Internal waves occur in the interface between two layers of fluids with density stratification. In order to better understand the characteristics of continuous internal waves, a series of experiments were conducted in a laboratory tank. The upper and lower layers are fresh water of 15 cm thick and salt water of 30 cm thick, respectively. The periods of internal waves are 2.5, 5.5 and 6.6 sec. A micro-ADV is used to measure velocity profiles. Wave profiles at the density interface and the free surface are monitored respectively by an ultrasonic and capacitance wave gauges. Our results indicate that particle velocities (u and w) above and below the density interface have opposite directions. The speed is peaked near the density interface and it becomes weaker further away from the interface. Empirical Mode Decomposition is used to remove noise from the observed particle velocities, and the period is consistent with those derived from the interface elevations. The observed particle velocities also compare favorably with the theoretical results. When internal waves propagate without the interference of a sloping bottom, the turbulence induced is rather insignificant. The turbulence is more significant only near the density interface. With the existence of a sloping bottom, the internal waves gradually shoal and deform, the crest becomes sharp and steep, finally the waves become unstable, break and overturn. In this study the effect of bottom slope and the steepness of internal waves on the reflectivity of incoming waves are investigated. The reflectivity is smaller with gentler slope, and it increases and reaches a constant value with steeper slopes. The observed energy dissipation rate£`is higher near the slope. Three methods were used to estimate the energy dissipation rate and shear stress; namely, the inertial dissipation, the TKE and auto-correlation method. The£` estimated from the auto-correlation method is larger than that from the other two methods, but their trend is similar. The energy dissipation rate is found to increase with a gentler sloping bottom.

Animal cells are often cultured in stirred tank reactors. Having no cell wall, these animal cells are very sensitive to the fluid mechanical stresses that result from agitation by the impeller and from the rising and bursting of bubbles, which are generated within the culture medium in the stirred tank to supply oxygen by mass transfer to the cells. If excessive, these fluid mechanical stresses can result in damage/death of animal cells. Stress due to the rising and bursting of bubbles can be avoided by using a gas-permeable membrane, in the form of a long coiled tube (with air passing through it) within the stirred tank, instead of air-bubbles to oxygenate the culture medium. Fluid mechanical stress due to impeller agitation can be controlled using appropriate impeller rotational speeds. The aim of this study was to lay the foundations for future work in which a correlation would be developed between cell damage/death and the fluid mechanical stresses that result from impeller agitation and bubbling. Such a correlation could be used to design stirred-tank reactors at any scale and to determine appropriate operating conditions that minimise cell damage/death due to fluid mechanical stresses. Firstly, a validated CFD model of a baffled tank stirred with a Rushton turbine was developed to allow fluid mechanical stresses due to impeller agitation to be estimated. In these simulations, special attention was paid to the turbulence energy dissipation rate, which has been closely linked to cell damage/death in the literature. Different turbulence models, including the k-ε, SST, SSG-RSM and the SAS-SST models, were investigated. All the turbulence models tested predicted the mean axial and tangential velocities reasonably well, but under-predicted the decay of mean radial velocity away from the impeller. The k-ε model predicted poorly the generation and dissipation of turbulence in the vicinity of the impeller. This contrasts with the SST model, which properly predicted the appearance of maxima in the turbulence kinetic energy and turbulence energy dissipation rate just off the impeller blades. Curvature correction improved the SST model by allowing a more accurate prediction of the magnitude and location of these maxima. However, neither the k-ε nor the SST models were able to properly capture the chaotic and three-dimensional nature of the trailing vortices that form downstream of the blades of the impeller. In this sense, the SAS-SST model produced more physical predictions. However,this model has some