1,720,968 research outputs found
Three-Dimensional Analysis of Rotordynamic Forces on Whirling and Cavitating Inducers
No full textThis paper investigates the linearized dynamics of three-dimensional bubbly cavitating flows in helical inducers. The purpose is to understand the impact of the bubble response on the radial and tangential rotordynamic forces exerted by the fluid on the rotor and stator stages of whirling turbomachines under cavitating conditions. The flow in the inducer annulus is modeled as a homogeneous inviscid mixture, containing vapor bubbles with a small amount of noncondensable gas. The effects of several contributions to the damping of the bubbly dynamics are included in the model. The governing equations of the inducer flow are written in "body-fitted" orthonormal helical Lagrangian coordinates, linearized for small-amplitude perturbations about the mean flow, and solved by modal decomposition. The whirl excitation generates finite-speed propagation and resonance phenomena in the two-phase flow within the inducer. These, in turn, lead to a complex dependence of the lateral rotordynamic fluid forces on the excitation frequency, the void fraction, the average size of the cavitation bubbles, and the turbopump operating conditions (including, rotational speed, geometry, flow coefficient and cavitation number). Under cavitating conditions the dynamic response of the bubbles induces major deviations from the noncavitating flow solutions, especially when the noncondensable gas content of the bubbles is small and thermal effects on the bubble dynamics are negligible. Then, the quadratic dependence of rotordynamic fluid forces on the whirl speed, typical of cavitation-free operation, is replaced by a more complex behavior characterized by the presence of different regimes where, depending on the whirl frequency, the fluid forces have either a stabilizing or a destabilizing effect on the inducer motion. Results are presented to illustrate the influence of the relevant flow parameters
Linear Analysis of Vortex-Shedding Induced Longitudinal Oscillations in Segmented Solid Rocket Motors
No full textA linear model for the analysis of longitudinal fluid dynamic oscillations in solid rocket motors is developed and applied to the Ariane 5 boosters. The natural frequencies and mode shapes are corrected as proposed by Culick in order to account for the effects of mean flow, mass addition from the burning grain. pressure coupling, nozzle admittance. and internal geometry of (he combustion chamber. In segmented solid rocket motors, the main driving source of acoustic oscillations is often represented by the coupling between the vortices shed by (he inhibitors at the intersegments and the acoustic field in the combustion chamber. This interaction is analyzed using Flandro's model, which provides the vortex-related contribution to the amplification/damping of the acoustic oscillations. The natural acoustic frequencies are considered as known inputs for vortex development in the shear layer, which is described by means of classical linear stability theory of parallel flows. The risk assessment of vortex-induced fluid dynamic instabilities in a small scale and full size configuration of the Ariane 5 boosters is carried out for the leading modes at several burn times. Model predictions are in good agreement with the available experimental results for the scaled configuration
On the Inviscid Stability of Parallel Bubbly Flows
Full text linkThis paper investigates the effects of bubble dynamics on the stability of parallel bubbly flows of low void fraction. The equations of motion for the bubbly mixture are linearized for small perturbations and the parallel flow assumption is used to obtain a modified Rayleigh equation governing the inviscid stability problem. This is then used for the stability analysis of two-dimensional shear layers, jets and wakes. Inertial effects associated with the bubble response and energy dissipation due to the viscosity of the liquid, the heat transfer between the two phases, and the liquid compressibility are included. Numerical solutions of the eigenvalue problems for the modified Rayleigh equation are obtained by means of a multiple shooting method. Depending on the characteristic velocities of the various flows, the void fraction, and the ambient pressure, the presence of air bubbles can induce significant departures from the classical stability results for a single-phase fluid
Stability of Parallel Bubbly and Cavitating Flows
No full textThis paper examines the bubble dynamic effects on the stability of parallel bubbly and cavitating flows of low void fraction. Inertial effects associated with the bubble response and energy dissipation due to the viscosity of the liquid, the heat transfer between the two phases, and the liquid comprcssibilily arc included. The equations of motion arc linearized for small perturbations and a modified Rayleigh equation for the inviscid stability of the two-dimensional parallel flow is derived. Numerical solutions of the characteristic problem for the modified Rayleigh equation of a free shear layer are obtained by means of a multiple shooting method. Depending on the dispersion of the gaseous phase in the bubbly mixture, the ambient pressure and the free stream velocities, the presence of air bubbles can induce significant departures from the classical solution for a single-phase fluid. Results arc presented to illustrate the influence of the relevant flow parameters
The Effects of Vapor/Gas Bubbles on the Rotodynamic Forces in Bearings
No full textThis paper presents an anylytical investigation of the effects that vapor/gas bubbles can have on the fluid-induced rotordynamic forces in a liquid-filled annulus between a cylindrical rotor and a surrounding cylindrical stator. It is demonstrated that such cavitation (vaporous or gaseous) can have important consequences in altering the rotordynamic characteristics of devices such as long journal bearings or long squeeze-film dampers. A linearized analysis which includes bubble dynamic effects is used to evaluate the rotordynamic effects caused by a small amplitude whirl motion of the rotor in both the high and low Reynolds number regimes of fluid motion. In the former case the Euler equations for a bubbly mixture are employed while, in the latter, a modified Reynolds lubrication equation is used. These are combined with a Rayleigh-Plesset analysis of the bubble dynamics which includes various bubble damping effects. It is shown that, in certain parametric regimes, the normal and tangential fluid-induced rotordynamic forces acting on the rotor can deviate substantially from their classical forms in single-phase flow
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Transient and stability analysis in single-phase natural circulation
No full textThis paper presents the mathematical modeling of single-phase natural circulation of the
University of Genoa’s rectangular loop (LOOP#1) by a computer program and using
RELAP5 system code. The mass, momentum and energy conservation equations in transient
form were solved numerically using the finite difference method. One-dimensional linear stability
analysis was performed for the single-phase natural circulation loop and the numerical
perturbation technique was used in this analysis. The Nyquist criterion was employed to find
the stability map of the LOOP#1. The obtained transient results using the first order upwind
scheme of the fluid temperatures in various sectors of the LOOP#1 are compared with the
results obtained from the RELAP5 system code and available experimental data
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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