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On the equivalence of total stress and pressure gradient formulations for predicting land subsidence above compacting gas/oil fields
Get PDFThe solution of the poroelastic equations for predicting land subsidence above productive gas/oil fields may be addressed by the principle of virtual works using either the effective intergranular stress, with the pore pressure gradient regarded as
a distributed body force, or the total stress incorporating the pore pressure. In the finite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the
strength source related to the pore pressure seems to generate different local forces on the element nodes. The two formulations are briey reviewed and discussed for triangular and tetrahedral finite elements. They are shown to yield different results
at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modification to both formulations is suggested which allows to correctly solve the problem of a finite
reservoir with an infinite pressure gradient, i.e. with a pore pressure discontinuity on its boundary
Flusso tridimensionale e viscoso nei ventilatori assiali: confronto fra risultati sperimentali e numerici
Get PDFLa conoscenza del flusso tridimensionale a valle della girante dei ventilatori assiali è di
fondamentale importanza per valutare la distribuzione delle perdite fluidodinamiche nella
macchina e per intervenire con proposte di modifiche della conformazione dei condotti palari in
fase di progettazione. Per raggiungere questo obiettivo è particolarmente utile poter affiancare
ai metodi di indagine sperimentale codici di calcolo evoluti ed affidabili.
Per verificare la validità di tale impostazione nel presente lavoro si è effettuato il confronto
fra i risultati dell’indagine sperimentale condotta con sonde ad alta risposta in frequenza e
quelli derivanti dalla simulazione del flusso con un codice di calcolo su un ventilatore assiale di
tipo industriale.The knowledge of detailed 3-D flow field downstream of rotor blade in industrial axial flow fan
is important both to evaluate the aerodynamic losses and to improve the aerodynamic design of
these machines.
To achieve these goals it is very important to use a reliable fluid dynamic computational code.
In this paper a comparison between experimental and numerical results downstream an axial
fan are presented
Phase Shift Plus Interpolation: a scheme for high performance echo-reconstructive imaging
Get PDFEcho-reconstruction techniques for non-intrusive imaging have wide application, from subsurface and underwater imaging to medical and industrial diagnostics. The techniques are based on experiments in which a collection of short acoustic or electromagnetic impulses, emitted at the surface, illuminate a certain volume and are backscattered by inhomogeneities of the medium.
The inhomogeneities act as reecting surfaces or interfaces which cause signal echoing; the echoes are then recorded at the surface and processed through a "computational lens" defined by a propagation model to yield an image of the same inhomogeneities. The most sophisticated of these processing techniques involve simple acoustic imaging in seismic exploration, for which the huge data sets and stringent performance requirements make high performance computing essential.
Migration, based on the scalar wave equation, is the standard imaging
technique for seismic applications [1]. In the migration process, the recorded
pressure waves are used as initial conditions for a wave field governed by the
scalar wave equation in an inhomogeneous medium. Any migration technique begins with an a priori estimate of the velocity field obtained from
well logs and an empirical analysis of seismic traces. By interpreting migrated data, comparing the imaged interfaces with the discontinuities of the
estimated velocity model, insuficiencies of the velocity field can be detected
and the estimate improved [2], allowing the next migration step to image
more accurately. The iterative process (turnaround) of correcting to a velocity model consistent with the migrated data can last several computing
weeks, and is particularly crucial for imaging complex geological structures,
including those which are interesting for hydrocarbon prospecting.
Subsurface depth imaging, being as it is the outcome of repeated steps of
3D seismic data migration, requires Gbytes of data which must be reduced,
transformed, visualized and interpreted to obtain meaningful information.