drawbacks for modelling stirred tanks, such as the large number of modelled revolutions required to obtain good statistical averaging for calculating turbulence quantities. Taking into consideration both accuracy and solution time, the SSG-RSM model was the least satisfactory model tested for predicting turbulent flow in a baffled stirred tank with a Rushton turbine. In the second part of the work, experiments to determine suitable oxygen transfer rates for culturing cells were carried out in a stirred tank oxygenated using either a sparger to bubble air through the culture medium or a gas-permeable membrane. Results showed that the oxygen transfer rates for both methods of oxygenation were always above the minimum oxygen requirements for culturing animal cells commonly produced in industry, although the oxygen transfer rate for air-bubbling was at-least 10 times higher compared with using a gas-permeable membrane. These results pave the way for future experiments, in which animal cells would be cultured in the stirred tank using bubbling and (separately) a gas-permeable membrane for oxygenation so that the effect of rising and bursting bubbles on cell damage/death rates can be quantified. The effect of impeller agitation on cell damage/death would be quantified by using the gas permeable membrane for oxygenation (to remove the detrimental effects of bubbling), and changing the impeller speed to observe the effect of agitation intensity. In the third and final part of this work, the turbulent flow in the stirred tank used in the oxygenation experiments was simulated using CFD. The SST turbulence model with curvature correction was used in these simulations, since it was found to be the most accurate model for predicting turbulence energy dissipation rate in a stirred tank. The predicted local maximum turbulence energy dissipation rate of 8.9x10¹ m2/s3 at a rotational speed of 900 rpm was found to be substantially less than the value of 1.98x10⁵ m2/s3 quoted in the literature as a critical value above which cell damage/death becomes significant. However, the critical value for the turbulence energy dissipation rate quoted in the literature was determined in a single-pass flow device, whereas animal cells in a stirred tank experience frequent exposure to high turbulence energy dissipation rates (in the vicinity of the impeller) due to circulation within the stirred tank and long culture times. Future cell-culturing experiments carried out in the stirred tank of this work would aim to determine a more appropriate critical value for the turbulence energy dissipation rate in a stirred tank, above which cell damage/death becomes a problem.

Submitted by Regina Correa (rehecorrea@gmail.com) on 2016-09-19T19:31:52Z No. of bitstreams: 1 TeseHS.pdf: 6507848 bytes, checksum: 467139021a2d6e49272a3197b75c3216 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T12:29:38Z (GMT) No. of bitstreams: 1 TeseHS.pdf: 6507848 bytes, checksum: 467139021a2d6e49272a3197b75c3216 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T12:29:45Z (GMT) No. of bitstreams: 1 TeseHS.pdf: 6507848 bytes, checksum: 467139021a2d6e49272a3197b75c3216 (MD5) / Made available in DSpace on 2016-09-21T12:29:53Z (GMT). No. of bitstreams: 1 TeseHS.pdf: 6507848 bytes, checksum: 467139021a2d6e49272a3197b75c3216 (MD5) Previous issue date: 2016-03-28 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Taylor-Vortex reactor (TVB) is fast becoming the next bioreactor to culture animal cells due to milder shear and homogeneous flow structures through-out the bioreactor in comparison to the traditional stirred vessels. However, there is little information in the literature for the TVB on the viscous energy dissipation rate (VEDR), which is considered the ideal parameter to characterize the cell death, and its geometrical aspects, which may affect the culture of animal cells resulting in poor efficiency. Consequently, this work focuses on: the estimation of the VEDR of mean flow and turbulent kinetic energy (TKE) using an experimental 2D particle image velocimetry (PIV) method and a computational fluid dynamics (CFD) method using different turbulence models, principally the direct numerical simulation (DNS) model; and, the impact of the off-bottom clearance area and the external cylinder’s bottom shape on the flow structures of TVB. Both numerical and experimental methods confirm that the bulk zone comprising of the 80 % of the gap-width, where the cell cultures will spend most of the time, has a near constant velocity magnitude of around 50 % of the tip velocity and VEDR values which are around 10 times lower