A numerical methodology for the multi-objective optimization of the DI Diesel engine combustion

DI Diesel engine are widely used both for industrial and automotive applications due to their durability and fuel economy. Nonetheless, increasing environmental concerns force that type of engine to comply with increasingly demanding emission limits, so that, it has become mandatory to develop a robust design methodology of the DI Diesel combustion system focused on reduction of soot and NOx simultaneously while maintaining a reasonable fuel economy. In recent years, genetic algorithms and CFD three-dimensional combustion simulations have been successfully applied to that kind of problem. However, combining GAs optimization with actual CFD three-dimensional combustion simulations can be too onerous since a large number of calculations is usually needed for the genetic algorithm to converge, resulting in a high computational cost and, thus, limiting the suitability of this method for industrial processes. In order to make the optimization process less time-consuming, CFD simulations can be more conveniently used to generate a training set for the learning process of an artificial neural network which, once correctly trained, can be used to forecast the engine outputs as a function of the design parameters during a GA optimization performing a so-called virtual optimization. In the current work, a numerical methodology for the multi-objective virtual optimization of the combustion of an automotive DI Diesel engine, which relies on artificial neural networks and genetic algorithms, was developed.

Ottimizzazione aerodinamica del rotore principale dell'elicottero RC Goblin Speed

L'elimodellismo è una passione che lega un numero sempre maggiore di persone: nuove manifestazioni vengono organizzate in tutto il mondo e nuove discipline vengono continuamente proposte. Questo è il caso della disciplina speed in cui i piloti si sfidano a far volare i propri elimodelli alle massime velocità. L'azienda SAB Heli Division s.r.l., come produttore di pale per elimodelli e della serie di elicotteri Goblin, ha interesse a sostenere i propri piloti con le proprie macchine, facendo sì che siano veloci e competitive. Per questo ha voluto sviluppare una pala che, montata sul proprio elicottero specifico per questa disciplina, possa vincere la concorrenza con l'ambizione di stabilire un primato di velocità a livello internazionale. Il problema è quindi quello di sviluppare una pala che ottimizzasse al meglio le caratteristiche dell'elimodello Goblin Speed, in modo da sfruttare al meglio la potenza installata a bordo. Per via dei limiti sui mezzi a disposizione l'ottimizzazione è stata portata avanti mediante la teoria dell'elemento di pala. Si è impostato il calcolo determinando la potenza media su una rotazione del rotore in volo avanzato a 270 km/h e quindi attraverso gli algoritmi di ottimizzazione globale presenti nel codice di calcolo MATLAB si è cercato il rotore che permettesse il volo a tale velocità al variare del raggio del disco del rotore, dello svergolamento della pala e della distribuzione di corda lungo la pala. Per far sì che si abbiano risultati più precisi si sono sfruttati alcuni modelli per stimare il campo di velocità indotta o gli effetti dello stallo dinamico. Inoltre sono state stimate altre grandezze di cui non sono noti i dati reali o di cui è troppo complesso, per le conoscenze a disposizione, avere un dato preciso. Si è tuttavia cercato di avere stime verosimili. Alcune di queste grandezze sono le caratteristiche aerodinamiche del profilo NACA 0012 utilizzato, ottenute mediante analisi CFD bidimensionale, i comandi di passo collettivo e ciclico che equilibrano il velivolo e la resistenza aerodinamica dell'intero elimodello. I risultati del calcolo sono stati confrontati innanzitutto con le soluzioni già adottate dall'azienda. Quindi si è proceduto alla realizzazione della pala e mediante test di volo si è cercato di valutare le prestazioni della macchina che monta la pala ottenuta. Nonostante le approssimazioni adottate si è osservato che la pala progettata a partire dai risultati dell'ottimizzazione rispecchia la filosofia adottata: per velocità paragonabili a quelle ottenute con le pale prodotte da SAB Heli Division, le potenze richieste sono effettivamente inferiori. Tuttavia non è stato possibile ottenere un vero e proprio miglioramento della velocità di volo, presumibilmente a causa delle stime delle caratteristiche aerodinamiche delle diverse parti del Goblin Speed.

