Hot wire manufacturing and resolution effects in high Reynolds number flows

Lo studio della turbolenza è di fondamentale importanza non solo per la fluidodinamica teorica ma anche perchè viene riscontrata in una moltitudine di problemi di interesse ingegneristico. All'aumentare del numero di Reynolds, le scale caratteristiche tendono a ridurre le loro dimensioni assolute. Nella fluidodinamica sperimentale già da lungo tempo si è affermata l'anemometria a filo caldo, grazie ad ottime caratteristiche di risoluzione spaziale e temporale. Questa tecnica, caratterizzata da un basso costo e da una relativa semplicità, rende possibile la realizzazione di sensori di tipo artigianale, che hanno il vantaggio di poter essere relizzati in dimensioni inferiori. Nonostante l'ottima risoluzione spaziale degli hot-wire, infatti, si può verificare, ad alto numero di Reynolds, che le dimensioni dell'elemento sensibile siano superiori a quelle delle piccole scale. Questo impedisce al sensore di risolvere correttamente le strutture più piccole. Per questa tesi di laurea è stato allestito un laboratorio per la costruzione di sensori a filo caldo con filo di platino. Sono in questo modo stati realizzati diversi sensori dalle dimensioni caratteristiche inferiori a quelle dei sensori disponibili commercialmente. I sensori ottenuti sono quindi stati testati in un getto turbolento, dapprima confrontandone la risposta con un sensore di tipo commerciale, per verificarne il corretto funzionamento. In seguito si sono eseguite misure più specifiche e limitate ad alcune particolari zone all'interno del campo di moto, dove è probabile riscontrare effetti di risoluzione spaziale. Sono stati analizzati gli effetti della dimensione fisica del sensore sui momenti statistici centrali, sugli spettri di velocità e sulle funzioni di densità di probabilità.

A numerical study of temperature effects on hot wire measurements inside turbulent channel flows

A way to investigate turbulence is through experiments where hot wire measurements are performed. Analysis of the in turbulence of a temperature gradient on hot wire measurements is the aim of this thesis work. Actually - to author's knowledge - this investigation is the first attempt to document, understand and ultimately correct the effect of temperature gradients on turbulence statistics. However a numerical approach is used since instantaneous temperature and streamwise velocity fields are required to evaluate this effect. A channel flow simulation at Re_tau = 180 is analyzed to make a first evaluation of the amount of error introduced by temperature gradient inside the domain. Hot wire data field is obtained processing the numerical flow field through the application of a proper version of the King's law, which connect voltage, velocity and temperature. A drift in mean streamwise velocity profile and rms is observed when temperature correction is performed by means of centerline temperature. A correct mean velocity pro�le is achieved correcting temperature through its mean value at each wall normal position, but a not negligible error is still present into rms. The key point to correct properly the sensed velocity from the hot wire is the knowledge of the instantaneous temperature field. For this purpose three correction methods are proposed. At the end a numerical simulation at Re_tau =590 is also evaluated in order to confirm the results discussed earlier.

Caratteristiche micromeccaniche di giunzioni friction welding

Studio della caratterizzazione micromeccanica di provini in alluminio 6082 T6 saldati con la tecnica della rotary friction welding, tecnica innovativa nell’ambito delle tecnologie di giunzione solida, che sfrutta il principio della saldatura per attrito. Dopo un'introduzione teorica sulle principali tematiche affrontate e sui principali sistemi di misura usati, verranno effettuate micrografie dei giunti e prove di microdurezza sugli stessi. Si termina riportando i grafici delle microdurezze trovate, in modo da poter confrontare i risultati ottenuti con le caratteristiche del materiale base. Da questi si osserva che: nonostante la qualità accettabile dei giunti, le elevate temperature raggiunte durante la saldatura hanno alterato il trattamento termico del materiale base, che da T6 assume caratteristiche più simili a un T4. In altre parole il materiale saldato presenta caratteristiche micromeccaniche inferiori a quelle nominalmente assegnabili al materiale base.

Analisi di microdurezza su giunti friction welding

Il presente lavoro di tesi si pone come obiettivo la caratterizzazione microstrutturale di una serie di giunti realizzati tramite un innovativo processo di saldatura allo stato solido denominato Rotary Friction Welding. In tale ricerca si descrive il percorso seguito per arrivare a tale caratterizzazione partendo da una discussione delle principali tecniche di saldatura, in particolare quelle per attrito, per passare poi ad un’analisi dettagliata della fase di creazione dei giunti saldati e della successiva fase dedicata alla verifica microstrutturale. I risultati ottenuti, ci forniranno indici di confronto sulla qualità del materiale, sui processi tecnologici, sui trattamenti e su alcune fondamentali proprietà meccaniche

Simulazione FEM e validazione sperimentale del processo di friction welding

Nel seguente elaborato si propone lo sviluppo di un modello agli elementi finiti (FEM) del processo di friction welding del quale, attraverso i dati rilevati da prove sperimentali di validazione termica, vengono valutati i parametri ottimali di simulazione. Ai risultati così ottenuti vengono applicati anche algoritmi per la valutazione della microstruttura e della qualità della saldatura, sviluppati originariamente per l'analisi dell'estrusione: in entrambi i casi a seguito del confronto con le analisi metallografiche dei provini è stato possibile validare ulteriormente il modello creato.