than at the walls. Qualitatively, the DNS model predicted well the flow structure of both mean and turbulence parameters in comparison with the experimental PIV predictions. However, quantitatively only the mean velocity predictions are in good agreement with the PIV data with certain amount of under-estimation of the turbulence parameters. Among different turbulence models, the large eddy simulation (LES) - wall adapting local eddy-viscosity (WALE) model presented best comparison with the DNS model data for all the flow parameters; while, the Reynolds stress model and the LES-Smagorinsky models were the poorest. On the other hand, the Reynolds averaged Navier-Stokes (RANS) based two equation models estimated well the mean velocity components in comparison with the DNS model data, but could not capture well the flow structures of the turbulence components. The geometrical features of curved surface of outer bottom and off-bottom clearance area which are of practical importance in stirred vessels, impact adversely the flow structures in the TVB due to poor axial velocity component. In comparison with the spinner vessel, a stirred tank type bioeactor but with lower shear, for similar Re/ReT ratio, the maximum and mean VEDR were always found to be of lower magnitude values, and due to much less difference between the maximum and the mean values, the TVB presents more uniform structures in comparison to the spinner vessel. / O biorreator de Vórtices de Taylor (TVB) está se tornando uma nova descoberta, devido ao seu cisalhamento mais suave e fluxo homogêneo em comparações com os biorreatores de tanque agitados. Na literatura acadêmica há pouca informação sobre este biorreator quanto a taxa de dissipação de energia viscosa (VEDR), que é o parâmetro ideal para caracterizar a morte celular, e seus aspectos geométricos, que afetam o cultivo das células animais, resultando em baixa eficiência. A presente pesquisa, portanto, objetivou focar na estimativa da VEDR de fluxo médio e de energia cinética turbulenta (TKE) no TVB usando os métodos: experimental de 2D de velocimetria das partículas por imagem (PIV) e numérico de dinâmica de fluídos computacional (CFD) com diferentes modelos de turbulência, principalmente a simulação numérica direta (DNS). E focar nos aspectos geométricos do impacto da área de apuramento entre o cilindro interno e externo e na forma da base do cilindro externo na estrutura de fluxo do TVB. Os dois métodos experimental e numérico demonstraram que, em aproximadamente 80 % da área lateral entre os cilindros interno e externo onde as células vão passar a maior parte do tempo, a magnitude de velocidade é de cerca de 50 % da máxima e os valores de VEDR são 10 vezes menores do que nas paredes. Qualitativamente, o DNS mostrou boas comparações dos fluxos médios e dos parâmetros turbulentos em relação aos resultados apresentados pelo PIV para o TVB. No entanto, quantitativamente, apenas as previsões médias de velocidade estão em boa concordância com os dados do PIV, pois os parâmetros turbulentos foram sub-estimados. Entre os diferentes modelos de turbulência utilizados, o modelo simulação de grande escala (LES) - Wall Adapting Local Eddy-Viscosity apresentou a melhor comparação com os dados do DNS para todos os parâmetros do fluxo. O modelo de estresse Reynolds e LES - Smagorinsky, por sua vez, apresentaram as piores comparações. Os modelos de duas equações de RANS, entretanto, apesar de estimarem bem os componentes de velocidade média em comparação com os dados do modelo DNS, não captaram bem as estruturas de fluxo dos componentes de turbulência. Quanto aos aspectos geométricos, as alterações nas características da área de apuramento entre o cilindro interno e externo e a estrutura curva da base do cilindro externo, que são de importância prática em tanque agitados, neste estudo, afetaram negativamente o fluxo no TVB devido ao seu baixo componente de velocidade axial. Por fim, a comparação entre o TVB e o Spinner Flask, considerado também um biorreator com baixo cisalhamento, demostrou que para Re/ReT semelhante, os valores máximo e médio do VEDR foram sempre inferiores, e devido à diferença muito menor entre o os valores máximo e médio, o TVB apresenta estruturas mais uniformes em comparação com o Spinner Flask. / processo nº 140756/2012-4 ; processo nº - 241739/2012-8)