Towards Centrifugal Compressor Stages Virtual Testing

Flow features inside centrifugal compressor stages are very complicated to simulate with numerical tools due to the highly complex geometry and varying gas conditions all across the machine. For this reason, a big effort is currently being made to increase the fidelity of the numerical models during the design and validation phases. Computational Fluid Dynamics (CFD) plays an increasing role in the assessment of the performance prediction of centrifugal compressor stages. Historically, CFD was considered reliable for performance prediction on a qualitatively level, whereas tests were necessary to predict compressors performance on a quantitatively basis. In fact "standard" CFD with only the flow-path and blades included into the computational domain is known to be weak in capturing efficiency level and operating range accurately due to the under-estimation of losses and the lack of secondary flows modeling. This research project aims to fill the gap in accuracy between "standard" CFD and tests data by including a high fidelity reproduction of the gas domain and the use of advanced numerical models and tools introduced in the author's OEM in-house CFD code. In other words, this thesis describes a methodology by which virtual tests can be conducted on single stages and multistage centrifugal compressors in a similar fashion to a typical rig test that guarantee end users to operate machines with a confidence level not achievable before. Furthermore, the new "high fidelity" approach allowed understanding flow phenomena not fully captured before, increasing aerodynamicists capability and confidence in designing high efficiency and high reliable centrifugal compressor stages.

Wave driven devices for the oxygenation of bottom layers

This thesis discusses the design of a system to use wave energy to pump oxygen-rich surface water towards the bottom of the sea. A simple device, called OXYFLUX, is proposed in a scale model and tested in a wave flume in order to validate its supposed theoretical functioning. Once its effectiveness has been demonstrated, a overset mesh, CFD model has been developed and validated by means of the physical model results. Both numerical and physical results show how wave height affects the behavior of the device. Wave heights lower than about 0.5 m overtop the floater and fall into it. As the wave height increases, phase shift between water surface and vertical displacement of the device also increases its influence on the functioning mechanism. In these situations, with wave heights between 0.5 and 0.9 m, the downward flux is due to the higher head established in the water column inside the device respect to the outside wave field. Furthermore, as the wave height grows over 0.9 m, water flux inverts the direction thanks to depression caused by the wave crest pass over the floater. In this situation the wave crest goes over the float but does not go into it and it draws water from the bottom to the surface through the device pipe. By virtue of these results a new shape of the floater has been designed and tested in CFD model. Such new geometry is based on the already known Lazzari’s profile and it aims to grab as much water as possible from the wave crest during the emergence of the floater from the wave field. Results coming from the new device are compared with the first ones in order to identify differences between the two shapes and their possible areas of application.

Hydrodynamic induced by an array of wave energy converters. Experimental and numerical analysis

The thesis analyses the hydrodynamic induced by an array of Wave energy Converters (WECs), under an experimental and numerical point of view. WECs can be considered an innovative solution able to contribute to the green energy supply and –at the same time– to protect the rear coastal area under marine spatial planning considerations. This research activity essentially rises due to this combined concept. The WEC under exam is a floating device belonging to the Wave Activated Bodies (WAB) class. Experimental data were performed at Aalborg University in different scales and layouts, and the performance of the models was analysed under a variety of irregular wave attacks. The numerical simulations performed with the codes MIKE 21 BW and ANSYS-AQWA. Experimental results were also used to calibrate the numerical parameters and/or to directly been compared to numerical results, in order to extend the experimental database. Results of the research activity are summarized in terms of device performance and guidelines for a future wave farm installation. The device length should be “tuned” based on the local climate conditions. The wave transmission behind the devices is pretty high, suggesting that the tested layout should be considered as a module of a wave farm installation. Indications on the minimum inter-distance among the devices are provided. Furthermore, a CALM mooring system leads to lower wave transmission and also larger power production than a spread mooring. The two numerical codes have different potentialities. The hydrodynamics around single and multiple devices is obtained with MIKE 21 BW, while wave loads and motions for a single moored device are derived from ANSYS-AQWA. Combining the experimental and numerical it is suggested –for both coastal protection and energy production– to adopt a staggered layout, which will maximise the devices density and minimize the marine space required for the installation.