Effetti della risoluzione spaziale nella stima della dissipazione con sensore hot-wire

L'obiettivo primario di questo elaborato di tesi è quello di stimare l'effetto della risoluzione spaziale e quindi della lunghezza finita del sensore di un anemometro a filo caldo, sul calcolo delle derivate che contribuiscono a valutare la dissipazione.

Simulazione numerica del processo rotary friction welding

Costruzione di un modello numerico base per la simulazione del processo Rotary Friction Welding.

Experimental characterization of a planar jet by hot-wire anemometry

The study of turbulence is also nowadays a problem that does not have solution from the mathematical point of view due to the lack of solution to link the mean part of the flow with the fluctuating one. To solve this problem, in the CICLoPE laboratory of Predappio, experiments on different type of jets are performed in order to derive a closure model able to close our mathematical model. One of the most interesting type of jet that could be studied is the planar turbulent free jet which is a two dimensional canonical jet characterized by the self-similarity condition of the velocity profiles. To study this particular jet, a new facility was built. The aim of this project is to characterize the jet at different distances from the nozzle exit, for different values of Reynolds number, to demonstrate that the self-similarity condition is respected. To do that, the evaluation of quantities such as spreading rate, centerline velocity decay and relation between fluctuations and mean part of the flow has to be obtain. All these parameters could be detected thanks to the use of single and X hot-wire anemometry with which it is possible to analyzed the fluctuating behaviour of the flow by associating to an electric signal a physical variable expressed in terms of velocity. To justify the data obtain by the measures, a comparison with results coming from the literature has to be shown.

Computational design of a new type of sandwich cross-section for a defined solution of a tree-like column produced by wire-and-arc additive manufacturing

When it comes to designing a structure, architects and engineers want to join forces in order to create and build the most beautiful and efficient building. From finding new shapes and forms to optimizing the stability and the resistance, there is a constant link to be made between both professions. In architecture, there has always been a particular interest in creating new shapes and types of a structure inspired by many different fields, one of them being nature itself. In engineering, the selection of optimum has always dictated the way of thinking and designing structures. This mindset led through studies to the current best practices in construction. However, both disciplines were limited by the traditional manufacturing constraints at a certain point. Over the last decades, much progress was made from a technological point of view, allowing to go beyond today's manufacturing constraints. With the emergence of Wire-and-Arc Additive Manufacturing (WAAM) combined with Algorithmic-Aided Design (AAD), architects and engineers are offered new opportunities to merge architectural beauty and structural efficiency. Both technologies allow for exploring and building unusual and complex structural shapes in addition to a reduction of costs and environmental impacts. Through this study, the author wants to make use of previously mentioned technologies and assess their potential, first to design an aesthetically appreciated tree-like column with the idea of secondly proposing a new type of standardized and optimized sandwich cross-section to the construction industry. Parametric algorithms to model the dendriform column and the new sandwich cross-section are developed and presented in detail. A catalog draft of the latter and methods to establish it are then proposed and discussed. Finally, the buckling behavior of this latter is assessed considering standard steel and WAAM material properties.

Development and Automatization of a Planar Jet Wind Tunnel for the X-wire Calibration

The scope of this study is to design an automatic control system and create an automatic x-wire calibrator for a facility named Plane Air Tunnel; whose exit creates planar jet flow. The controlling power state as well as automatic speed adjustment of the inverter has been achieved. Thus, the wind tunnel can be run with respect to any desired speed and the x-wire can automatically be calibrated at that speed. To achieve that, VI programming using the LabView environment was learned, to acquire the pressure and temperature, and to calculate the velocity based on the acquisition data thanks to a pitot-static tube. Furthermore, communication with the inverter to give the commands for power on/off and speed control was also done using the LabView VI coding environment. The connection of the computer to the inverter was achieved by the proper cabling using DAQmx Analog/Digital (A/D) input/output (I/O). Moreover, the pressure profile along the streamwise direction of the plane air tunnel was studied. Pressure tappings and a multichannel pressure scanner were used to acquire the pressure values at different locations. Thanks to that, the aerodynamic efficiency of the contraction ratio was observed, and the pressure behavior was related to the velocity at the exit section. Furthermore, the control of the speed was accomplished by implementing a closed-loop PI controller on the LabView environment with and without using a pitot-static tube thanks to the pressure behavior information. The responses of the two controllers were analyzed and commented on by giving suggestions. In addition, hot wire experiments were performed to calibrate automatically and investigate the velocity profile of a turbulent planar jet. To be able to analyze the results, the physics of turbulent planar jet flow was studied. The fundamental terms, the methods used in the derivation of the equations, velocity profile, shear stress behavior, and the effect of vorticity were reviewed.

