Diffuser rotating stall (in both cases of a vaneless or a vaned configuration) is still one of the open questions which has never been fully understood because of the complexity of the phenomenon and the experimental difficulties to get reliable measurements in such a complex environment.
Under this perspective, Computational Fluid Dynamics (CFD) is an interesting tool to "see" the flow and provide a basic understanding of the associated physics. Several published works have shown that a simplified model of the diffuser without the upstream impeller and the downstream return channel, with realistic boundary conditions entering the diffuser, can provide a qualitative analysis of the stall onset.
This research applies CFD methods to understand the onset mechanisms of rotating stall in centrifugal compressor diffusers, comparing vaneless and vaned configurations to identify design factors that influence stability and operating range.
Nel corso degli ultimi anni si è assistito a continui miglioramenti nelle capacità predittive dei codici CFD, grazie alla combinazione dell'aumento della potenza di calcolo e di software sempre più efficaci ed efficienti.
Nel presente articolo sarà illustrata la storia delle potenze di calcolo disponibili, e cosa ciò ha comportato per il progetto aerodinamico, per poi discutere le prospettive per i prossimi anni e le loro implicazioni.
Questi impressionanti miglioramenti hanno ovviamente comportato che la CFD ha assunto un ruolo sempre più importante nell'ambito del progetto aerodinamico. Non solo: grazie alla maggiore accuratezza dei risultati, ormai notevole, e alla rapidità con la quale questi sono ottenuti, si è assistito anche a un rilevante incremento del ruolo dell'aerodinamica nel progetto generale.
Un aspetto da tenere in considerazione, per non rimanere "spiazzati" dai futuri sviluppi nel campo del calcolo, è la previsione delle future capacità di calcolo, in modo da non trovarsi impreparati e iniziare già a immaginare le modifiche all'approccio del progetto aerodinamico che saranno necessarie. Sono quindi descritte le prospettive di sviluppo delle capacità di calcolo, attraverso un'analisi dei futuri processori e delle possibili architetture che i software dovranno gestire nei prossimi anni.
L'uso di GPU (Graphic Processor Unit), possibile nel codice ANSYS-FLUENT, appare molto promettente, ed è stato testato in via preliminare su delle macchine di limitata potenza. I risultati sono promettenti, e questa sembra una strada che offre buone possibilità di forti incrementi nelle capacità computazionali.
Già ora è possibile risolvere flussi complessi con grande accuratezza nella rappresentazione dei dettagli. In un futuro prossimo modelli matematici come le LES e le DES diventeranno lo standard, e si potranno affrontati problemi sempre più complessi.
Giovanni Lombardi, Marco Maganzi, Sandra Baldini, Walter Rosellini
2015
Cubit, Piaggio, Università di Pisa
Abstract
The flow around a scooter is analysed by means of the CFD. In a first phase the results are compared with those obtained through a wind tunnel test campaign, showing a good data accordance. Therefore, the CFD approach can be considered a useful engineering tool in the scooter design, and, with respect to the experimental approach, it has the advantage of providing a complete description of the flow field. This aspect appears particularly important when a modification of the geometry must be verified from the aerodynamics point of view. Finally, as an example, the procedure is applied to the analysis of the effects of deflectors, introduced to increase the comfort. From the CFD a complete description of the flows, with and without the spoiler, are obtained and compared. The results show a significant increase in the comfort grade, coupled with a decrease in the drag of the scooter.
Authors: Giovanni Lombardi, Marco Maganzi, Sandra Baldini, Walter Rosellini
Numerical optimization procedures are today a key tool in engineering design, especially when integrated with parametric CAD, automatic mesh generation, CFD solvers, and advanced search algorithms. This approach enables a systematic exploration of large design spaces, making it possible to identify configurations that significantly improve aerodynamic performance. Applications range from aeronautics to automotive engineering, from sailing and yacht design to wind energy systems, including the development of airfoils, rear diffusers, transonic air intakes, wind turbines, and unconventional lifting surfaces. A central aspect is the comparative optimization of alternative solutions, which allows objective evaluations by bringing each configuration to the same level of design maturity. The integration of CFD-based optimization into development processes makes it possible to reduce industrial time and costs, limit the need for physical prototypes, and support more robust, efficient, and informed engineering decisions.
Authors: G. Lombardi
Conference/Journal: ModeFrontier International User Meeting, Trieste (IT), 2014
Alessio Tosini, Enrico Cardile, Giovanni Lombardi, Marco Maganzi
2014
Cubit, Ferrari, Università di Pisa
Abstract
The experimental data corrections on automotive wind tunnel tests usually benefit of pre-test corrections methodology, which can break down working time. This methodology is based on the use of CFD simulations and yields a global correction term, sum of different contributions. It is interesting to analyze such contributions and their possible dependencies by the involved parameters, in order to have a better comprehension of the phenomenon.
In the present paper, the main purpose is to describe, with the support of a CFD analysis, the influence of the wind tunnel walls, supports and Reynolds number effects, to the global correction factor. A high performances car was taken as reference and RANS equations were used as fluid dynamic model. The CFD methodology and its validation, through a comparison with experimental data, are preliminarily presented. Afterward, the contributions to the correction term are shown and discussed. Finally, is evaluated their effect on different elements of the car. The car was split in body, wheels, underbody, radiators, rear wing and internal volumes.