Numerical and experimental investigation of turbulent dissipation

Turbulent energy dissipation is presented in the theoretical context of the famous Kolmogorov theory, formulated in 1941. Some remarks and comments about this theory help the reader understand the approach to turbulence study, as well as give some basic insights to the problem. A clear distinction is made amongst dissipation, pseudo-dissipation and dissipation surrogates. Dissipation regulates how turbulent kinetic energy in a flow gets transformed into internal energy, which makes this quantity a fundamental characteristic to investigate in order to enhance our understanding of turbulence. The dissertation focuses on experimental investigation of the pseudo-dissipation. Indeed this quantity is difficult to measure as it requires the knowledge of all the possible derivatives of the three dimensional velocity field. Once considering an hot-wire technique to measure dissipation we need to deal with surrogates of dissipation, since not all the terms can be measured. The analysis of surrogates is the main topic of this work. In particular two flows, the turbulent channel and the turbulent jet, are considered. These canonic flows, introduced in a brief fashion, are often used as a benchmark for CFD solvers and experimental equipment due to their simple structure. Observations made in the canonic flows are often transferable to more complicated and interesting cases, with many industrial applications. The main tools of investigation are DNS simulations and experimental measures. DNS data are used as a benchmark for the experimental results since all the components of dissipation are known within the numerical simulation. The results of some DNS were already available at the start of this thesis, so the main work consisted in reading and processing the data. Experiments were carried out by means of hot-wire anemometry, described in detail on a theoretical and practical level. The study of DNS data of a turbulent channel at Re=298 reveals that the traditional surrogate can be improved Consequently two new surrogates are proposed and analysed, based on terms of the velocity gradient that are easy to measure experimentally. We manage to find a formulation that improves the accuracy of surrogates by an order of magnitude. For the jet flow results from a DNS at Re=1600 of a temporal jet, and results from our experimental facility CAT at Re=70000, are compared to validate the experiment. It is found that the ratio between components of the dissipation differs between DNS and experimental data. Possible errors in both sets of data are discussed, and some ways to improve the data are proposed.

Studio preliminare di flusso attorno ad un pantografo per treno ad alta velocità

In questo elaborato di tesi viene presentata la comparazione tra due codici CFD, rispettivamente Fluent e OpenFOAM, mediante simulazioni che sono alla base di uno studio numerico di flusso attorno ad un pantografo per treno ad alta velocità. Si è apprezzato quindi la facilità d’uso di un software venduto tramite licenza e la difficoltà di un software open source come OpenFOAM, il quale però ha vantaggi in termini di adattamento ai casi più specifici. Sono stati quindi studiati due casi, scambio termico in regime laminare attorno ad un cilindro bidimensionale e flusso turbolento completamente sviluppato in un canale. Tutte le simulazioni numeriche hanno raggiunto convergenza e sono state validate positivamente mediante confronto con dati sperimentali. Il primo caso prevede un cilindro investito da un flusso a temperatura minore rispetto alla temperatura della superficie del cilindro; per avere più riscontri, sono state condotte diverse prove a valori differenti del numero di Prandtl, e per ogni simulazione è stato ricavato il corrispettivo numero di Nusselt, successivamente comparato con i dati sperimentali per la validazione delle prove. A partire dalla creazione della griglia di calcolo, è stato effettuato uno studio del fenomeno in questione, creando così una griglia di calcolo sviluppata a valle del cilindro avente maggior densità di celle a ridosso della parte del cilindro. In aggiunta, svolgendo le prove con schemi numerici sia del primo che del secondo ordine, si è constatata la miglior sensibilità degli schemi numerici del secondo ordine rispetto a quelli del primo ordine. La seconda tipologia di simulazioni consiste in un flusso turbolento completamente sviluppato all’interno di un canale; sono state svolte simulazioni senza e con l’uso delle wall functions, e quindi usate griglie di calcolo differenti per i due tipi di simulazioni, già disponibili per entrambi i software. I dati ottenuti mostrano uno sforzo computazionale maggiore per le simulazioni che non prevedono l’uso delle wall functions, e quindi una maggiore praticità per le simulazioni con le wall functions. Inoltre, le simulazioni di questo secondo caso sono state svolte con diversi modelli di turbolenza; in Fluent sono stati utilizzati i modelli k-ε e RSM mentre in OpenFOAM è stato utilizzato solo il modello k-ε in quanto il modello RSM non è presente. La validazione dei risultati è affidata alla comparazione con i dati sperimentali ricavati da Moser et all mediante simulazioni DNS, mettendo in risalto la minor accuratezza delle equazioni RANS.

