1,721,592 research outputs found
Influence of the Spool Velocity on The Performance of a Directional Hydraulic Valve
No full textIn this paper an accurate numerical method has been used to verify the influence of the spool velocity on the performance
of a directional hydraulic valve (4/3, closed center): the flow during the opening phase of the valve has been
solved by Direct Numerical Simulation (DNS), using an Immersed-Boundary (IB) technique.
The present results have been compared with the ones of a previous study, based on the same numerical method, but
with a stationary spool. The numerical comparisons prove that the "quasi-stationary" hypothesis is approximately correct
for present commercial devices, but it is not suitable for future high-speed valves. However it is shown that, even
inside the range of the spool velocities currently adopted, for small pressure drops Δp and small openings s more significant
differences arise on the axial forces
Analysis of a directional hydraulic valve by a direct numerical simulation using an immersed-boundary method
No full textThe improvement of the hydraulic valves depends on the careful analysis
of the coherent structures driving the motion of the working fluid. In the
past those devices have been studied by experimental tests; during the last
15 years also several numerical works have been presented, solving the flow
on body-fitted computational grids by RANS methods.
In this study a different approach is proposed for the axisymmetric analysis
of a directional valve (4/3, closed centre): whereas the RANS techniques are
based on the time-averaged equations of the flow, in the present work the unsteady Navier-Stokes equations have been solved using the Direct Numerical
Simulation (DNS); the time evolution of the physics is simulated, providing
important details on the instantaneous structures of the flow, affecting the valve performance. Furthermore, while in the previous numerical studies the
computational domain has been discretized by conformal grids, in this case
the fluid-body interaction has been represented by an immersed-boundary
(IB) method on a Cartesian grid, more suitable for unsteady eddy-resolving
simulations, as DNS.
The analysis of the discharge coefficient and the flow forces for different openings s and pressure drops ∆p is presented in this paper. The behaviour of
those global parameters is justified also considering the time-averaged and
the instantaneous fields. For small openings and pressure drops the flow is
steady and attached to the wall of the discharge chamber on the side of the
restricted section. When s and ∆p are increased the jet separates at the restricted section and it re-attaches downstream (Coanda effect), keeping the
steady state. Finally, for large openings and pressure drops the flow becomes
strongly unsteady: it is organized like a free jet and is dominated by large
vortices
Simulazione diretta del flusso in un distributore oleodinamico mediante un metodo ai contorni immersi
No full textViene presentata l’analisi del flusso all’interno di una valvola distributrice 4/3 a centro chiuso mediante un codice, basato sul metodo dei contorni
immersi, che risolve le equazioni di Navier-Stokes mediante una DNS.
I risultati ottenuti sono presentati sia in termini di campi istantanei e medi, evidenziando la manifestazione dell’effetto Coanda alle piccole aperture,
sia in termini di parametri globali, quali il coefficiente di efflusso e di forza assiale.In this paper a directional valve (4\/3, closed center) is analyzed using a code based on the immersed boundary method and solving the Navier-. Stokes equations by a Direct Numerical Simulation (DNS).. The results are presented in terms of instantaneous and time-averaged fields, showing the Coanda effect for small valve openings, and global parameters,. such as the discharge coefficient and the flow force coefficient
Large-eddy simulations of a mixed-flow pump at off-design conditions
No full textreduced flow-rates in turbopumps produce significant unsteady phenomena, characterized by separation and back-flow. In this study an LES approach coupled with an immersed-boundary methodology is utilized to investigate the changes in the flow physics, when compared to nominal flow-rates. The present methodology has been already validated for the design case through comparison with PIV experiments in the literature. It will be shown that for a reduced flow rate (40\% of the design one) separation phenomena are generated on the suction side of the rotor blades and on the pressure side of the stator ones. Significant spanwise non-uniformity is produced in the diffuser channels, with a displacement of the flow towards the hub side and back-flow on the shroud side. The values of turbulent kinetic energy are increased by an order of magnitude at off-design conditions and the main source of turbulence is not anymore the flow from the suction side and the trailing edge of the rotor blades: most turbulence is generated now at the leading edge of the diffuser blades. The increased interaction between rotating and stationary parts implies also a stronger dependence of the flow features on the relative position between impeller and diffuser blades
Simulazione diretta del Flusso in un Distributore Oleodinamico mediante un Metodo ai Contorni Immersi
No full text
Investigation of a pressure compensated vane pump
Full text linkVariable displacement hydraulic machines offer a very promising alternative and energy saving solution for many applications in mobile machines, robots and other applications. In the present paper a vane pump will be theoretically analyzed using the software AMESim and/or MATLAB/Simulink, in order to estimate the friction forces and volumetric efficiency loss without hardworking experimental tests. The friction forces taken into account by our model are the friction between vanes and rotor slots and that between vanes and eccentric pump stator. The same design parameters affect the volumetric and mechanical efficiency, whose behavior has been reported and discussed. The behavior of the machine is analyzed and discussed at different angular velocities and pressure regimes
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
Hydroacoustic analysis of a marine propeller using large-eddy simulation and acoustic analogy
No full textThe acoustic analogy is adopted to characterise the signature of a seven-bladed submarine propeller, relying on a high-fidelity large-eddy simulation, performed on a computational grid consisting of 840 million points. Results demonstrate that the nonlinear terms of the Ffowcs-Williams and Hawkings equation quickly become dominant moving away from the propeller along the direction of its wake development. While the linear terms experience a decay moving downstream, the nonlinear terms grow in the near wake, as a result of the development of wake instability. In particular, this growth affects frequencies lower than the blade frequency. Therefore, the acoustic signature of the propeller is mainly tonal in the near field only, due to the thickness and loading components of noise from the surface of the propeller and the periodic perturbation caused by its tip vortices. They develop instability at a faster rate, compared with the hub vortex, triggering the process of energy cascade towards higher frequencies and contributing in this way to broadband noise
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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