Search

An efficient and flexible air interface is the necessary condition to achieve the ambitious goals of next generation broadband wireless networks. Multicarrier code division multiple access (MC-CDMA) is a practical answer to this need, but the considerable vulnerability to nonlinear distortions induced by high power amplifiers at the transmitter side hinders its appealing features, thereby demanding for adequate countermeasures. The current paper contributes to this issue with a novel multiuser data predistortion (MUDP) procedure aimed at mitigating the performance degradation of a MC-CDMA forward-link in the presence of a nonlinear channel. The proposed MUDP resides entirely at the transmitter of the base station, and simply requires a set of modified constellations, which are jointly and iteratively optimized so that the distortions experienced by each active user are significantly reduced. The required computational complexity is then kept at affordable levels by resorting to a suboptimal yet effective scheme based on the identification of the strongest interfering users combined with the set-partitioning and the symmetry properties of their constellations. Numerical simulation results corroborate the effectiveness of the MUDP in improving the link performance under typical demanding multiple-access operating conditions. [ABSTRACT FROM PUBLISHER]

The sound generated by the flow around a circular cylinder is numerically investigated by using a finite-element method. In particular, we study the acoustic emissions generated by the flow past the bluff body at low Mach and Reynolds numbers. We perform a global stability analysis by using the compressible linearized Navier-Stokes equations. The resulting direct global mode provides detailed information related to the underlying hydrodynamic instability and data on the acoustic field generated. In order to recover the intensity of the produced sound, we apply the self-consistent model for non-linear saturation proposed by Mantič-Lugo, Arratia, and Gallaire [“Self-consistent mean flow description of the nonlinear saturation of the vortex shedding in the cylinder wake,” Phys. Rev. Lett. 113, 084501 (2014)]. The application of this model allows us to compute the amplitude of the resulting linear mode and the effects of saturation on the mode structure and acoustic field. Our results show excellent agreement with those obtained by a full compressible simulation direct numerical simulation and those derived by the application of classical acoustic analogy formulations. [ABSTRACT FROM AUTHOR]

The research focused on a collection of 37 Late Antique glass fragments, found at the archaeological site of San Giusto (southern Italy). The analyses performed by SEM-EDS, EMPA and LA-ICP-MS showed that all samples are constituted by natron-based soda-lime silica glass and allowed clarifying the distinction between Egyptian HIMT1 and HIMT2 productions. The comparison with the composition of the available glass reference groups was verified through three ternary diagrams (provided here for the first time). The results grouped San Giusto samples into HIMT1 (7 samples), HIMT2 (8 samples) and Levantine (13, plus maybe other 3 samples) productions. A convincing provenance assignment was not feasible for six samples only. The isotopic analyses (87Sr/86Sr and 143Nd/144Nd), performed on a selected set of ten samples, provided further information both in terms of glass technology and provenance, as well as in terms of reference groups characterisation. Particularly, the results demonstrated that nine out of a total of ten samples were made using a limestone-bearing sand enriched with variable amount of marine shells and provided further evidence for the distinction of the HIMT1 and HIMT2 groups; the latter overlapping the Levantine field in the 87Sr/86Sr versus εNd binary diagram. From a technological point of view, the importance of separating the likely 'fresh' materials from those intentionally recycled or coloured was put into clear evidence, especially in relation to provenance assignments. Lastly, an interesting technological feature has been observed in sample no. 9, where the composition of the main body of the vessel turned out to be different from that of the handle. This result provided clear evidence of the fact that different glass batches could have been used for the different parts of a single vessel body. [ABSTRACT FROM AUTHOR]

• LIFUS5 separate effect test facility investigated the HLM-water interaction. • Experiments of Steam Generator Tube Rupture event in HLMFR. • Twenty-years of experimental programme. The LIFUS5 facility is a separate effect test facility aimed at investigating the heavy liquid metal-water interaction. It has been designed and constructed to withstand high pressure and temperature (i.e. up to 200 bar and up to 500 °C) and to record the fast pressure transients in heavy liquid metal melt. This type of transient is typical of a Steam Generator Tube Rupture event in a pool type Gen IV Heavy Liquid Metal cooled Fast Reactor system, and should be investigated from the safety point of view because it could potentially induce, beyond the damaging of the internal structures (HX tube bundle, above core structures, Fuel Assembly, Control Rods, etc.) several negative effects on the operation of the reactor. These include an insertion of positive reactivity into the system or reduced cooling efficiency due to steam dragging into the core. It will also have an effect on the chemistry control of the cooling. All these effects compromise the safety and the reliability of the system. The twenty years experimental programme and the different LIFUS5 configurations are presented in the paper, as well as the experimental campaigns and test matrix characterizing the heavy liquid metal-water interaction phenomena and data for codes validation. [ABSTRACT FROM AUTHOR]