Geomorphological and statistical analysis on Ravenna dune fields changes, based on Terrestrial Laser Technology

Coastal sand dunes represent a richness first of all in terms of defense from the sea storms waves and the saltwater ingression; moreover these morphological elements constitute an unique ecosystem of transition between the sea and the land environment. The research about dune system is a strong part of the coastal sciences, since the last century. Nowadays this branch have assumed even more importance for two reasons: on one side the born of brand new technologies, especially related to the Remote Sensing, have increased the researcher possibilities; on the other side the intense urbanization of these days have strongly limited the dune possibilities of development and fragmented what was remaining from the last century. This is particularly true in the Ravenna area, where the industrialization united to the touristic economy and an intense subsidence, have left only few dune ridges residual still active. In this work three different foredune ridges, along the Ravenna coast, have been studied with Laser Scanner technology. This research didn’t limit to analyze volume or spatial difference, but try also to find new ways and new features to monitor this environment. Moreover the author planned a series of test to validate data from Terrestrial Laser Scanner (TLS), with the additional aim of finalize a methodology to test 3D survey accuracy. Data acquired by TLS were then applied on one hand to test some brand new applications, such as Digital Shore Line Analysis System (DSAS) and Computational Fluid Dynamics (CFD), to prove their efficacy in this field; on the other hand the author used TLS data to find any correlation with meteorological indexes (Forcing Factors), linked to sea and wind (Fryberger's method) applying statistical tools, such as the Principal Component Analysis (PCA).

Modellazione CAD e ricostruzione virtuale di autobus pubblicitario d'epoca

L’oggetto della presente tesi è sviluppare la ricostruzione virtuale di un autobus pubblicitario progettato negli anni 50, un simbolo del design italiano legato allo sviluppo industriale del secondo dopoguerra. La tesi si sviluppa inizialmente con un’attività di ricerca e documentazione legata all'autobus, con particolare attenzione alla raccolta di immagini e disegni progettuali d’epoca, punto di partenza per la ricostruzione virtuale del mezzo. Si procede quindi alla modellazione CAD della carrozzeria cercando di replicare in modo quanto più dettagliato l’originale. Dal modello ottenuto, sono stati ricavati due prototipi di diverse dimensioni e materiali: un primo di dimensioni ridotte, realizzato tramite la tecnica del rapid prototyping, ed un secondo di dimensioni maggiori, in ureol fresato. Successivamente si procede ad uno studio preliminare di ricostruzione del mezzo con lo scopo di analizzare le principali problematiche di un’eventuale futura costruzione e messa in strada del mezzo. Tale studio parte dalla normativa vigente sulla costruzione e sulle norme di circolazione degli autobus, comprendente sia le leggi italiane sia comunitarie, riguardanti le caratteristiche dimensionali, prestazionali e di sicurezza che qualsiasi autobus con licenza di circolazione deve rispettare. Quindi verrà compiuta la scelta del telaio base da utilizzare tra quelli proposti dai maggiori costruttori europei e verrà realizzato il modello CAD anche del telaio con l’obiettivo di collegarlo alla carrozzeria e ottenere così il modello completo in tutte le sue parti. Per realizzare la scocca, verranno prese in esame le soluzioni costruttive più utilizzate dai progettisti di autobus, e viene svolto un breve studio riguardo i materiali utilizzabili a tale scopo. Sulla base di tali considerazioni si procederà alla modellazione della stessa. In ultimo si procede ad un preliminare studio fluidodinamico attraverso l’analisi CFD, e ad un preliminare studio del comportamento meccanico della struttura reticolare che sostiene la carrozzeria.

