Diego Angeli

A Comprehensive Review of Natural Convection in Triangular Enclosures

Natural convection in triangular enclosures is an important problem. It displays well thegeneric attributes of this class of convection, with its dependence on enclosure geometry,orientation and thermal boundary conditions. It is particularly rich in its variety of flowregimes and thermal fields as well as having significant practical application. In this pa-per, a comprehensive view of the research area is sought by critically examining the ex-perimental and numerical approaches adopted in studies of this problem in the literature.Different thermal boundary conditions for the evolution of the flow regimes and thermalfields are considered. Effects of changes in pitch angle and the Rayleigh number on theflow and thermal fields are examined in detail. Although most of the past studies are inthe laminar regime, the review extends up to the recent studies of the low turbulent re-gime. Finally, areas of further research are highlighted. [DOI: 10.1115/1.4004290]Keywords: buoyancy-induced flows, flow fields, triangular enclosures

Mathematical Modelling of a Two Streams Coanda Effect Nozzle

This paper analyses the ACHEON Coanda effect based propulsion nozzle for aircraft propulsion based on the dynamic equilibrium of two jet streams. It presents a large bibliographic analysis and the ACHEON concept and, in particular, the HOMER Nozzle, that is its main component. The Constructal optimization process that allows defining this architecture has presented. A preliminary mathematical model of a 2D case of the system has presented, focusing on the combined effect of the mixing effect of the two streams and the Coanda Effect Adhesion over a convex surface. A CFD preliminary validation has presented in uncompressible regime. The results have been evaluated in 2D cases.Copyright

Characterization and modeling of the thermal and electrical properties of transparent silver nanowire thin-film heaters

In this study, we present the characterization and the modeling of transparent silver nanowire thin-film heaters in terms of their transient thermal response when subjected to Joule heating and their electrical properties. The electrical properties, which showed a conductance-temperature dependence that is reduced down to a factor of 2 compared to the value for bulk silver, could be modeled accurately by simulation results. In addition, our transparent electrode deposition technique, i.e. spray-coating, allowed for an excellent reproducibility and provided homogeneous and large films that compare to state-of-the-art silver nanowire transparent electrode performance.

Appearance of quasiperiodicity within a period doubling route to chaos of a swaying thermal plume

The birth, evolution and disappearance of quasiperiodic dynamics in buoyancy-driven flow arising from an enclosed horizontal cylinder are analysed here, by numerical means, in the limit of the 2D approximation. The governing equations are solved on orthogonal Cartesian grids, giving special treatment to the internal, non-aligned boundaries. Thanks to the adoption of a high level of refinement of the Rayleigh number range, quasiperiodicity was observed to emerge from a periodic limit cycle (P1), and to turn into its omologous orbit with doubled period (P2), eventually evolving into a classical period-doubling route to chaos, for further increases of the Rayleigh number. The present study gives a deeper insight to what appears to be an imperfect period doubling bifurcation through a quasiperiodic T2-torus. The approach used is based on the classical tools for time series analysis. The distribution of the power spectral densities is used to search for and characterise the existence of relations between the frequencies of the P1, T2 and P2 dynamics. The topology of the orbits, as well as their evolution within the quasiperiodic window, are analysed with the aid of phase space representation and Poincare maps.

Parametric Analysis of a Large-scale Cycloidal Rotor in Hovering Conditions

In this work, four key design parameters of cycloidal rotors, namely the airfoil section; the number of blades; the chord-to-radius ratio; and the pitching axis location, are addressed. The four parameters, which have a strong effect on the rotor aerodynamic efficiency are analyzed with an analytical model and a numerical approach. The numerical method is based on a finite-volume discretization of two-dimensional Unsteady Reynolds Averaged Navier-Stokes equations on a multiple sliding mesh, are proposed and validated against experimental data. A parametric analysis is then carried out considering a large-scale cyclogyro, suitable for payloads above 100 kg, in hovering conditions. Results demonstrate that the airfoil thickness significantly affects the rotor performance; such a result is partly in contrast with previous findings for small- and micro-scale configurations. Moreover, it will be shown that increasing the number of blades could result in a decrease of the rotor efficiency. The effect of chord-to-radius will demonstrate that values of around 0.5 result in higher efficiency. Finally it is found out that for these large systems, in contrast with micro-scale cyclogyros, the generated thrust increases as the pitching axis is located away from the leading edge, up to 35% of chord length. Further the shortcomings of using simplified analytical tools in the prediction of thrust and power in non-ideal flow conditions will be highlighted and discussed.

