arXiv subjects

Andrea Ferrantelli

Publications and source records attributed to Andrea Ferrantelli.

11 recordsLinked to original sources

arXiv2019

Thermomechanical surface instability at the origin of surface fissure patterns on heated circular MDF samples

When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primary crack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary to common belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that this results from thermomechanical instability. In the present paper, new experimental data are presented for circular samples of the same MDF material. The sample was exposed to radiant heating at 20 or 50 kW/m2, and completely different crack patterns with independent Eigenmodes were observed at the two heat fluxes. We show that the two patterns can be reproduced with a full 3-D thermomechanical surface instability model of a hot layer adhered to an elastic colder foundation in an axisymmetric domain. Analytical and numerical solutions of a simplified 2-D formulation of the same problem provide excellent qualitative agreement between observed and calculated patterns. Previous data for square samples together with the results reported in the present paper for circular samples confirm the validity of the model for qualitative predictions, and indicate that further refinements can be made to improve its quantitative predictive capability.

cond-mat.soft↗

arXiv2019

Energy analysis in ice hockey arenas and analytical formula for the temperature profile in the ice pad with transient boundary conditions

The energy efficiency of ice hockey arenas is a central concern for the administrations, as these buildings are well known to consume a large amount of energy. Since they are composite, complex systems, solutions to such a problem can be approached from many different areas, from managerial to technological to more strictly physical. In this paper we consider heat transfer processes in an ice hockey hall, during operating conditions, with a bottom-up approach based upon on-site measurements. Detailed heat flux, relative humidity and temperature data for the ice pad and the indoor air are used for a heat balance calculation in the steady-state regime, which quantifies the impact of each single heat source. We then solve the heat conduction equation for the ice pad in transient regime, and obtain a generic analytical formula for the temperature profile that can be used in practical applications. We then apply this formula to the resurfacing process for validation, and find good agreement with an analogous numerical solution. Since it is given with implicit initial condition and boundary conditions, it can be used not only in ice hockey halls, but in a large variety of engineering applications.

physics.gen-ph↗

arXiv2018

Gravitino phenomenology and cosmological implications of supergravity

Gravitino production in the primordial Universe is investigated into details. After briefly reviewing inflation, supersymmetry and supergravity, we first study the scattering of massive W bosons in the thermal bath of particles, during the period of reheating. It is found that the process generates in the cross section terms which eventually lead to unitarity breaking above a certain scale. This happens by virtue of the supergravity vertex. We show that the longitudinal polarizations of the on-shell W become strongly interacting in the high energy limit, and that the inclusion of diagrams with off-shell scalars of the MSSM does not cancel the divergences. Next, we consider the dynamics and the decay into gravitinos of a scalar field S, which starts oscillating in its potential at the end of inflation. We embed S in a model of gauge mediation with metastable vacua, where the hidden sector is of the O'Raifeartaigh type. By demanding that the gravitinos thus produced provide with the observed Cold Dark Matter density, we modify previous results in the literature, and find that it is easy to account for gravitino Dark Matter with an arbitrarily low reheating temperature.

hep-ph↗

arXiv2017

Reheating, thermalization and non-thermal gravitino production in MSSM inflation

In the framework of MSSM inflation, matter and gravitino production are here investigated through the decay of the fields which are coupled to the udd inflaton, a gauge invariant combination of squarks. After the end of inflation, the flat direction oscillates about the minimum of its potential, losing at each oscillation about 56% of its energy into bursts of gauge/gaugino and scalar quanta when crossing the origin. These particles then acquire a large inflaton VEV-induced mass and decay perturbatively into the MSSM quanta and gravitinos, transferring the inflaton energy very efficiently via instant preheating. Regarding thermalization, we show that the MSSM degrees of freedom thermalize very quickly, yet not immediately by virtue of the large vacuum expectation value of the inflaton, which breaks the $SU(3)_C\times U(1)_Y$ symmetry into a residual $U(1)$. The energy transfer to the MSSM quanta is very efficient, since full thermalization is achieved after only $\mathcal{O}(40)$ complete oscillations. The udd inflaton thus provides an extremely efficient reheating of the Universe, with a temperature $T_{reh}=\mathcal{O}(10^8\mathrm{GeV})$ that allows for instance several mechanisms of baryogenesis. We also compute the gravitino number density from the perturbative decay of the flat direction and of the SUSY multiplet. We find that the gravitinos are produced in negligible amount and satisfy cosmological bounds such as the Big Bang Nucleosynthesis (BBN) and Dark Matter (DM) constraints.

hep-ph↗

arXiv2017

A thermo-mechanical explanation for the topology of crack patterns observed on the surface of charred wood and particle fibreboard

In the assessment of wood charring, it was believed for a long time that physicochemical processes were responsible for the creation of cracking patterns on the charring wood surface. This implied no possibility to rigorously explain the crack topology. In this paper we show instead that below the pyrolysis temperatures, a primary global macro-crack pattern is already completely established by means of a thermomechanical instability phenomenon. First we report experimental observations of the crack patterns on orthotropic (wood) and isotropic (Medium Density Fibreboard) materials in inert atmosphere. Then we solve the 3D thermomechanical buckling problem numerically by using the Finite Element Method, and show that the different crack topologies can be explained qualitatively by the simultaneous thermal expansion and softening, taking into account the directional dependence of the elastic properties. Finally, we formulate a 2D model for a soft layer bonded to an elastic substrate, and find an equation predicting the inter-crack distance in the main crack-pattern for the orthotropic case. We also derive a formula for the critical thermal stress above which the plane surface will wrinkle and buckle. The results can be used for finding new ways to prevent or delay the crack formation, leading to improved fire safety of wood-based products.