Design of a GNSS receiver for the ESEO mission

This thesis was carried out inside the ESA's ESEO mission and focus in the design of one of the secondary payloads carried on board the spacecraft: a GNSS receiver for orbit determination. The purpose of this project is to test the technology of the orbit determination in real time applications by using commercial components. The architecture of the receiver includes a custom part, the navigation computer, and a commercial part, the front-end, from Novatel, with COCOM limitation removed, and a GNSS antenna. This choice is motivated by the goal of demonstrating the correct operations in orbit, enabling a widespread use of this technology while lowering the cost and time of the device’s assembly. The commercial front-end performs GNSS signal acquisition, tracking and data demodulation and provides raw GNSS data to the custom computer. This computer processes this raw observables, that will be both transferred to the On-Board Computer and then transmitted to Earth and provided as input to the recursive estimation filter on-board, in order to obtain an accurate positioning of the spacecraft, using the dynamic model. The main purpose of this thesis, is the detailed design and development of the mentioned GNSS receiver up to the ESEO project Critical Design Review, including requirements definition, hardware design and breadboard preliminary test phase design.

Studio e ricostruzione delle distribuzioni granulometriche interne al grano in motori a propellente solido

The main purposes of this essay were to investigate in detail the burning rate anomaly phenomenon, also known as "Hump Effect", in solid rocket motors casted in mandrel and the mechanisms at the base of it, as well as the developing of a numeric code, in Matlab environment, in order to obtain a forecasting tool to generate concentration and orientation maps of the particles within the grain. The importance of these analysis is due to the fact that the forecasts of ballistics curves in new motors have to be improved in order to reduce the amount of experimental tests needed for the characterization of their ballistic behavior. This graduate work is divided into two parts. The first one is about bidimensional and tridimensional simulations on z9 motor casting process. The simulations have been carried out respectively with Fluent and Flow 3D. The second one is about the analysis of fluid dynamic data and the developing of numeric codes which give information about the concentration and orientation of particles within the grain based on fluid strain rate information which are extrapolated from CFD software.

Studio di fattibilità di una nuova tipologia di argano per il sollevamento di carichi nel settore off-shore

Studio di una nuova tipologia di argano per il sollevamento di carichi nel settore off-shore. In particolare, un argano funzionalmente identico ad uno in fase di realizzazione in azienda è stato modellato al CAD 3D e verificato strutturalmente utilizzando Ansys Workbench, software adatto per analisi F.E.M.. Ottenuti i risultati, ne è stato fatto un confronto con quelli relativi all'argano esistente (argano di ‟Moho‟); nello specifico sono stati comparati gli stress ottenuti nelle varie parti dei due argani (tamburo, spalla, flange, alette di rinforzo, albero) in seguito all‟applicazione di 4 casi di carico differenti (E‟, E‟‟, P‟, P‟‟).

