Enrico Nobile

Physical and numerical modelling of a solar chimney-based ventilation system for buildings

Abstract This paper describes an experimental and numerical study to analyse the thermal performance of a bio-climatic building prototype in Nigeria. The roof performs as a solar chimney, generating an air flow through the living space of the building to provide cooling. Experimental tests on a 1:12 small-scale model of the prototype are outlined, and the results, bith qualitative and quatitative, are used to validate a two-dimensional flow simulation model, in which the steady state conservation equations of mass, momentum and thermal energy are solved using a finite volume formulation. The experimental and numerical results, expressed in terms of temperature and velocity fields, for two different window geometries are critically evaluated and compared with good agreement.

Geometric Parameterization and Multiobjective Shape Optimization of Convective Periodic Channels

In this article we describe a general procedure for the geometric parameterization and multiobjective shape optimization of periodic wavy channels, representative of the repeating module of an ample variety of heat exchangers. The two objectives considered are the maximization of heat transfer rate and the minimization of friction factor. Since there is no single optimum to be found, we use a multiobjective genetic algorithm and the so-called Pareto dominance concept. The optimization of the two-dimensional periodic channel is obtained, by means of an unstructured finite-element solver, for a fluid of Prandtl number Pr = 0.7, assuming fully developed velocity and temperature fields, and steady laminar conditions. The geometry of the channel is parameterized either by means of simple linear-piecewise profiles, or by nonuniform rational B-splines, and in the latter case their control points represent the design variables. The results obtained are very encouraging, and the procedure described can be applied, in principle, to even more complex problems.

DNS study of turbulent transport at low Prandtl numbers in a channel flow

Direct numerical simulations of the velocity and temperature fields for turbulent flow in a channel are used to examine the influence of Prandtl number Pr on turbulent transport. The Reynolds number, based on the half-height of the channel and the friction velocity, is Re τ = 150. Prandtl numbers of 1.0, 0.3, 0.1, 0.05, 0.025 were studied. The bottom and the top walls were kept at constant temperatures of + T w and − T w . The influence of Pr on Reynolds transport, on the turbulent diffusivity, α τ , and on the spectral density function of the temperature fluctuations was studied. The observation that spatial variations of the ratio of the turbulent diffusivity to the value observed at Pr = 1.0 are not large is used to propose a method for calculating average temperature fields. The decrease in α τ with decreasing Pr is related to observations of the increased damping of high-wavenumber temperature fluctuations. Molecular conductivity, at smaller Pr , is pictured to act as a filter that renders high-frequency velocity fluctuations ineffective in transporting heat.

Numerical Predictions of Cavitating Flow around Model Scale Propellers by CFD and Advanced Model Calibration

The numerical predictions of the cavitating flow around two model scale propellers in uniform inflow are presented and discussed. The simulations are carried out using a commercial CFD solver. The homogeneous model is used and the influence of three widespread mass transfer models, on the accuracy of the numerical predictions, is evaluated. The mass transfer models in question share the common feature of employing empirical coefficients to adjust mass transfer rate from water to vapour and back, which can affect the stability and accuracy of the predictions. Thus, for a fair and congruent comparison, the empirical coefficients of the different mass transfer models are first properly calibrated using an optimization strategy. The numerical results obtained, with the three different calibrated mass transfer models, are very similar to each other for two selected model scale propellers. Nevertheless, a tendency to overestimate the cavity extension is observed, and consequently the thrust, in the most severe operational conditions, is not properly predicted.

Simulation of time-dependent flow in cavities with the additive-correction multigrid method, part II: Applications

An additive-correction multigrid method for the prediction of two-dimensional unsteady flows, described in a previous article, is applied to selected cavity flow problems. The cases considered are the lid-driven cavity problem and the buoyant flow in differentially heated cavities. Accurate results are obtained for the lid-driven cavity, where fine-grid, high-Reynolds-number calculations, indicate that the steady flow bifurcates to a periodic regime for a Reynolds value in the range 7,500--10,000. The results for side-heated rectangular enclosures are presented first for a Prandtl number equal to zero, and corresponding values of Grashof number of 1.2 {times} 10{sup 5} and 1.6 {times} 10{sup 5}. In addition, a Prandtl number of 0.71 is considered, with values of the Rayleigh number of 1 {times} 10{sup 8}, 2 {times} 10{sup 8}, and 2 {times} 10{sup 9}. The study demonstrates that the additive-correction multigrid method is computationally efficient, and is capable of performing accurate simulations of time-dependent, and possibly chaotic, flows in enclosures.

