bluuebunny/crossref_metadata_2025_split huggingface dataset:
Projection algorithm for simulation of fluid flow around moving objects with immersed boundary method
Authors: Abbati Alessia , Chierici Andrea , Chirco Leonardo , Da Vià Roberto , Manservisi Sandro 
Published: May 2019

Abstract
The simulations of fluid flow around moving objects are usually done using body conforming meshes, but recently there is growing interest in algorithms using non conforming grids, known as immersed boundary methods (IB). In particular the resolution of the flow around objects with moving boundary may involve a high meshing work that has to be repeated at each time step. In the present work the immersed boundary method is applied to an in-house finite element code, in order to simulate transient applications involving the movement of immersed solid interfaces. The distinguishing feature of this method is that the entire simulation is carried out on a grid which cover the entire domain and do not conform to the geometry of the rigid objects. A second mesh independent from the first one and built on the immersed body is considered. The presented method consists in the projection of fields, including boundary conditions, from the second mesh mentioned to the first one, that cover the whole domain. The projection is carried out considering the movement of the immersed objects, through the handling of the second mesh. The proposed algorithm is built in order to satisfy the mass conservation in the entire fluid region and to avoid any numerical instability. For the projection of such fields we develop a multiphysics open-source coupling platform, based on the open-source SALOME platform. MED (Modèle d’Échange de Donné) data structure has been used for all field operations, allowing the algorithm to be used with all the numerical codes where a MED duplicate of the field solution can be created. Applications of this approach will be presented and we provide results that show the consistency of the proposed algorithm.
Distributed optimal control applied to Fluid Structure Interaction problems
Authors: Chierici A , Chirco L , Da Vià R , Manservisi M , Magnaniand S 
Published: May 2019

Abstract
In this paper we focus on an optimal control problem based on Lagrangian multipliers and adjoint variables applied to the steady state Fluid-structure interaction problem. FSI systems consist of one or more solid structures that deform by interacting with a surrounding fluid flow. We consider the Fluid-structure interaction problem written in variational monolithic form in order to balance automatically solid and liquid forces at the interface. The objective of the control problem is the minimization of a velocity matching functional and it is obtained with a distributed force control that deforms the shape of the solid domain. This distributed control for Fluid-structure interaction systems can be easily used to control the shape of the solid domain and the flow effects of the fluid-solid interactions. The optimality system is derived from the first order optimality conditions by taking the Fréchet derivatives of the Lagrangian with respect to all the variables involved. The optimal solution is then obtained through a steepest descent algorithm applied to the optimality system. To support this approach we report a few numerical tests where the distributed force deforms the solid structure in order to match the desired velocity profile. The results obtained show the feasibility and robustness of this approach and its possible use in many industrial relevant applications.
A new surface tension VOF evaluation by using variational representation and Galerkin interpolation projection
Authors: Chirco L. , Chierici A. , Da Vià R. , Manservisi S. 
Published: May 2019

Abstract
In this work we propose a new algorithm for studying two-phase interface advection problems dominated by surface tension. We use a Volume Of Fluid (VOF) algorithm for studying the evolution of the two-phase interface on a Cartesian grid and a finite element numerical scheme for the velocity-pressure state. The velocity field that drives the evolution of this interface is obtained by solving the weak form of the Navier-Stokes equation where the surface tension force is not defined in a singular way. With standard numerical approaches that solve the strong form of the Navier-Stokes equations the surface force is determined by taking the divergence of the surface tension tensor. The computation of the divergence term results in a force which is non-convergent when the grid is refined since the tensor is computed in a discontinuous cell-by-cell way. In the past this approach was proposed with artificial different smoothing schemes in order to compute such a singular force. In this work we use the variational formulation of the Navier-Stokes equation and avoid differentiation. The tensor which is a function of the unit normal is evaluated by a Galerkin projection over regular Sobolev spaces. This allows the piece-wise continuous representation of the surface tensor and the unit normal based on the VOF reconstruction. Tests on convergence for two and three-dimension in the static and dynamical cases are reported to show the correct representation in the desired spaces. This method is also natural for coupling non uniform grid computation of the fluid with Cartesian grid of the VOF algorithm.
Optimal Control of the Wilcox turbulence model with lifting functions for flow injection and boundary control
Authors: Chirco L , Chierici A , Da Vià R , Giovacchini V , Manservisi S 
Published: May 2019