Hydrodynamics and solid dosage form disintegration/dissolution : immediate release tablets and novel in situ polyelectrolyte gastroretentive drug delivery systems

Solid oral dosage form disintegration in the human stomach is a highly complex process dependent on physicochemical properties of the stomach contents as well as on physical variables such as hydrodynamics and mechanical stress. Understanding the role of hydrodynamics and forces in disintegration of oral solid dosage forms can help to improve in vitro disintegration testing and the predictive power of the in vitro test. The aim of this work was to obtain a deep understanding of the influence of changing hydrodynamic conditions on solid oral dosage form performance. Therefore, the hydrodynamic conditions and forces present in the compendial PhEur/USP disintegration test device were characterized using a computational fluid dynamics (CFD) approach. Furthermore, a modified device was developed and the hydrodynamic conditions present were simulated using CFD. This modified device was applied in two case studies comprising immediate release (IR) tablets and gastroretentive drug delivery systems (GRDDS). Due to the description of movement provided in the PhEur, the movement velocity of the basket-rack assembly follows a sinusoidal profile. Therefore, hydrodynamic conditions are changing continually throughout the movement cycle. CFD simulations revealed that the dosage form is exposed to a wide range of fluid velocities and shear forces during the test. The hydrodynamic conditions in the compendial device are highly variable and cannot be controlled. A new, modified disintegration test device based on computerized numerical control (CNC) technique was developed. The modified device can be moved in all three dimensions and radial movement is also possible. Simple and complex moving profiles can be developed and the influence of the hydrodynamic conditions on oral solid dosage form performance can be evaluated. Furthermore, a modified basket was designed that allows two-sided fluid flow. CFD simulations of the hydrodynamics and forces in the modified device revealed significant differences in the fluid flow field and forces when compared to the compendial device. Due to the CNC technique moving velocity and direction are arbitrary and hydrodynamics become controllable. The modified disintegration test device was utilized to examine the influence of moving velocity on disintegration times of IR tablets. Insights into the influence of moving speed, medium viscosity and basket design on disintegration times were obtained. An exponential relationship between moving velocity of the modified basket and disintegration times was established in simulated gastric fluid. The same relationship was found between the disintegration times and the CFD predicted average shear stress on the tablet surface. Furthermore, a GRDDS was developed based on the approach of an in situ polyelectrolyte complex (PEC). Different complexes composed of different grades of chitosan and carrageenan and different ratios of those were investigated for their swelling behavior, mechanical stability, and in vitro drug release. With an optimized formulation the influence of changing hydrodynamic conditions on the swelling behavior and the drug release profile was demonstrated using the modified disintegration test device. Both, swelling behavior and drug release, were largely dependent on the hydrodynamic conditions. Concluding, it has been shown within this thesis that the application of the modified disintegration test device allows for detailed insights into the influence of hydrodynamic conditions on solid oral dosage form disintegration and dissolution. By the application of appropriate test conditions, the predictive power of in vitro disintegration testing can be improved using the modified disintegration test device. Furthermore, CFD has proven a powerful tool to examine the hydrodynamics and forces in the compendial as well as in the modified disintegration test device. rn

Entwicklung eines schnellen, piezobasierten Frequenztuners für supraleitende CH-Kavitäten

