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.

Heat-transfer enhancement with pulsed jets in turbulent pipe flow at high-Reynolds number

Improving heat transfer is a critical area of ​​research in various fields such as thermal engineering, energy conversion and aeronautical engineering. The aim of this thesis is to present the design, construction and testing of an experimental setup for the study of heat transfer enhancement in a turbulent boundary layer using cross-flow pulsed jets. The set-up is designed to generate and control pulsed jets, measure heat transfer and acquire all parameters related to wind tunnel flow and is also capable of varying the parameters of the pulsed jets, such as frequency, amplitude and the duty cycle, in order to study the effects on the increase in heat transfer. The thesis describes the design phases, the construction process and the final successful testing of the plant. The test results verify the functionality and accuracy of the set-up and ensure that it can be used to perform a full experimental campaign to investigate heat transfer enhancement using cross-flow pulsed jets in a turbulent layer boundary.

Experimental analysis of a separation control device based on plasma technology

In this thesis effects of plasma actuators based on Dielectric Barrier Discharge (DBD) technology over a NACA 0015 bidimensional airfoil have been analyzed in an experimental way, at low Reynolds number. Work developed on thesis has been carried on in partnership with the Department of Electrical Engineering of Università di Bologna, inside Wind Tunnel of the Applied Aerodynamic Laboratory of Aerospace Engineering faculty. In order to verify the effectiveness of these active control devices, the analysis has shown how actuators succeed in prevent boundary layer separation only in certain conditions af angle of attack and Reynolds numbers. Moreover, in this thesis actuators’ chordwise position effect has been also analyzed, together with the influence of steady and unsteady operations.

A LES study of a modified Ahmed body geometry

A numerical study using Large Eddy Simulation Coherent Structure Model (LES-CSM), of the flow around a simplified Ahmed body, has been done in this work of thesis. The models used are two salient geometries from the experimental investigation performed in [1], and consist, in particular, in two notch-back body geometries. Six simulation are carried out in total, changing Reynolds number and back-light angle of the model’s rear part. The Reynolds numbers used, based on the height of the models and the free stream velocity, are Re = 10000, Re = 30000 and Re = 50000. The back-light angles of the slanted surface with respect to the horizontal roof surface, that characterizes the vehicle, are taken as B = 31.8◦ and B = 42◦ respectively. The experimental results in [1] have shown that, depending on the parameter B, asymmetric and symmetric averaged flow over the back-light and in the wake for a symmetric geometry can be observed. The aims of the present work of master thesis are principally two. The first aim is to investigate and confirm the influence of the parameter B on the presence of the asymmetry of the averaged flow, and confirm the features described in the experimental results. The second important aspect is to investigate and observe the influence of the second variable, the Reynolds number, in the developing of the asymmetric flow itself. The results have shown the presence of the mentioned asymmetry as well as an influence of the Reynolds number on it.

Separation control by means of oriented DBD plasma actuators: an experimental analysis in a wind tunnel

This thesis provides an experimental analysis of the effectiveness of oriented DBD plasma actuators over a NACA 0015 airfoil at low Reynolds numbers. Tests were performed in partnership with the Department of Electrical Engineering of Bologna University, in the wind tunnel of the Applied Aerodynamics Laboratory of Aerospace Engineering faculty. Lift coefficient measurements were carried out in order to verify how an oriented plasma jet succeeds in prevent boundary layer separation. Both actuators’ chord wise position and plasma jet orientation angle have been investigated to examine which configurations lead to the best results. A particular attention has been paid also to the analysis of results in steady and unsteady plasma actuation. Questa tesi offre un’analisi sperimentale sull’efficacia di attuatori al plasma orientabili, basati su una tecnologia DBD, installati su un profilo alare NACA 0015, a bassi numeri di Reynolds. Le prove sono state condotte in collaborazione con il Dipartimento di Ingegneria Elettrica dell’Università di Bologna, nella galleria del vento del Laboratorio di Aerodinamica Applicata della Facoltà di Ingegneria Aerospaziale di Forlì. Per verificare come un getto orientabile di plasma riesca a prevenire la separazione dello strato limite, sono state eseguite misure sul coefficiente di portanza. Sono state indagate sia la posizione degli attuatori lungo la corda che l’angolo con cui è orientato il getto di plasma, per vedere quali configurazioni conducono ai migliori risultati. Una particolare attenzione è stata riservata all’analisi dei risultati ottenuti con plasma continuo e pulsato.

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.

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.

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.

Impact of convective motions on pollutant dispersion in urban canyons: an analysis through RANS simulations

Since the majority of the population of the world lives in cities and that this number is expected to increase in the next years, one of the biggest challenges of the research is the determination of the risk deriving from high temperatures experienced in urban areas, together with improving responses to climate-related disasters, for example by introducing in the urban context vegetation or built infrastructures that can improve the air quality. In this work, we will investigate how different setups of the boundary and initial conditions set on an urban canyon generate different patterns of the dispersion of a pollutant. To do so we will exploit the low computational cost of Reynolds-Averaged Navier-Stokes (RANS) simulations to reproduce the dynamics of an infinite array of two-dimensional square urban canyons. A pollutant is released at the street level to mimic the presence of traffic. RANS simulations are run using the k-ɛ closure model and vertical profiles of significant variables of the urban canyon, namely the velocity, the turbulent kinetic energy, and the concentration, are represented. This is done using the open-source software OpenFOAM and modifying the standard solver simpleFoam to include the concentration equation and the temperature by introducing a buoyancy term in the governing equations. The results of the simulation are validated with experimental results and products of Large-Eddy Simulations (LES) from previous works showing that the simulation is able to reproduce all the quantities under examination with satisfactory accuracy. Moreover, this comparison shows that despite LES are known to be more accurate albeit more expensive, RANS simulations represent a reliable tool if a smaller computational cost is needed. Overall, this work exploits the low computational cost of RANS simulations to produce multiple scenarios useful to evaluate how the dispersion of a pollutant changes by a modification of key variables, such as the temperature.