Abstract
This paper deals with boundary optimal control problems for the Navier-Stokes equations and Wilcox turbulence model. In this paper we study adjoint optimal control problems for Navier-Stokes equations to improve the advantages of using simulations where turbulence models play a significant role in designing engineering devices. We assess first distributed optimal control problems with the purpose to control the fluid behavior by injecting a flow on the boundary solid region to obtain a desired control over the fluid velocity and the kinetic turbulence energy in specific parts of the domain. Then, with the same purpose, we use lifting functions and boundary control. For this reason we reformulate the boundary optimal control problem into a distributed problem through a lifting function approach. The stronger regularity requirements which are imposed by standard boundary control approaches can then be avoided. The state, adjoint and control equations are derived and the optimality system solved for some simple cases with a finite element. Furthermore, we propose numerical strategies that allow to solve the coupled optimality system in a robust way for a large number of unknowns. The approach presented in this work is general and can be used to assess different objectives and types of control.
Numerical modelling of flashing flow phase change in convergent-divergent nozzle: A sensitivity analysis
Authors: Quang Dang Le , Mereu Riccardo , Besagni Giorgio , Dossena Vincenzo , Inzoli Fabio 
Published: May 2019

AbstractA sensitivity analysis of a computational fluid dynamics (CFD) model of flashing flow phenomenon including the thermal non-equilibrium effect is proposed in the present paper. The model uses a two-phase mixture approach and the phase-change process is based on the difference between the vaporization pressure and the vapour partial pressure. Thermal non-equilibrium effect is included in the model by a sub-routine for the boiling delay. A two-dimensional axisymmetric convergent-divergent nozzle is used as benchmark for the proposed study. Such geometry is representative of well-known applications in nuclear, refrigeration and energy engineering (e.g., primary flow in the motive nozzle of ejectors). The results are compared to experimental data and previous numerical results available in literature. Present results show good agreement with global and local experimental fluid dynamic quantities used to validate the model. The paper includes in the first part a brief description of physical phenomenon of flashing flow, the experimental benchmark geometry and operating conditions. In the second part, the physical model and the numerical modeling approach are reported, showing the validation and the results related to the sensitivity analyses of the artificial coefficients characterizing the modelling approach.
Comparison between cooling strategies for power electronic devices: fractal mini-channels and arrays of impinging submerged jets
Authors: Baraldi N , Fregni A , Sabato M , Stalio E , Brusiani F , Tranchero M , Baritaud T 
Published: May 2019

Abstract
Power electronic devices like Insulated Gate Bipolar Transistors (IGBTs) and diodes are often characterized by power densities and dimensions that could result in very high heat flux densities. In order to guarantee the expected performance and lifetime for these components, dedicated active cooling devices are usually adopted. In the present paper, the comparison between two different cooling strategies for power electronics is presented: fractal-channel design and submerged impinging jets. Each cooling strategy is tested on two different geometrical configurations. Water is used as coolant in all cases. Assessment of the considered cooling methods is done through application of the selected configuration in a simplified system composed by a rectangular chip (heat source) separated from the coolant by a solid block.
Three-dimensional conjugated heat transfer simulations are performed by using RANS solver implemented in OpenFOAM and two-equations turbulence models, resolving also the viscous sublayer. Numerical results allow to compare the cooling strategies in terms of maximum chip temperature, overall chip-to-coolant thermal resistance, and pumping power required. In summary, the fractal-channel design shows limitations in guaranteeing low chip temperatures at an affordable pumping power. The submerged impinging jets approach shows very high local heat transfer coefficient by which it is possible to tailor the cooling effect on specific hot spots.
Finite heating duration based-sensitivity analysis in transient heat conduction
Authors: D’Alessandro G , de Monte F 
Published: May 2019