Understanding the physics of mixing for two fluids is a complicated problem and has always been an interesting phenomenon to study. Surface discharge is the oldest, least expensive and simplest way of discharging industrial or domestic wastewater into rivers and estuaries. Because of the lower degree of dilution in surface discharges, critical conditions are more likely to occur. Having a better understanding of the mixing phenomenon in these cases will help to predict the environmental effects more accurately. In this study, surface discharges of jets into waterbodies with or without crossflows were investigated numerically and experimentally. Three-dimensional (3-D) Computational Fluid Dynamics (CFD) models were developed for studying the surface discharge of jets into water bodies using different turbulence models. Reynolds stress turbulence models and spatially filtered Large Eddy Simulation (LES) were used in the numerical models. The effects of inclusion of free surface water in the CFD models on the performance of the numerical model results were investigated. Numerical model results were compared with the experimental data in the literature as well as the experimental works performed in this study. Experimental works for buoyant and non-buoyant surface discharge of jets into crossflow and stagnant water were conducted in this study. A new setup was designed and built in the Civil Engineering Hydraulics Laboratory at the University of Ottawa to perform the desired experiments. Stereoscopic Particle Image Velocimetry (Stereo-PIV) was used to measure the instantaneous spatial and temporal 3-D velocity distribution on several planes of measurement downstream of the jet with the frequency of 40 Hz. Averaged 3-D velocity distribution was extracted on different planes of measurement to show the transformation of the velocity vectors from a “jet-like” to a “plume-like” flow regime. Averaged 3-D velocity distribution and streamlines illustrated the flow transformation of the surface jets. Experimental results detected the formation and evolution of vortices in the surface jet’s flow structure over the measurement zone. Additional turbulent flow characteristics such as the turbulent kinetic energy (k), turbulent kinetic energy dissipation rate (ϵ), and turbulent eddy viscosity (υt) were calculated using the measured time history of the 3-D velocity field.

No description available.

Depuis une dizaine d’années, l’évolution de la puissance des ordinateurs a permis de développer l’utilisation, dans les études d’ingénierie, des simulations 3D CFD (Computational Fluid Dynamics) pour l’étude de l’atmosphère à petite échelle, en particulier pour la dispersion de polluants sur des sites industriels et urbains complexes. Compte tenu de la complexité des domaines à étudier et des ressources de calcul généralement disponibles, ces études sont la plupart du temps réalisées à l’aide des modèles RANS (Reynolds Averaged Navier-Stokes), et particulièrement avec le modèle de fermeture k – e. Différents travaux de validation de l’approche RANS k – e ont mis en évidence quelques limitations à reproduire la dynamique de l’écoulement et de la dispersion dans des configurations géométriques complexes. Le travail de recherche réalisé dans le cadre de cette thèse a pour objectif une caractérisation expérimentale fine de l’écoulement et de la dispersion turbulente autour d’un obstacle bidimensionnel placé dans une couche limite de surface, afin d’évaluer la validité des modèles RANS en vue de leur application pour l’étude de la dispersion atmosphérique.Dans un premier temps, nous avons utilisé des techniques d’anémométrie à fil chaud, d’anémométrie laser Doppler et d’anémométrie par image de particules, pour déterminer le champ de vitesse dans une couche limite de surface rugueuse et autour d’un obstacle bidimensionnel de section carrée. Une attention particulière a été portée sur l’analyse des termes de l’équation évolutive de l’énergie cinétique turbulente (ECT) et sur la détermination de la viscosité turbulente vt. Différentes approches ont également été utilisées pour estimer le taux de dissipation e de l’énergie cinétique turbulente. Nous avons mis en évidence que ces différentes approches fournissent des résultats comparables dans le cas de la couche limite, tandis que seule la technique estimant e comme le résidu de l’ECT est applicable dans le sillage de l’obstacle. De plus, nos mesures ont permis d’évaluer les paramétrisations du modèle k – e et de montrer que la valeur du coefficient Cμ = 0.09 ne semble pas adaptée dans le cas de la couche limite, conduisant à une surestimation de vt, alors que cette valeur apparait plus adaptée dans le cas de l’obstacle. Une étude de sensibilité, portant la détermination de la constante σk du modèle k – e, indique une contribution non négligeable des termes de corrélation entre la vitesse et la pression dans le sillage