1,721,016 research outputs found
Modified gravitational waves across galaxies from macroscopic gravity
Full text linkWe analyze the propagation of gravitational waves in a medium containing bounded subsystems ("molecules") able to induce significant macroscopic gravity effects We establish a precise constitutive relation between the average quadrupole and the amplitudes of a vacuum gravitational wave, via the geodesic deviation equation. Then we determine the modified equation for the wave inside the medium and the associated dispersion relation. A phenomenological analysis shows that anomalous polarizations of the wave emerge with an appreciable experimental detectability if the medium is identified with a typical galaxy. Both the modified dispersion relation (wave velocity less than the speed of light) and anomalous oscillations modes could be detectable by the incoming LISA or pulsar timing array experiments, having the appropriate size to see the concerned wavelengths (larger than the molecular size) and the appropriate sensitivity to detect the expected deviation from vacuum general relativity
Independent analysis for the Assessment of Multidimensional Features of the MARS Code
No full textA request was formulated to University of Pisa by the Government regulatory body in Korea KINS (Korean Institute for Nuclear Safety) to assess the three dimensional capabilities of the MARS code developed in Korea. An innovative approach was proposed by University of Pisa including the use of both system thermal-hydraulic code (having, as far as possible, qualified 3D capabilities) and a commercial qualified Computational Fluid-Dynamic (CFD) code. An experiment in one German facility where the three dimensional nature of flow was measured was simulated by the three codes (i.e. the MARS, the RELAP5/3D and the CFX). The adopted approach involved the characterization of a quantitative measure of accuracy including spatial distribution of parameters at the basis of a figure of Merit and the use of the Fast Fourier Transform based method (FFTBM) for the time variation of the selected parameters.
The activity, based on the comparison with experimental data, demonstrated the capabilities of the MARS code (i.e. same level-of-error as other state-of-art-recognized computational tools)
Scalar modes in extended hybrid metric-Palatini gravity: weak field phenomenology
Full text linkWe investigate the nature of additional scalar degrees of freedom contained in extended hybrid metric-Palatini gravity, outlining the emergence of two coupled dynamical scalar modes. In particular, we discuss the weak field limit of the theory, both in the static case and from a gravitational waves perspective. In the first case, performing an analysis at the lowest order of the postparametrized Newtonian structure of the model, we stress the settling of Yukawa corrections to the Newtonian potential. In this respect, we show that one scalar field can have long range interactions as used in the principle for mimicking dark matter effects. Concerning the gravitational waves propagation, instead, we demonstrate that it is possible to have well-defined physical degrees of freedom, provided by suitable constraints on model parameters. Moreover, the study of the geodesic deviation points out the presence of breathing and longitudinal polarizations due to these novel scalar waves, which on peculiar assumptions can give rise to beating phenomena during their propagation
Identification of relevant experimental data
No full textThe EC- (European Commission) EURATOM TACIS project 2.02/02 aims at safety evaluation of VVER-1000 Russian Design, Gidropress, pressurized water reactor. University of Pisa was partner for the project leaded by AREVA in Germany. Mixing of boron in the vessel constitutes a safety issue to be addressed in the activity. To this aim qualified computational tools shall be available. In order to demonstrate the quality level, experimental data are needed. This document provides a review of experimental data suitable to perform code validation and demonstrate capabilities of the codes to be adopted for safety analysis in VVER-1000
Autogenous healing on the recovery of mechanical performance of High Performance Fibre Reinforced Cementitious Composites (HPFRCCs): Part 2 – Correlation between healing of mechanical performance and crack sealing
Full text linkThis paper is the second part of a companion paper study focused on the autogenous self-healing capacity
of High Performance Fibre Reinforced Cementitious Composites (HPFRCCs). In part 1 investigation
has focused on the capacity of the material to completely or partially re-seal the cracks, as a function of
its composition, maximum crack width and exposure conditions. Different flow induced alignment of
fibres, with respect to the applied bending stresses have been also considered. The outcomes of the selfhealing
phenomenon, have been analyzed in terms of recovery of stiffness, strength and ductility, as
measured by means of 4-point bending tests, performed before (pre-cracking) and after the conditioning
exposure. In a durability-based design framework, self-healing indices quantifying the recovery of mechanical
properties were also defined and their significance cross-checked. In this paper the crack closure
will be evaluated, both through visual image analysis of the healed cracks as well as through a tailored
indirect method, proposed by the first authors in a previous study. This method is based on the
