In this thesis the turbulence in a turbulent channel flow was investigated to guarantee the fully developed channel flow condition. The experimental facility is the TU Wien Turbulent Water Channel, consisting of a closed water channel and the experiments are conducted at the 180,360 and 720 friction Reynolds. The velocity measurements are conducted employing two-dimensional Particle Image Velocimetry (PIV), optic technique able to obtain instantaneous velocity fields through the evaluation of particle displacements. Thus four window sizes and three time resolutions for the PIV measurements are investigated, resulting in twelve combinations for low and intermediate Reynolds numbers and four for high Reynolds number. To perform the assessment of fully developed channel condition for the TU channel, turbulence statistics are derived from the measurements: mean streamwise velocity profiles, streamwise and normal velocity fluctuations, Reynolds stresses, skewness and flatness profiles of the fluctuations were computed and compared to the current data available in the literature. The results show a good agreement with available DNS and experimental data: any statistic discrepancy related to the uncertainty of the measurement technique is then discussed in the thesis. In addition, three experiments for fiber laden flow were conducted at the same friction Reynolds and consequently a fiber reconstruction from acquired images was proposed for the case of Reτ=200. The fibers are neutrally buoyant rods, with an aspect ratio of 120 and show a certain curvature. The reconstruction consists of two phases: a discrimination phase and a modeling phase. First the fibers were discriminated, i.e. they are detected in the recorded image despite the high density of tracers and other spurious elements in the channel through intensity and area thresholds. Consequently the fiber is reconstructed using a MATLAB in-house built code based on light intensity and orientation of the fiber.
In questa tesi è stata studiata la turbolenza in un flusso di canale turbolento per garantire la condizione fully developed channel flow. Il canale analizzato è il TU Wien Turbulent Water Channel, costituito da un canale ad acqua chiuso. Gli esperimenti sono stati condotti ad un valore di friction Reynolds di 180,360 e 720. Le misurazioni di velocità sono state condotte utilizzando Particle Image Velocimetry (PIV) bidimensionale, tecnica ottica in grado di ottenere campi di velocità istantanei attraverso la valutazione degli spostamenti delle particelle. Dunque, sono state studiate quattro dimensioni per la finestra di interrogazione e tre risoluzioni temporali per le misurazioni PIV, risultando in dodici combinazioni per i numeri di Reynolds basso ed intermedio e quattro per il numero di Reynolds più alto. Per dimostrare la condizione di fully developed channel flow per il canale della TU, le statistiche turbolente sono state ottenute attraverso le misurazioni: i profili di velocità media, le fluttuazioni della velocità normale e assiale, le sollecitazioni di Reynolds, i profili di asimmetria e planarità delle fluttuazioni turbolente sono stati calcolati e confrontati con gli attuali dati disponibili in letteratura. I risultati mostrano una buona concordanza con i dati DNS e sperimentali disponibili attualmente: qualsiasi discrepanza delle statistiche correlata all'incertezza della tecnica di misurazione viene poi discussa nella tesi. Inoltre, sono stati condotti tre esperimenti per il caso di flusso turbolento in presenza di fibre a valori comparabili di friction Reynolds e poi è stata proposta una ricostruzione della fibra dalle immagini acquisite per il caso di Reτ=200. Le fibre sono a galleggiabilità neutra, con un aspect ratio di 120 e mostrano una certa curvatura. La ricostruzione consiste in due fasi: una fase di discriminazione e una fase di modellazione. Prima le fibre sono state discriminate, cioè vengono rilevati nell'immagine registrata nonostante l'elevata densità di traccianti e altri elementi spuri nel canale attraverso soglie di intensità e area. Infine, la fibra viene ricostruita utilizzando un codice MATLAB in base all'intensità della luce e all'orientamento della fibra.
Experimental investigation of microfibers dispersion in fully developed turbulent channel flow
Sambuca, Silvia
2021/2022
Abstract
In this thesis the turbulence in a turbulent channel flow was investigated to guarantee the fully developed channel flow condition. The experimental facility is the TU Wien Turbulent Water Channel, consisting of a closed water channel and the experiments are conducted at the 180,360 and 720 friction Reynolds. The velocity measurements are conducted employing two-dimensional Particle Image Velocimetry (PIV), optic technique able to obtain instantaneous velocity fields through the evaluation of particle displacements. Thus four window sizes and three time resolutions for the PIV measurements are investigated, resulting in twelve combinations for low and intermediate Reynolds numbers and four for high Reynolds number. To perform the assessment of fully developed channel condition for the TU channel, turbulence statistics are derived from the measurements: mean streamwise velocity profiles, streamwise and normal velocity fluctuations, Reynolds stresses, skewness and flatness profiles of the fluctuations were computed and compared to the current data available in the literature. The results show a good agreement with available DNS and experimental data: any statistic discrepancy related to the uncertainty of the measurement technique is then discussed in the thesis. In addition, three experiments for fiber laden flow were conducted at the same friction Reynolds and consequently a fiber reconstruction from acquired images was proposed for the case of Reτ=200. The fibers are neutrally buoyant rods, with an aspect ratio of 120 and show a certain curvature. The reconstruction consists of two phases: a discrimination phase and a modeling phase. First the fibers were discriminated, i.e. they are detected in the recorded image despite the high density of tracers and other spurious elements in the channel through intensity and area thresholds. Consequently the fiber is reconstructed using a MATLAB in-house built code based on light intensity and orientation of the fiber.| File | Dimensione | Formato | |
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ExtendedAbstract_SilviaSambuca.pdf
Open Access dal 27/11/2023
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Master_Thesis_SilviaSambuca .pdf
Open Access dal 27/11/2023
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9.11 MB
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9.11 MB | Adobe PDF | Visualizza/Apri |
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https://hdl.handle.net/10589/196516