de l’obstacle.Dans un deuxième temps, nous avons étudié la dispersion d’un scalaire passif, en mesurant les différents moments statistiques de la concentration, au moyen d’un détecteur à ionisation de flamme. Nous avons également déterminé les flux turbulents de masse, par un couplage entre les mesures de vitesse et de concentration, en prenant soin de contrôler les influences réciproques des deux techniques de mesure. Ces mesures nous ont permis de tester la validité de différents modèles de fermeture de l’équation d’advection-diffusion pour estimer les flux dans le sens vertical et dans le sens longitudinal. Nous avons également pu déterminer expérimentalement le coefficient de diffusivité turbulente Dt, nous permettant d’évaluer un nombre de Schmidt turbulent Sct, afin de mettre en évidence que la valeur Sct = 0.7 est adaptée à la majorité des zones étudiées, excepté dans la zone de recirculation induite par l’obstacle. Enfin, nous nous sommes intéressés aux différents termes de l’équation de la variance de la concentration et plus particulièrement à son taux de dissipation. À nouveau, les mesures nous ont permis de tester un modèle de fermeture disponible dans la littérature et de montrer la bonne cohérence entre le modèle et l’expérience. / In the last decades, there has been an increasing use of Computational Fluid Dynamics (CFD)simulations to evaluate the impact of atmospheric pollutants dispersion in within industrial and urban sites. Given the high geometrical complexity of these sites, these simulations are mainly performed adopting a Reynolds Averaged Navier-Stokes (RANS) approach and using k−e closure models. As is well known from previous studies, RANS k−e simulations are affected by some limitations that prevent them correctly reproducing the dynamics of the flow and the pollutant dispersion in complex geometrical configurations. The aim of the PhD is to provide a detailed experimental characterization of the flow and the turbulent dispersion around an idealized two-dimensional obstacle placed within a boundary layer flow. This is subsequently used to analyse the reliability of RANS closure models as predictive tools for the atmospheric dispersion of airborne pollutants. Initially we focus on the flow dynamics of a boundary layer flow developing over a rough wall and in the wake of a 2D obstacle. The velocity field is investigated experimentally by means of different measurement techniques, namely Hot Wire Anemometry (HWA), Laser Doppler Anemometry (LDA) and Stereo-Particle Imagery Velocimetry (PIV). A particular attention was devoted to the estimate of the turbulent viscosity nt as well as of the terms composing the turbulent kinetic energy budget (TKE), including its rate of dissipation e which was determined adopting different approaches. These measurements allowed us to analyse the accuracy of the parameterizations included in a standard k−e closure model. Our analysis show that a value of the coefficient Cμ = 0.09 leads to significant overestimation of nt in a boundary layer flow. Conversely, adopting Cμ = 0.09 provides a good agreement between modeled and direct estimates of nt in the wake of the obstacle. As a second step, we studied the dispersion of a passive scalar emitted by a ground level line source. To that purpose we measured the first four order moments of the concentration probability density function by mean of a flame ionization detector (FID). Furthermore, the coupling of the FID system with the LDA or HWA system allowed us to directly measure the turbulent mass transfer, i.e. the correlation between velocity and concentration fluctuations. The combination of these two techniques was carefully analyzed, in order to quantify eventual mutual disturbances of one measurement technique on the other. The measurements of the velocity/concentration correlations allowed us to determine experimentally the turbulent diffusivity Dt and the turbulent Schmidt number Sct , and therefore to test the accuracy of different closure models of the advection-distribution equation. Our results show that the value of the turbulent Schmidt number is approximately equal to 0.7 in most of the domain, except in the recirculation zone on the wake of the obstacle. Experimental data provide also a complete description of the spatial distribution of the concentration variance, and of the term composing its budget (with a focus on its dissipation). As for the velocity field, we test the reliability of different closure model proposed in the literature of the turbulent mass transfer terms, enlightening the shortcomings of simple gradient-law closer models.