Simulation of Time-dependent Flow in Cavities with the Additive Correction Multigrid Method, Part I: Mathematical Formulation

A time-accurate, additive-correction multigrid method for the prediction of two-dimensional unsteady flows is presented in this article. The method makes use of the additive-correction multigrid strategy, which, originally proposed for steady-flow problems, is extended to the calculation of time-dependent and chaotic flows at high Reynolds or Rayleigh numbers. The numerical algorithm guarantees absolute, to machine accuracy, mass conservation at every time step, and it is characterized by second-order accuracy in space and time. In a companion article, the method is applied to the calculation of unsteady flow in cavities, i.e., the lid-driven cavity problem at high Reynolds number, and the buoyant flow in differentially heated cavities at high values of the Rayleigh number. Although the method has been implemented and tested for two-dimensional flows, it can also be extended to three-dimensional problems.

Multi-objective Optimization for Problems Involving Convective Heat Transfer

In this chapter, focused on Computational Fluid Dynamics (CFD)-based optimization for problems involving convective heat transfer, we present our approach for the multi-objective shape optimization of periodic wavy channels, representative of the repeating module of many heat exchangers.

Numerical simulation of flow in a high head Francis turbine with prediction of efficiency, rotor stator interaction and vortex structures in the draft tube

The paper presents numerical simulations of flow in a model of a high head Francis turbine and comparison of results to the measurements. Numerical simulations were done by two CFD (Computational Fluid Dynamics) codes, Ansys CFX and OpenFOAM. Steady-state simulations were performed by k- and SST model, while for transient simulations the SAS SST ZLES model was used. With proper grid refinement in distributor and runner and with taking into account losses in labyrinth seals very accurate prediction of torque on the shaft, head and efficiency was obtained. Calculated axial and circumferential velocity components on two planes in the draft tube matched well with experimental results.

Microtomography-based CFD Analysis of Transport in Open-Cell Aluminum Metal Foams

Nowadays, the need for developing more effective heat exchange technologies and innovative materials, capable of increasing performances while keeping power consumption, size and cost at reasonable levels, is well recognized. Under this perspective, metal foams have a great potential for enhancing the thermal efficiency of heat transfer devices, while allowing for the use of smaller and lighter equipments. However, for practical applications, it is necessary to compromise between the augmented heat transfer rate and the increased pressure drop induced by the tortuous flow passages. For design purposes, the estimation of the flow permeability and the thermal conductivity of the foam is fundamental, but far from simple. From this perspective, besides classical transport models and correlations, computational fluid dynamics (CFD) at the pore scale, although challenging, is becoming a promising approach, especially if coupled with a realistic description of the foam structure. For precisely recovering the microstructure of the foams, a 3D X-ray computed microtomography (μ-CT) can be adopted. In this work, the results of μ-CT-based CFD simulations performed on different open-cell aluminum foams samples, for laminar flow regime, will be discussed. The results demonstrate that open-cell aluminum foams are effective means for enhancing heat transfer.

Optimization of a Boomerang shape using modeFRONTIER

The paper describes the optimization of a boomerang, simulating its trajectory by a dynamic model coupled to CFD analysis to compute aerodynamic coefficients. The optimization process flow and formulation is built within the commercial process integration and optimization software modeFRONTIER. The design variables involved are primarily the geometric parameters to change the shape of the boomerang. To steer the complete process of optimization, from variables variation to performance evaluation, modeFRONTIER is coupled to Catia v5 software for geometry modification and mass properties evaluation, to MATLAB for dynamic simulation, and to the commercial Computational Fluid Dynamics (CFD) tool StarCCM+ for aerodynamic analysis. In addition, dedicated RSM (Response Surfaces Methods), available in modeFRONTIER, are used to extrapolate the aerodynamic coefficients as a function of the angle of incidence and velocity, as required by the dynamic model, through a reduced number of CFD analyses (database) for each geometric configuration. Different design simulations are therefore executed automatically by modeFRONTIER following a dedicated optimization strategy, until the optimal configuration of the boomerang is found, accordingly to the specified requirements, such as minimum energy required for the launch, maximum accuracy in returning, and a guaranteed minimum range. The physical complexity of this, apparently simple, problem, and the not standard application case, has been selected as an interesting benchmark that can be disclosed in full to test the multi-objective and multidisciplinary capabilities of the optimization environment modeFRONTIER.