• The paper discusses a subchannel modeling approach developed with RELAP5-3D for lead and lead–bismuth eutectic (LBE) cooled wire-wrapped fuel bundle. • Code results are compared to experiment data coming from the NACIE-UP facility, hosted at ENEA Brasimone Research Center. • The influence on numerical results of several thermal hydraulic phenomena was evaluated (pressure drops along bundle, crossflow between adjacent subchannels, sweeping mixing induced by helical wire and radial thermal conduction). • The comparison between simulation outcomes and experimental data highlighted a satisfactory prediction obtained by the code. The paper deals with the presentation of a subchannel modeling approach developed with RELAP5-3D for lead and lead–bismuth eutectic (LBE) cooled wire-wrapped fuel bundle. For the modeling assessment, the code-to-experiment analysis is performed using experimental data from the NACIE-UP facility, hosted at ENEA Brasimone Research Center. Three relevant thermal hydraulic phenomena must be accounted in case of wire-wrapped fuel bundle: pressure drops along bundle, crossflow between adjacent subchannels, and sweeping mixing induced by helical wire. Furthermore, in case of liquid metal system, the effect of radial thermal conduction must be evaluated. All these phenomena are considered in the modeling methodology. A thermal hydraulic nodalization of the whole facility has been developed, with particular attention to the fuel pin bundle simulator (modelled with a detailed subchannel approach). The main outcomes of the activity are the assessment of satisfactory capabilities of the proposed subchannel modeling and the identification of some open issues to be investigated in the future works. [ABSTRACT FROM AUTHOR]

The study of the stability of a dynamical system described by a set of partial differential equations (PDEs) requires the computation of unstable states as the control parameter exceeds its critical threshold. Unfortunately, the discretization of the governing equations, especially for fluid dynamic applications, often leads to very large discrete systems. As a consequence, matrix based methods, like for example the Newton–Raphson algorithm coupled with a direct inversion of the Jacobian matrix, lead to computational costs too large in terms of both memory and execution time. We present a novel iterative algorithm, inspired by Krylov-subspace methods, which is able to compute unstable steady states and/or accelerate the convergence to stable configurations. Our new algorithm is based on the minimization of the residual norm at each iteration step with a projection basis updated at each iteration rather than at periodic restarts like in the classical GMRES method. The algorithm is able to stabilize any dynamical system without increasing the computational time of the original numerical procedure used to solve the governing equations. Moreover, it can be easily inserted into a pre-existing relaxation (integration) procedure with a call to a single black-box subroutine. The procedure is discussed for problems of different sizes, ranging from a small two-dimensional system to a large three-dimensional problem involving the Navier–Stokes equations. We show that the proposed algorithm is able to improve the convergence of existing iterative schemes. In particular, the procedure is applied to the subcritical flow inside a lid-driven cavity. We also discuss the application of Boostconv to compute the unstable steady flow past a fixed circular cylinder (2D) and boundary-layer flow over a hemispherical roughness element (3D) for supercritical values of the Reynolds number. We show that Boostconv can be used effectively with any spatial discretization, be it a finite-difference, finite-volume, finite-element or spectral method. [ABSTRACT FROM AUTHOR]

• The simulation activity was aimed at analyzing the TH performances of the DEMO WCLL BB PHTS during accidental conditions. • A complete model of the system was prepared by using RELAP5/Mod3.3 code. • Locked Rotor/Shaft Seizure was selected as PIE and investigated when involving either a BZ or a FW pump. • A preliminary management strategy, based on the consolidated PWR experience, was proposed and implemented for some reactor components. • The simulation outcomes proved the appropriateness of the current blanket and PHTS design in withstanding such accidental conditions. The EU-DEMO Water-Cooled Lithium-Lead (WCLL) Breeding Blanket (BB) main subsystems to be cooled are the Breeder Zone (BZ) and the First Wall (FW). Each subsystem will be equipped with an independent Primary Heat Transfer System (PHTS). Within the framework of the EUROfusion Work Package Breeding Blanket research program, several accidents belonging to the category of "Decrease in Coolant System Flow Rate" were studied. The activity was aimed at evaluating the blanket and primary cooling systems thermal-hydraulic performances during such transient conditions. A complete model including the BB and related PHTS circuits has been developed at Sapienza University of Rome. A modified version of RELAP5/Mod3.3 system code has been used to perform the calculations. The simulation results showed that a locked rotor/shaft seizure of a BZ or a FW main coolant pump is the most challenging scenario. BZ and FW system behavior has been analyzed following this initiating event with the goal of the design improvement and to individuate the need for preventive measures. The influence of loss of off-site power on the accident evolution has also been investigated. Moreover, management strategies have been proposed for different reactor components. Calculations demonstrate that the current blanket and PHTS design is appropriate to cope with these kinds of accident scenarios. [ABSTRACT FROM AUTHOR]