Abstract
In the parameter estimation the sensitivity analysis of the temperature to the unknown parameter (such as thermal conductivity, heat capacity) plays a fundamental role. In the experimental setup for thermal properties measurements of solid materials, the heater in contact with the specimen can be modelled through an high conductivity thin layer. In such a case the one dimensional finite rectangular body, representing the sample, is subject to a boundary condition of the 4th kind at the heated boundary. Also, a constant heat flux applied for a finite period of time is considered in the analysis and the sample backside is assumed insulated. The temperature field within the finite body is obtained by using the superposition principle. Then, the scaled sensitivity coefficients are computed analytically for two different locations: at the interface between the heater and the sample, and at the sample backside. The results show that the sensitivity coefficients with respect to the thermal conductivity and to the volumetric heat capacity of the sample are uncorrelated.
Occupancy modelling of buildings based on CO2 concentration measurements: an experimental analysis
Authors: Franco Alessandro , Leccese Francesco , Marchi Lorenzo 
Published: May 2019

Abstract
The problem of real-time estimation of occupancy of buildings (number of people in various zones at every time instant) is relevant to a number of emerging applications that achieve high energy efficiency through feedback control. The measurement of CO2 concentration can be considered an important indicator that allows to define the occupation of closed and crowded spaces. Interesting cases can be school buildings and other buildings used in civil and residential (shopping centers, hospitals, etc.). This paper starting from an experimental analysis conducted of a University campus in different operating conditions, with different model for occupancy simulation in multi-zone multi-occupant buildings and its preliminary validation from experimental data acquired in different cases. The acquired data are used to present some graphical models and perform occupancy estimates. Starting from an accurate analysis of the data, attempts are made to define a preliminary estimation method through the development of a mathematical models of occupancy dynamics in a building, which show promising results.
Experimental study of PM emissions from wood pellet stoves with an innovative burning pot
Authors: Polonini L F , Petrocelli D , Parmigiani S P , Lezzi A M 
Published: May 2019

Abstract
In this study, results of an extensive set of experiments on a wood pellet stove model equipped with innovative burning pots are presented. Tests have been performed to investigate the relations between design data and operation parameters of the stove and its emissions. In particular, carbon monoxide (CO) and particulate matter (PM) emissions are correlated to the burning-pot depth.
The burning pot A, adopted by AICO S.p.A, allows to obtain near-to-zero CO emissions and low PM emissions: however, PM emissions reduction with respect to standard burning pots is not as enhanced as CO emission reduction. Here it is shown that a further reduction of PM emissions (down to 6-16 mg/Nm3) can be attained increasing the burning pot depth.
Experiments show also an increment of PM emissions in subsequent tests in the same day: there is an apparent dependence of PM release on the time from ignition. It is likely that this effect is due to ashes accumulation on the bottom of the burning pot and to their interaction with the primary air stream that flows over them.
Experimental analysis of steam condensation patterns within a pressure suppression system operating at sub-atmospheric pressure conditions
Authors: Lo Frano R , Aquaro D , Mazed D , Del Serra D , Giambartolomei G 
Published: May 2019