Wind and buoyancy driven natural ventilation in double skin façades

ABSTRACT The computational fluid dynamics (CFD) modelling activity presented in this work aims at investigating the reliability of the assumptions employed in a double skin façades (DSFs) simplified model, developed for the integration of naturally ventilated DSFs in building simulation (BS) tools. The simplified fluid-dynamic model considers both the wind action, by means of pressure coefficients (Cp) on the openings, and the buoyancy inside the ventilated channel. Both the BS and the CFD models have been assessed using the database of an experimental campaign carried out in a full scale test facility, named ‘The Cube’, and made available by the Department of Civil Engineering of Aalborg University. The results show a good agreement between the CFD and BS models in terms of predicted temperature increase, with a maximum deviation of 15%. Both models exhibit a high sensitivity to the imposed differential wind pressure, which depends on the Cp source employed. The determination of proper Cp values for the specific case is, hence, a crucial aspect. Moreover, the CFD analysis offers a deeper insight on surface heat transfer and suggests the need to take into account the interplay between thermal and wind-driven convection, which ultimately gives rise to a mixed convection phenomenon.

Heat transfer along the route to chaos of a swaying thermal plume

Detailed analyses have been recently reported on the low order dynamics of a thermal plume arising from a horizontal cylindrical heat source concentric to an air-filled isothermally cooled square enclosure, together with those of the related flow structures, in the limit of the 2D approximation. In particular, within the range of 0 < Ra < 3Racr, with Racr corresponding to the loss of stability of the stationary buoyant plume, the entire evolution from a periodic limit cycle (P1) to the birth of chaos through a period-doubling cascade has been fully explored. With this respect, special attention has been given to the window of quasiperiodic dynamics onto a T2-torus that is observed to separate the monoperiodic dynamics from the biperiodic dynamics onto a P1 and a P2-limit cycle, respectively. The results of these analyses hint at the bimodal nature of the overall dynamics, in general, and of the subharmonic cascade, in particular, which are still under investigation. Although relevant on a dynamical perspective, a with a main reflection on the laminar-turbulent transition, the observed oscillations appear to be characterised by comparable amplitudes and to be determined by similar evolutions of the flow pattern evolutions, so that their role on the overall heat transfer rate is expected to be marginal. Within this frame, the present study aims at reporting the influence played by the observed dynamics of the thermal plume and of the flow structures on the global heat transfer rate. In particular, the aim is the assessment of the correlation between the Rayleigh number and the average Nusselt number on the cylinder surface, as well as the effect on the latter of the observed series of bifurcations.

Natural convection in asymmetric triangular enclosures heated from below

Triangular enclosures are typical configurations of attic spaces found in residential as well as industrial pitched-roof buildings. Natural convection in triangular rooftops has received considerable attention over the years, mainly on right-angled and isosceles enclosures. In this paper, a finite volume CFD package is employed to study the laminar air flow and temperature distribution in asymmetric rooftop-shaped triangular enclosures when heated isothermally from the base wall, for aspect ratios (AR) 0.2 ≤ AR ≤ 1.0, and Rayleigh number (Ra) values 8 × 105 ≤ Ra ≤ 5 × 107. The effects of Rayleigh number and pitch angle on the flow structure and temperature distributions within the enclosure are analysed. Results indicate that, at low pitch angle, the heat transfer between the cold inclined and the hot base walls is very high, resulting in a multi-cellular flow structure. As the pitch angle increases, however, the number of cells reduces, and the total heat transfer rate progressively reduces, even if the Rayleigh number, being based on the enclosure height, rapidly increases. Physical reasons for the above effect are inspected.

Application of Recurrence Analysis to the period doubling cascade of a confined buoyant flow

Recurrence Analysis (RA) is a promising and flexible tool to identify the behaviour of nonlinear dynamical systems. The potentialities of such a technique are explored in the present work, for the study of transitions to chaos of buoyant flow in enclosures. The case of a hot cylindrical source centred in a square enclosure, is considered here, for which an extensive database of results has been collected in recent years. For a specific value of the system aspect ratio, a sequence of period doublings has been identified, leading to the onset of chaos. RA is applied here to analyse the different flow regimes along the route to chaos. The qualitative visual identification of patterns and the statistics given by the quantitative analysis suggest that this kind of tool is well suited to the study of transitional flows in thermo-fluid dynamics.

A fast algorithm for Direct Numerical Simulation of natural convection flows in arbitrarily-shaped periodic domains

A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy- induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme. The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method.