Temperature measurements in turbulent pipe flow with cold wire anemometry

Passive scalars measurements in turbulent pipe flows are difficult to perform and only few experimental data are available in literature. The present thesis deals with the experimental acquisition and study of the first turbulent temperature profile inside the CICLoPE wind tunnel through cold wire anemometry technique at Reτ = 6000 and Reτ = 9500. This type of measurements provides not only useful data on temperature (and passive scalars) behaviour and statistics in turbulent pipe flows, but could be used also for temperature correction of turbulent velocity profiles. In the present work, subsequent acquisitions of temperature and velocity profiles has been performed at the same Reynolds number and in the same points, through cold wire and hot wire techniques respectively. Taking as reference data from both DNS and experimental campaigns, the activity has been carried out obtaining satisfactory results. We have verified the presence of turbulent temperature profile inside the CICLoPE wind tunnel and then studied its statistical and spectral behaviours obtaining results in agreement with existing data from Hishida, Nagano, and Ferro. Cold wire temperature data were then used to correct hot wire velocity data, obtaining a slightly improvement in the near wall region.

Film di poli(esametilene 2,5- furanoato) contenenti filler lignocellulosici per imballaggio alimentare attivo e sostenibile

Nel seguente lavoro di tesi sperimentale è stato svolto uno studio su film di poli(esametilen furanoato) additivato con filler antiossidanti estratti da una materia prima lignocellulosica, la corteccia di betulla. Tale studio ha lo scopo di incrementare le proprietà meccaniche e soprattutto conservative dei film di PHF per applicazioni nel campo del packaging alimentare. Il poli(esametilen furanoato) è un poliestere i cui monomeri di sintesi possono essere ottenuti da fonti rinnovabili, tale caratteristica lo rende completamente bio-based e di elevato interesse per l’ottenimento di materiali sostenibili. Nella fase iniziale dello studio è stato sintetizzato il polimero in esame tramite una sintesi di tipo solvent-free, in accordo con le attuali strategie sintetiche che mirano a ridurre l’impatto del solvente. Tale polimero è stato quindi caratterizzato tramite NMR e GPC. Sono state poi preparate quattro miscele di polimero additivato, due differenti composizioni per ciascuno dei due filler disponibili. Le miscele sono state preparate tramite solvent casting e in seguito stampate tramite pressofusione per ottenere dei film. È stata svolta una caratterizzazione dei film ottenuti, di tipo morfologica (SEM), termica (TGA e DSC), meccanica, comportamento barriera e con analisi antiossidanti. I filler hanno mostrato una buona miscibilità con l’omopolimero e non hanno causato interferenze nel comportamento termico. È stato osservato un miglioramento nella flessibilità dei film in tutte le miscele studiate e un aumento dell’allungamento a rottura nelle composizioni con quantità di filler pari al 5%. Le proprietà barriera si sono mantenute in linea con quelle dell’omopolimero e ancora migliori dei poliesteri attualmente in commercio. Infine, l’aggiunta del filler ha reso il film attivo per lo scavenging di radicali, valutato attraverso il test con DPPH, confermando il trasferimento delle proprietà antiossidanti dei filler alle miscele polimeriche.

Design, fabrication and mechanical characterization of a structural node made using Wire and Arc Additive Manufacturing (WAAM) technology

Additive Manufacturing (AM), also known as “3D printing”, is a recent production technique that allows the creation of three-dimensional elements by depositing multiple layers of material. This technology is widely used in various industrial sectors, such as automotive, aerospace and aviation. With AM, it is possible to produce particularly complex elements for which traditional techniques cannot be used. These technologies are not yet widespread in the civil engineering sector, which is slowly changing thanks to the advantages of AM, such as the possibility of realizing elements without geometric restrictions, with less material usage and a higher efficiency, in particular employing Wire-and-Arc Additive Manufacturing (WAAM) technology. Buildings that benefit most from AM are all those structures designed using form-finding and free-form techniques. These include gridshells, where joints are the most critical and difficult elements to design, as the overall behaviour of the structure depends on them. It must also be considered that, during the design, the engineer must try to minimize the structure's own weight. Self-weight reductions can be achieved by Topological Optimization (TO) of the joint itself, which generates complex geometries that could not be made using traditional techniques. To sum up, weight reductions through TO combined with AM allow for several potential benefits, including economic ones. In this thesis, the roof of the British Museum is considered as a case study, analysing the gridshell structure of which a joint will be chosen to be designed and manufactured, using TO and WAAM techniques. Then, the designed joint will be studied in order to understand its structural behaviour in terms of stiffness and strength. Finally, a printing test will be performed to assess the production feasibility using WAAM technology. The computational design and fabrication stages were carried out at Technische Universität Braunschweig in Germany.