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
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)
High-speed catamaran design requires accurate prediction of performance and dynamic behavior under both calm and rough sea conditions. Advanced CFD techniques enable the simultaneous analysis of hydrodynamics, aerodynamics, and vessel motion, providing detailed insight into stability, loads, and propulsion efficiency.
A fully unsteady numerical approach with free-surface modeling and six-degrees-of-freedom rigid body motion allows realistic simulation of hull dynamics in waves. Sensitivity analyses on grid resolution highlight the trade-off between computational cost and solution accuracy, while refined meshes improve the representation of wave-hull interaction.
The numerical results provide quantitative information on speed, trim, accelerations, dominant motion frequencies, and the influence of weight and inertial properties. Simulations in rough sea conditions show modified oscillation amplitudes and frequencies, while preserving harmonic motion characteristics. The methodology demonstrates the capability of CFD to support the design and optimization of high-performance marine platforms.
Conference/Journal: STAR European Conference, London, 2010
Alessandro Mariotti, Giovanni Lombardi, Marco Maganzi
2010
Cubit, Università di Pisa
Abstract
Advanced CFD techniques make it possible to investigate the complex interaction between aerodynamics, hydrodynamics, and rigid-body motion in sailing yachts operating in realistic sea conditions. Free-motion simulations that couple unsteady multiphase flow models with six-degree-of-freedom dynamics enable the evaluation of yacht performance in up-wind sailing under rough sea states. This integrated approach combines turbulence modeling, volume-of-fluid wave representation, and dynamic mesh handling to capture both sail aerodynamics and hull–wave interactions. The methodology allows detailed analysis of boat motions, thrust generation, hydrodynamic loads, and their correlations with velocity, heel, and pitch, providing insight into off-design behavior that cannot be obtained from simplified or steady configurations. Although computationally demanding, such simulations offer a powerful tool to support yacht design and performance assessment, improving the understanding of real operating conditions and enabling more informed engineering choices in high-performance sailing applications.
Authors: G. Lombardi, M. Maganzi, A. Mariotti
Conference/Journal: STAR European Conference, London, 2010
Enrico Cardile, Ferdinando Cannizzo, Giovanni Lombardi, Marco Maganzi
2010
Cubit, Ferrari, Università di Pisa
Abstract
Un'analisi aerodinamica che consideri gli effetti di tutti i parametri potrebbe sembrare difficile. Nell'analisi tramite ottimizzazione numerica un codice aerodinamico è accoppiato ad una routine di ottimizzazione per gestire automaticamente i valori delle variabili di progetto, con l'obiettivo di minimizzare una funzione obiettivo assegnata. Questo approccio è estremamente flessibile ed in grado di rispondere ad esigenze multidisciplinari.
Il progetto ha dimostrato l'applicabilità delle procedure di ottimizzazione nel contesto del settore automobilistico, utilizzando un codice CFD per l'aerodinamica. L'integrazione dell'ottimizzazione aerodinamica in fase di progettazione permette ai progettisti di interagire velocemente con gli altri gruppi. Diventa possibile cercare soluzioni che garantiscano le prestazioni cercate, che non influenzino negativamente lo stile dell'auto, e che al contempo diano un elevato grado di efficienza e sicurezza.
Enrico Cardile, Ferdinando Cannizzo, Giovanni Lombardi, Marco Maganzi
2010
Cubit, Ferrari, Università di Pisa
Abstract
Il comfort termico in abitacolo rappresenta un aspetto sempre più rilevante nella progettazione automobilistica ad alte prestazioni, risultando dall'interazione complessa tra fenomeni termo-fluidodinamici e percezioni fisiologiche soggettive.
Viene presentata una procedura integrata per la valutazione del comfort termico basata su simulazioni CFD, finalizzata alla definizione di indici quantitativi rappresentativi delle condizioni ambientali interne al veicolo. L'approccio combina l'analisi del campo di velocità, temperatura, umidità e irraggiamento con modelli di bilancio termico del corpo umano, introducendo un indice globale di comfort ottenuto dalla ponderazione di contributi locali legati a equilibrio termico, raffiche, gradienti verticali e laterali di temperatura.
Le simulazioni includono flussi freddi e caldi, meccanismi di conduzione, convezione e irraggiamento solare, su geometrie realistiche di abitacolo. La procedura è validata mediante prove sperimentali in galleria climatica e test soggettivi, mostrando una buona correlazione tra risultati numerici e dati sperimentali.