Severe performance requirements have led in the direction of high performance computing hardware and techniques. In addition, an enormous effort has historically gone into simplifying the migration model so as to reduce the cost of the operation while retaining the essential features of the wave propagation. The phase-shift-plus-interpolation (PSPI) algorithm can be an effective method for seismic migration using the "one-way" scalar wave equation; it is particularly well suited to data parallelism because of, among other things, its decoupling of the problem in the frequency domain.126-132Pubblicat
Sustaining the paper metaphor with dynamic HTML
Get PDFWe describe prototypes that sustain the paper metaphor in two new on-line scenarios:
(1) reading annotated works of foreign literature, and (2) paperless medical reporting
[Brelstaff & Chessa, “D-HTML paper metaphors”, submitted to Demo’s at HCI’98]
Mapping basin scale variable source areas from multitemporal remotely sensed observations of soil moisture behavior
Get PDFSoil moisture is an important and highly variable component of the hydrologic cycle. Active microwave remote sensing offers the potential for frequent observation of soil moisture at basin and regional scales. Notwithstanding recent advances, the goal of obtaining accurate and reliable measurements or maps of soil moisture from these instruments remains elusive. The main difficulties for active sensors such as synthetic aperture radar (SAR) are the combined effects of soil moisture, surface roughness, and vegetation on the backscattered signal. We show that it is possible to separate soil moisture information from the other physical factors that dominate the radar backscattering, such as topography and land cover, through a principal component analysis of a time series of eight European Remote Sensing (ERS) SAR images. The soil moisture patterns observed in one of the principal components are consistent with the rainfall-runoff dynamics of a catchment and reflect the variable source areas occuring in the vicinity of the river network.3235–3244Pubblicat
A distributed heterogeneous image server
Get PDFDigital image transmission is now ubiquitous across computer networks and thus there
is increasing pressure to allow access to medical image data at sites remote from PACS
locations. In fact, it may soon make economic sense to outsource medical image
services - to dedicated service providers at geographical locations outside of the
traditional radiology department or HIS’s. The technical challenge faced by system
developers is to produce client-Viewer/server-PACS configurations that can
realistically span the network. In particular, the systems must provide the performance
usually expected by client medics and it must be flexible to the needs of the service
providers.
At CRS4 - BioMedical Applications - we are integrating various technologies derived
from the www-intranet field, and object-oriented middleware to prototype
technological solutions that address both the issues of performance and flexibility.
These are discussed in turn below; then we provide an overview of our system
Interactive virtual angioscopy
Get PDFVirtual angioscopy is a non invasive medical procedure for exploring parts of the human vascular system. We have developed an interactive tool that takes as input data acquired with standard medical imaging modalities and regards it as a virtual environment to be interactively inspected. The system supports real-time navigation with stereoscopic direct volume rendering and dynamic endoscopic camera control, interactive tissue classification, and interactive point picking for morphological feature measurement. In this paper, we provide an overview of the system, discuss the techniques used in our prototype, and present experimental results on human data sets. The accompanying video-tape illustrates our approach with interactive sequences showing the examination of a human carotid artery.435-43
3D spectral reverse time migration with no-wraparound absorbing conditions
Get PDFComparative studies of methods of reverse time migration (RTM) show that spectral methods for calculating the Laplacian impose the least stringent demands on discretization stepsize; thus with spectral methods, the
grid refinements often required by other methods can be avoided. Implemented with absorbing boundary conditions, which are energy-tuned to give good absorption at the boundaries, these spectral methods can be used
effectively for migration, without suffering the problems of wraparound which have traditionally plagued them (Furumyra and Takenaka, 1995).1925-192
Design and optimisation of a liquid metal spallation target for the energy amplifier prototype
Get PDFThe thermo-fluid-dynamic and structural analysis of the spallation target for the Energy Amplifier [1] (EA) is presented. The sensitivity of the system to the geometrical and physical parameters has been analysed through extensive numerical thermo-fluid-dynamic (TFD) simulations performed with the Star-CD commercial code [2]. Starting from the results obtained with the fluid dynamic analysis, a linear structural analysis of the window-pipe system has been performed with the MSC/Nastran finite-elements code [3].
The result of this work is a set of design guidelines for the final sizing of the target
Hyb2D (A hybrid grid generation in 2D). Algorithm overview and description
No full textIn this report we present the hybrid grid generation algorithm implemented in the generator Hyb2D. We will describe the numerical methods and the algorithmic components used by
the version 1.0 of this generator which combines the elliptic generation of boundary-fitted grids (BFGs) with an advancing front technique for unstructured mesh generation.
A companion report [6] contains the instructions on how to run the actual code. The report is organized as follows. First a brief review on mesh generation techniques for structured, unstructured and hybrid grids is given in section 1. Section 2 presents the grid
generation method used to create the structured grid. It is an elliptic generation technique which provides BFGs. Section 3 is devoted to the illustration of the unstructured mesh generation. In section 4 there is the description of the overall hybrid mesh generation strategy using the two presented algorithms for the structured and unstructured part. A short conclusion complete