Abstract
For some specific applications, the pressure suppression systems should be maintained at sub-atmospheric pressure, such as in fusion plants systems. This operation conditions differ considerably from those experienced in the suppression pools of BWR operating at atmospheric pressure.
The steam condensation in water at sub-atmospheric conditions is not sufficiently known, if not at all; therefore its effectiveness has to be investigated experimentally. In fact the steam condensation at sub-atmospheric pressure requires a subcooling capacity 2.5 times higher than that at atmospheric pressure.
In consideration of that, a low scale experimental facility was designed and built at the University of Pisa. More than 350 steam condensation tests were performed in broad influencing parameters range (e. g. pressure within the condensation tank, water temperature and steam mass flux) to study of effectiveness of the steam condensation. In doing that, different sparger configurations (with single or multiple holes) were considered to check possible interference of condensing steam jets and bubble coalescence.
Results indicate a high efficiency of condensation for all the examined conditions. The 3D map of the condensation regimes demonstrate they are dependent only on the downstream exit pressure, the water temperature and the steam mass flux per hole.
The ultrasounds as a mean for the enhancement of heat exchanger performances: an analysis of the available data
Authors: Franco Alessandro , Bartoli Carlo 
Published: May 2019

Abstract
The aim of this paper is to investigate the potential use of ultrasounds for improving the performances of heat exchangers. In a heat exchanger, the effect of ultrasonic vibrations is important both for heat transfer intensification and for the possibility of obtaining fouling reduction: the cumulative effect can determine an interesting advantage in term of overall heat transfer coefficient increase. After a general analysis of the topic, the paper analyse the results recently obtained in the two main fields of analysis, heat transfer coefficient enhancement and fouling reduction. The perspective of the use of acoustic fields and ultrasound for optimization of heat exchangers operation is some particular fields of application are discussed.
Design and assessment of an experimental facility for the characterization of flow boiling of azeotropic refrigerants in horizontal tubes
Authors: Pietro Maria Colombo Luigi , Lucchini Andrea , Nhan Phan Thanh , Molinaroli Luca , Niro Alfonso 
Published: May 2019

Abstract
An experimental facility was designed to measure pressure drop and heat transfer coefficient during flow boiling of azeotropic refrigerants in horizontal tubes. The apparatus is made of the refrigerant circuit, including the test section; the water circuit, to provide the power for evaporation; the glycol-water circuit, to fix the refrigerant pressure. The test section is a counterflow tube-in-tube heat exchanger (refrigerant inside, water outside). Plant assessment involved the measurement of pressure drop and heat transfer during R134a flow boiling in a smooth tube (outer diameter 9.56mm, inner diameter 8.92mm). Such a case study is a benchmark for the availability of a wide experimental database in the literature, which has been also summarized in many correlations. Then further tests involved a microfin tube. The experimental conditions were: evaporation temperature, 5°C; mass flux, 111÷333kg/m2s; average quality, 0.15÷0.93; heat flux, 8.8kW/m2 and 17.6kW/m2. The uncertainty affecting the pressure drop and the heat transfer coefficient resulted lower than 1% and 5% respectively. The comparison with the literature shows satisfactory agreement with the major findings and enables using the data for the assessment of the existing models to predict both the pressure drop and the heat transfer coefficient.
Experimental results on local heat transfer coefficient inside a rectangular channel with chevron-shaped ribs
Authors: Gramazio P , Vitali L , Fustinoni D , Vignati F , Niro A 
Published: May 2019

Abstract
A non-intrusive measurement technique of the heat transfer coefficient field, based on IR-thermography, is applied on a 12-mm-height rectangular channel of 1:10 aspect ratio, whose lower surface is enhanced by chevron-shaped square-cross-section ribs. The channel is operated at imposed heat flux while forced-air flows for Reynolds numbers ranging from 700 to 8000. In particular, the technique uses local temperature measurements to retrieve the radiative heat flux with a three-wavelength-band model for partially transparent enclosures, and the conductive flux towards the backside with a FEM model, which allow an edge-to-edge estimation of the local convective heat transfer. The resulting local Nusselt number maps are presented and compared to data obtained operating both at fixed temperature and inside a channel with flat surfaces.
Bulk viscosity in 2D relativistic uids: the effects of temperature and modifications to the Rayleigh-Brillouin spectrum
Authors: García-Perciante A L , Franco-Pérez L , Méndez A R 
Published: May 2019