comparative analysis of the damage evolution curves built for both the pre-cracked and the healed stages
from the evaluation of the flexural stiffness. Recovery of mechanical properties will hence be correlated
to the identified amount of crack closure. In the authors' opinion, this step represents a fundamental
contribution in order to reliably and consistently incorporate the effects of self-healing into tailored
durability-based design approaches, based, e.g., on a “healable” crack width threshold concept
On the use of crystalline admixtures in cement based construction materials: from porosity reducers to promoters of self healing
No full textThe project detailed in this paper aims at a thorough characterization of the effects of crystalline
admixtures, currently employed as porosity reducing admixtures, on the self-healing capacity of the
cementitious composites, i.e. their capacity to completely or partially re-seal cracks and, in case, also
exhibit recovery of mechanical properties. The problem has been investigated with reference to both
a normal strength concrete (NSC) and a high performance fibre reinforced cementitious composite
(HPFRCC). In the latter case, the influence of flow-induced fibre alignment has also been
considered in the experimental investigation. With reference to either 3-point (for NSC) or 4-point
(for HPFRCC) bending tests performed up to controlled crack opening and up to failure,
respectively before and after exposure/conditioning recovery of stiffness and stress bearing capacity
has been evaluated to assess the self-healing capacity. In a durability-based design framework, selfhealing
indices to quantify the recovery of mechanical properties will also be defined. In NSC,
crystalline admixtures are able to promote up to 60% of crack sealing even under exposure to open
air. In the case of HPFRCCs, which would already feature autogenous healing capacity because of
their peculiar mix compositions, the synergy between the dispersed fibre reinforcement and the
action of the crystalline admixture has resulted in a likely ‘chemical pre-stressing’ of the same
reinforcement, from which the recovery of mechanical performance of the material has greatly
benefited, up to levels even higher than the performance of the virgin un-cracked material
CFX simulation of ROCOM experiments
No full textThe EC- (European Commission) EURATOM TACIS project 2.02/02 aims at safety evaluation of VVER-1000 Russian Design, Gidropress, pressurized water reactor. University of Pisa was partner for the project leaded by AREVA in Germany. Mixing of boron in the vessel constitutes a safety issue to be addressed. To this aim qualified computational tools shall be available. In order to demonstrate the quality level of codes, experimental data are needed. This document deals with the comparison between boron mixing relevant data measured in the ROCOM test facility (available at the Rossendorf research center in Germany) and the commercial CFX, Computational Fluid Dynamic code, available at University of Pisa
Study of a Boiling Channel with System and CFD codes
No full textThe present report documents the system and CFD code validation activity carried out at University of Pisa, dealing with the simulation of forced convective boiling flows.
At first, a study of system codes capabilities in predicting boiling flow characteristics is presented; thanks to the expertise achieved by DIMNP researches in the use of these thermal-hydraulics tools, it was possible to perform steady state and transient analyses of a BWR fuel assembly with three different codes (namely RELAP5, CATHARE2 and KORSAR).
Since simulation of boiling flows through CFD approaches is a rather new issue for our Department, a simpler case was chosen as study problem: forced convective boiling flow in a vertical heated pipe, with water at high pressure. Calculations were performed with ANSYS CFX-11.0 (“beta” version, embedding the RPI wall boiling model). Several sensitivity studies were conduced, in order to assess the influence of mesh size and model parameters on code performances. Numerical results, in terms of axial distribution of void fraction and liquid temperature, are compared against measured data from Bartolomei’s experiment and system code predictions obtained with RELAP5. This comparison showed that the axial void distribution and the void fraction at the channel exit can be accurately predicted by CFX (at least within the experimental uncertainty range)
Relap5-3D©/V2.3.6 assessment against boron mixing experiments performed at ROCOM test facility
No full textThe EC- (European Commission) EURATOM TACIS project 2.02/02 aims at safety evaluation of VVER-1000 Russian Design, Gidropress, pressurized water reactor. University of Pisa was partner for the project leaded by AREVA in Germany. Mixing of boron in the vessel constitutes a safety issue to be addressed. To this aim qualified computational tools shall be available. In order to demonstrate the quality level of codes, experimental data are needed. This document deals with the comparison between boron mixing relevant data measured in the ROCOM test facility (available at the Rossendorf research center in Germany) and the Relap5/3D, system thermal-hydraulics code available at University of Pisa. This activity is complementary to the activity dealing with the application of the CFD code CFX to the same category of experiments
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