stability curve, and the dependence of its critical Reynolds number Re_c on the Hartmann number Ha. In contrast, the structure of the soft mode's growth rate contours in the (Re, alpha) plane, where alpha is the wavenumber, differs markedly between problems with small, but nonzero, Pm, and their counterparts in the inductionless limit. As derived from large wavelength approximations, and confirmed numerically, the soft mode's critical Reynolds number grows exponentially with Ha in inductionless problems. However, when Pm is nonzero the Lorentz force originating from the steady state current leads to a modification of Re_c(Ha) to either a sublinearly increasing, or decreasing function of Ha, respectively for problems with insulating and conducting walls. In the former, we also observe pairs of Alfven waves, the upstream propagating wave undergoing an instability at large Alfven numbers.Comment: 58 pages, 16 figure

A study of the relation between inflation and exchange rates in the Fiji Islands: a cointegration and vector error correction approach

The main purpose of this study is to examine the effects of the exchange rates, international prices, and the demand shocks on inflation in Fiji. How the domestic inflation in a pegged exchange rate system is aligned with international price shocks is an important monetarist idea, and this is tested in this study. This study employs annual data from 1975 to 2010. The multivariate cointegration tests are done after the unit root tests, and further, the Vector Error Correction (VEC) model shows that the changes in Fiji’s CPI are Granger caused by the long-term trends in all other variables, and the CPI in Australia, and devaluation-year dummies are used as exogenous variables in the VEC model, and the changes in exchange rate and changes in demand shocks are the independent variables but made endogenous in the VEC model. The impulse response function also shows that due to the exchange rate depreciation, inflation has increased for many years in Fiji. The policy implication of our study is that as a monetary policy instrument, the flexibility of the exchange rate policy is indispensable for Fiji to appropriately absorb the international supply and price shocks

A study of the pass - through effects of exchange rates and international prices shocks on inflation in the Fiji Islands

The main purpose of this study is to examine the effects of the exchange rates,international prices, and the demand shocks on inflation in Fiji Island. The study covers the annual data from 1975 to 2010.The variables are transformed into changes/per cent and then Ordinary Least Squares methodology (OLS) is used. The appropriateness of the OLS assumptions is tested including the normality of the residuals, autocorrelations,heteroskedasticity and functional forms. The main findings are that the Fijian dollar depreciation increases, the international price shocks in the form of Australian consumer prices increase, the Keynesian demand shocks increase, and the devaluation events increase the consumer price inflation in Fiji. As a monetary policy instrument the flexibility of the exchange rate policy is indispensable for Fiji to absorb appropriately the international supply and price shocks. We have been able to include the international supply shocks, the domestic demand shocks, the exchange rates, and devaluation dummies in our model. Though we have used a simple OLS model, the originality of our study is the comprehensiveness of the theoretical variables in our model. This study will have an important implication for the small open economy of Fiji especially its exchange rate policy

Preventing transition to turbulence: a viscosity stratification does not always help

In channel flows a step on the route to turbulence is the formation of streaks, often due to algebraic growth of disturbances. While a variation of viscosity in the gradient direction often plays a large role in laminar-turbulent transition in shear flows, we show that it has, surprisingly, little effect on the algebraic growth. Non-uniform viscosity therefore may not always work as a flow-control strategy for maintaining the flow as laminar.Comment: 9 pages, 8 figure

Quantum Cellular Neural Networks

We have previously proposed a way of using coupled quantum dots to construct digital computing elements - quantum-dot cellular automata (QCA). Here we consider a different approach to using coupled quantum-dot cells in an architecture which, rather that reproducing Boolean logic, uses a physical near-neighbor connectivity to construct an analog Cellular Neural Network (CNN).Comment: 7 pages including 3 figure