Conference/Journal: ATA, Vol.67, N. 3-4, 2014
Authors: G. Lombardi, A. Tosini, E. Cardile, D. Zinelli
Federico Cartoni, Giovanni Lombardi, Marco Maganzi
2014
Cubit, Università di Pisa
Abstract
The wing-sail is an interesting idea for sailboats, and the last America’s Cup shown that its use is possible and very effective. At the University of Pisa it was decided to use this type of sail on the small boat realised for the inter-university race “1001 vele”. A first wing sail has been used in 2013 with satisfactory results, but it was evident that high improvement in the performances can be obtained. The performances of this system are impressive, but the flow is complex and the number of geometrical parameters very high. In order to tackle this problem resort was made to a direct numerical optimization technique; this approach was attractive, as it made it possible to address the problem systematically, and offered flexibility in the choice of the design variables. The approach is capable of meeting multi-disciplinary requirements, but it requires, given the complexity of the flow, the use of sophisticated CFD solver; STAR CCM+ was then chosen for the aerodynamics evaluation in the optimisation loop. The optimization procedure is completely described in the paper, within the results and a critical analysis of the flow around the wing-sail, in order to have indication on the aerodynamics behaviour. This for a better understanding of the physical aspects and also to give to the sailors the indications for the more efficient set-up.
Authors: G. Lombardi, M. Maganzi
Conference/Journal: Star Global Conference 2014, Wien (AT)
Keywords: wing-sail, CFD, optimization, sailing, STAR CCM+
Enrico Cardile, Federico Cartoni, Giovanni Lombardi, Marco Maganzi
2014
Cubit, Ferrari, Università di Pisa
Abstract
The use of Computational Fluid Dynamics (CFD) provides an effective approach for the qualitative analysis of exhaust gas trajectories in high-performance vehicles under transient driving conditions. Complex flow interactions occurring during acceleration and braking phases require advanced numerical models capable of capturing unsteady effects on realistic geometries.
A CFD workflow based on automated mesh generation, unsteady simulations, and particle tracking techniques enables the investigation of exhaust plume behavior and its interaction with the vehicle underbody. The adoption of an Eulerian-Discrete Phase Model allows a reliable representation of exhaust dispersion while maintaining a balance between accuracy and computational cost, in line with industrial time constraints.
Large-scale simulations performed on HPC architectures support detailed time-dependent analyses using experimentally derived boundary conditions. The results provide valuable qualitative insight into critical flow regions potentially affecting cockpit comfort, supporting targeted experimental testing and local design modifications.
Conference/Journal: ANSYS User Group Meeting, Milano (IT)
When designing passenger cars, HVAC (Heating, Ventilation, Air Conditioning) plays an important role to the overall vehicle design process in order to satisfy thermal comfort criteria for occupants while reducing energy consumption required. In this context, a Ferrari car is analyzed with Computational Fluid Dynamics (CFD) methods to improve the internal flow inside the cockpit.
In the work detailed here the toolbox for HVAC applications available in the open source CFD software HELYX was employed. The HVAC module exploits an improved internal radiation model, a solar radiation module, support for humidity modelling, functionality to assess human comfort by parameters like PMV, PPD, DR, etc.
The paper shows preliminary results of aeroelastic analyses of two half-wing models, having curved and swept planform, carried out at the Aerospace Unit of the Department of Civil and Industrial Engineering of Pisa University. For a wing with a curved planform, as demonstrated in previous papers regarding rigid models of wings, the wave drag effects are strongly reduced in the transonic flight conditions. In the paper some results obtained by using Star-CCM+® 6.04.14 and Abaqus® 6.11 in "cosimulation" are summarized: for this reason the present numerical comparison, between a curved wing and a swept wing, includes the effects of structure's deformability (the wings have the same aspect ratio). The beneficial effects of the planform shape on drag polar curves are confirmed.
Authors: Giovanni Lombardi (Cubit), Mario Rosario Chiarelli (Università di Pisa)
Control surfaces, such as airplane elevators or rudders may use symmetric airfoil shapes which are mostly based on single airfoil geometry. For the 2013 America's Cup sailboat competition rigid sails were specified, based on tandem symmetrical airfoils of equal chord. Because of the unique geometry of this combination and because none of the traditional two-element airfoils were designed for this application, a more suitable airfoil shape was sought. Furthermore, control surfaces such as the rudder are not designed for high lift, while the rigid sail studied here is expected to operate near a lift coefficient of one. A parametric study, using numerical methods, on the effect of different geometrical variables led to the development of an improved sail geometry, compared with the initial baseline shape. Therefore, the first objective of the present study is to validate those predictions for this particular application. Because of the large dimensions of the actual sail, its operating Reynolds numbers are high compared with the available wind tunnel facility. The conservative approach in this study is based on the assumption that the smaller Reynolds number tests provide a satisfactory validation for the higher Reynolds number sailing conditions.
Authors: Giovanni Lombardi (Cubit), Joseph Katz (San Diego State University), Maurizio Foresta (Università di Pisa)