Im Rahmen der vorliegenden Arbeit wurde ein schnelles, piezobasiertes Frequenztuningsystem für aktuelle sowie zukünftige supraleitende (sl) CH-Kavitäten entwickelt. Die Grundlage des hierbei verwendeten Tuningkonzepts unterscheidet sich von bisherigen, konventionellen Tuningmethoden supraleitender Kavitäten grundlegend. Zum Ausgleichen von unerwünschten Frequenzverstimmungen während des Beschleunigerbetriebes werden sogenannte bewegliche Balgtuner in das Innere der Resonatorgeometrie geschweißt. Aufgrund ihrer variablen Länge können diese die Kapazität der Kavität und somit die Resonanzfrequenz gezielt beeinflussen. Die Antriebsmechanik, die für die Auslenkung bzw. Stauchung der Balgtuner sorgt, besteht aus einer langsamen, schrittmotorbetriebenen und einer schnellen, piezobasierten Tuningeinheit, welche auf der Außenseite des Heliummantels der jeweiligen CH-Kavität installiert wird. Zur Überprüfung dieses neuartigen Tuningkonzepts wurde in der Werkstatt des Instituts für Angewandte Physik (IAP) der Goethe Universität Frankfurt ein Prototyp der gesamten Tuningeinheit aus Edelstahl gefertigt. Die Funktionsweise der langsamen sowie schnellen Tuningeinheit konnten hierbei in ersten Messungen bei Raumtemperatur erfolgreich getestet werden. Somit stellt die in dieser Arbeit entwickelte Tuningeinheit eine vielversprechende Möglichkeit des dynamischen Frequenztunings supraleitender CH-Strukturen dar. rnDes Weiteren wurden im Rahmen der Arbeit mit Hilfe der Simulationsprogramme ANSYS Workbench sowie CST MicroWave Studio gekoppelte strukturmechanische und elektromagnetische Simulationen der sl 217 MHz CH sowie der sl 325 MHz CH-Kavität durchgeführt. Hierbei konnte zum einen der Frequenzbereich und somit der notwendige mechanische Hub der jeweiligen Tuningeinheit durch Bestimmung der Frequenzverstimmungen signifikant reduziert werden. Zum anderen war es möglich, die mechanische Stabilität der beiden Kavitäten zu untersuchen und somit plastische Deformationen von vornherein auszuschließen. Zur Überprüfung der Genauigkeit sämtlicher getätigter Simulationsrechnungen wurde das strukturmechanische Verhalten in Abhängigkeit äußerer Einflüsse und die daraus resultierenden Frequenzverstimmungen der CH-Kavitäten sowohl bei Raumtemperatur als auch bei kryogenen Temperaturen von 4.2 K gemessen. Hierbei zeigten sich zum Teil hervorragende Übereinstimmungen zwischen den simulierten und gemessenen Werten mit Diskrepanzen von unter 10%. Mit Hilfe dieser Ergebnisse konnte gezeigt werden, dass die gekoppelte Simulation ein essentielles Werkzeug während der Entwicklungsphase einer supraleitenden Beschleunigungsstruktur darstellt, so dass die für den Betrieb erforderliche mechanische Stabilität einer supraleitenden Kavität erreicht werden kann. rn

Testing of subgrid scale (SGS) models for large-eddy simulation (LES) of turbulent channel flow

Sub-grid scale (SGS) models are required in order to model the influence of the unresolved small scales on the resolved scales in large-eddy simulations (LES), the flow at the smallest scales of turbulence. In the following work two SGS models are presented and deeply analyzed in terms of accuracy through several LESs with different spatial resolutions, i.e. grid spacings. The first part of this thesis focuses on the basic theory of turbulence, the governing equations of fluid dynamics and their adaptation to LES. Furthermore, two important SGS models are presented: one is the Dynamic eddy-viscosity model (DEVM), developed by \cite{germano1991dynamic}, while the other is the Explicit Algebraic SGS model (EASSM), by \cite{marstorp2009explicit}. In addition, some details about the implementation of the EASSM in a Pseudo-Spectral Navier-Stokes code \cite{chevalier2007simson} are presented. The performance of the two aforementioned models will be investigated in the following chapters, by means of LES of a channel flow, with friction Reynolds numbers $Re_\tau=590$ up to $Re_\tau=5200$, with relatively coarse resolutions. Data from each simulation will be compared to baseline DNS data. Results have shown that, in contrast to the DEVM, the EASSM has promising potentials for flow predictions at high friction Reynolds numbers: the higher the friction Reynolds number is the better the EASSM will behave and the worse the performances of the DEVM will be. The better performance of the EASSM is contributed to the ability to capture flow anisotropy at the small scales through a correct formulation for the SGS stresses. Moreover, a considerable reduction in the required computational resources can be achieved using the EASSM compared to DEVM. Therefore, the EASSM combines accuracy and computational efficiency, implying that it has a clear potential for industrial CFD usage.