cond-mat.mtrl-sci↗

arXiv2013

Sensitivity studies of heat transfer: forced convection across a cylindrical pipe and duct flow

We consider two common heat transfer processes and perform a through sensitivity study of the variables involved. We derive and discuss analytical formulas for the heat transfer coefficient in function of film velocity, air temperature and pipe diameter. The according plots relate to a qualitative analysis of the multi-variable function $h$, according to functional optimization. For each process, we provide with graphs and tables of the parameters of interest, such as the Reynolds number. This method of study and the specific values can constitute a useful reference for didactic purposes.

physics.class-ph↗

arXiv2013

Energy optimization in ice hockey halls I. The system COP as a multivariable function, brine and design choices

This work is the first in a series of articles addressing the energy optimization in ice hockey halls. Here we adopt an analytical method, called functional optimization, to find which design and operating conditions maximize the Coefficient Of Performance of the entire cooling system (brine pumps and cooling tower), which we call ${\rm COP}_{sys}$. This is addressed as a function of several variables, like electric consumption and brine physical properties. By maximizing such function, the best configuration and brine choices for the system can thus be determined accurately and rigorously. We investigate the importance of pipe diameter, depth and brine type (ethylene glycol and ammonia) for average-sized ice rinks. An optimal brine density is found, and we compute the weight of the electric consumption of the brine pumps on ${\rm COP}_{sys}$. Our formulas are validated with heat flow measurement data obtained at an ice hockey hall in Finland. They are also confronted with technical and cost-related constraints, and implemented by simulations with the program COMSOL Multiphysics. The multivariable approach here discussed is general, and can be applied to the rigorous preliminary study of diverse situations in building physics and in many other areas of interest.

physics.gen-ph↗

arXiv2012

Symmetries, higher order symmetries and supersymmetries

We discuss a geometrical formulation for gauge field theories, called Gauge-Natural, which has been mostly developed at the University of Turin (Italy) and at the Masaryk University in Brno (Czech Republic). In this framework, the group of automorphisms of some suitable principal bundle P, called the structure bundle, acts on fields by means of gauge transformations. After showing that the classical Wess-Zumino model can be consistently described within this formalism, we consider minimal supergravity (mSugra). We find that to embed mSugra in a Gauge-Natural framework, further investigations are needed. This happens since both the covariance of the Rarita-Schwinger Lagrangian and the closure of the SUSY algebra occur on-shell.

hep-th↗

arXiv2011

Non-perturbative production of matter and rapid thermalization after MSSM inflation

A {\it gauge invariant} combination of LLe {\it sleptons} within the Minimal Supersymmetric Standard Model is one of the few inflaton candidates that can naturally explain population of the observable sector and creation of matter after inflation. After the end of inflation, the inflaton oscillates coherently about the minimum of its potential, which is a point of {\it enhanced gauged symmetry}. This results in bursts of non-perturbative production of the gauge/gaugino and (s)lepton quanta. The subsequent decay of these quanta is very fast and leads to an extremely efficient transfer of the inflaton energy to (s)quarks via {\it instant} preheating. Around 20% of the inflaton energy density is drained during every inflaton oscillation. However, all of the Standard Model degrees of freedom (and their supersymmetric partners) {\it do not} thermalize immediately, since the large inflaton vacuum expectation value breaks the electroweak symmetry. After about 100 oscillations -- albeit within one Hubble time -- the amplitude of inflaton oscillations becomes sufficiently small, and all of the degrees of freedom will thermalize. This provides by far the most efficient reheating of the universe with the observed degrees of freedom.

hep-ph↗

arXiv2010

Cosmological evolution of scalar fields and gravitino dark matter in gauge mediation at low reheating temperatures

We consider the dynamics of the supersymmetry-breaking scalar field and the production of dark matter gravitinos via its decay in a gauge-mediated supersymmetry breaking model with metastable vacuum. We find that the scalar field amplitude and gravitino density are extremely sensitive to the parameters of the hidden sector. For the case of an O'Raifeartaigh sector, we show that the observed dark matter density can be explained by gravitinos even for low reheating temperatures T_{R} < 10 GeV. Such low reheating temperatures may be implied by detection of the NLSP at the LHC if its thermal freeze-out density is in conflict with BBN.

hep-ph↗

arXiv2009

Scattering of massive W bosons into gravitinos and tree unitarity in broken supergravity

The WW scattering into gravitino and gaugino is here investigated in the broken phase, by using both gauge and mass eigenstates. Differently from what is obtained for unbroken gauge symmetry, we find in the scattering amplitudes new structures, which can lead to violation of unitarity above a certain scale. This happens because, in the annihilation diagram, the longitudinal degrees of freedom in the propagator of the gauge bosons disappear from the amplitude, by virtue of the SUGRA vertex. We show that the longitudinal polarizations of the on-shell W become strongly interacting in the high energy limit, and that the inclusion of diagrams with off-shell scalars of the MSSM does not cancel the divergences.

hep-ph↗