Abstract
The dependence of the bulk viscosity with the relativistic parameter z = kT/mc
2, obtained through the complete Boltzmann equation [1, 2], is thoroughly analyzed. A complete and rigorous examination of the relevant non-relativistic and ultra-relativistic limits is carried out in the case of a hard disk model and compared with the results obtained in the relaxation time approximation. The modifications of the non-vanishing bulk viscosity to the Rayleigh - Brillouin is briefly discussed.
From peace campaigns to peaceocracy: Elections, order and authority in Africa
Authors: Lynch Gabrielle , Cheeseman Nic , Willis Justin 
Published: May 2019

Abstract
Research on Kenya’s 2013 elections has suggested that a “peace narrative” was deliberately promoted by an establishment elite to delegitimize protest and justify the use of excessive force. It has also tended to see the Kenyan case as exceptional and to assume that such a narrative was only possible because of the 2007/2008 post-election violence. We agree that peace campaigns are often particularly intense in the wake of violence and that they can be manipulated to generate a “peaceocracy”, a system in which an emphasis on peace is used to prioritize stability and order to the detriment of democracy. However, by comparing Kenya to Ghana and Uganda, two countries that have had very different experiences of elections and election-related violence, we demonstrate that peace messaging is neither unique to countries that have experienced recent electoral conflict, nor a recent phenomenon. Instead, we highlight the pervasiveness of peace narratives across the sub-continent, which we show is due to a number of factors. These include but are not limited to the way that elections are used to assert and perform state autonomy and an associated ideal of elections as orderly processes; the capacity of multiple actors to instrumentalize the ideal of orderly elections; a popular fear of electoral violence even in countries where it is rare; a growing tendency to individualize responsibility for peace; and the availability of international funding. Taken together, these factors help to explain the rise of peace messaging. At the same time, we argue that the risk that this messaging will foster a “peaceocracy” varies markedly and that the likelihood of incumbent manipulation is greatest in countries with a recent history of civil conflict and where the quality of democracy is already low.
The expression of extracellular matrix metalloproteinase inducer (EMMPRIN) in the compression area during orthodontic relapse
Authors: Li Hanyue , Xia Lulu , Wang Shuo , Al-balaa Maher , Liu Wei , Hua Xianming 
Published: May 2019

Summary

Objective
This study investigated the expression of extracellular matrix metalloproteinase inducer (EMMPRIN) in the compression area during orthodontic relapse in rat molars.

Materials and methods
Thirty Wistar rats (6 weeks old) underwent orthodontic tooth movement (OTM) of the left first maxillary molar for 21 days, followed by removal of the force device. The contralateral maxillary molar served as a control with no mechanical force stimuli. Animals were sacrificed at 0, 1, 3, 7, and 14 days of relapse after force withdrawal. Tooth relapse and alveolar bone parameters were measured using microcomputed tomography (micro-CT). Maxilla sections were obtained for haematoxylin and eosin (HE), immunohistochemical staining [EMMPRIN, nuclear factor kappa B ligand (RANKL) and vascular endothelial growth factor (VEGF)] and tartrate-resistant acid phosphatase (TRAP). Correlation analyses were then performed.

Results
After force removal, nearly 79.88% of the total relapse occurred within the initial 3 days. The number of osteoclasts clearly increased while the alveolar bone density decreased on the pressure side on Day 3 of relapse. Moreover, the EMMPRIN expression level significantly increased on Day 1, peaked up on Day 3 and decreased on Days 7 and 14. Statistically, a strong positive correlation was found between EMMRPIN expression and the osteoclast number and RANKL and VEGF expression.

Conclusion
EMMPRIN was highly expressed on the pressure side during the orthodontic tooth relapse, which could be involved in osteoclastogenesis and alveolar bone resorption in association with RANKL and VEGF expression.