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.

Lorsqu’un corps flexible non-profilé est immergé dans un écoulement, les forces instationnaires associées au phénomène de détachement tourbillonnaire peuvent provoquer une réponse vibratoire de la structure. Ces vibrations induites par vortex (VIV) se produisent via un mécanisme de synchronisation entre l’instationnarité du sillage et le déplacement du corps, aussi appelé mécanisme de lock-in. Les VIV ont un effet néfaste sur beaucoup de systèmes industriels, mais elles peuvent également être utilisées comme convertisseur d’énergie mécanique pour l’extraction d’énergie. Dans ce travail, les VIV d’un cylindre circulaire rigide, monté sur un support flexible, sont étudiées dans différentes configurations sur la base de simulations numériques directes. (i) Les mécanismes d’interaction sont d’abord examinés au début du régime turbulent, où les VIV ont rarement été étudiées numériquement. Le nombre de Reynolds, basé sur le diamètre du cylindre et la vitesse incidente, est fixé à 3900. Une analyse conjointe des réponses structurelles et des forces fluides est réalisée sur un intervalle de vitesses réduites (inverse de la fréquence naturelle du corps), dans le cas où le cylindre est libre d’osciller dans les directions longitudinale et transverse, c.a.d les directions parallèle et perpendiculaire à l’écoulement incident. (ii) Le comportement du système lorsque le corps est libre d’osciller dans une direction seulement est également exploré, révélant les interactions possibles entre les déplacements longitudinal et transverse dans le cas à deux degrés de liberté (2-ddl) ; l’analyse montre par exemple comment des oscillations longitudinales de grandes amplitudes peuvent être induites par un déplacement transverse du corps. (iii) Le sillage tri-dimensionnel se développant en aval du cylindre est analysé dans le cas 2-ddl et dans le cas du corps fixe. Les structures d’écoulement dans la direction axiale (parallèle à l’axe du cylindre) sont analysées quantitativement à partir des amplitudes et longueurs d’onde des fluctuations de vorticité dans cette direction ; l’altération de ces structures lorsque le corps oscille diffère dans les couches cisaillées et plus en aval dans le sillage. (iv) La dernière configuration étudiée concerne un cylindre immergé dans un écoulement linéairement cisaillé dans la direction transverse. L’impact de la brisure de symétrie, induite par le cisaillement, sur le comportement du système fluide-structure, est exploré. Différents régimes d’interaction sont mis en évidence dans l’espace paramétrique défini par le taux de cisaillement et la vitesse réduite. Certains d’entre eux sont associés à une profonde reconfiguration du sillage et à une altération majeure des forces fluides. / When a flexible body with bluff cross-section is immersed in a flow, the unsteady fluid forces accompanying the vortex-shedding phenomenon may lead to structural vibrations. These vortexinduced vibrations (VIV) occur through a mechanism of synchronization between flow unsteadiness and body displacement, referred to as lock-in. VIV are detrimental to many industrial systems, but may also be used as mechanical energy converter in the context of flow energy harvesting. In the present work, the VIV of a rigid circular cylinder mounted on a elastic support are investigated in various configurations on the basis of direct numerical simulations. Four aspects are studied. (i) The interaction mechanisms are first examined in the early turbulent regime, where VIV have been rarely studied numerically. The Reynolds number, based on the cylinder diameter and oncoming flow velocity, is set to 3900. A combined analysis of the structural responses and fluid forcing in the case where the cylinder is free to oscillate in the in-line and cross-flow directions, i.e. the directions parallel and perpendicular to the oncoming flow, is performed over a range of the reduced velocity (inverse of the oscillator natural frequency). (ii) The system behavior when the body is free to oscillate in a single direction only is also explored, shedding some light on the possible interactions between in-line and cross-flow motions in the two-degree-of-freedom (2-dof) case; the analysis shows for instance how large-amplitude in-line oscillations may be induced by cross-flow motion. (iii) The three-dimensional wake developing downstream of the oscillating body is analyzed in the 2-dof case as well as in the fixed body case. The spanwise flow patterns, which are analyzed quantitatively in terms of wavelength and amplitude of vorticity fluctuations, are differently altered in the shear-layer and wake regions, when the body oscillates. (iv) The last physical configuration involves a cylinder immersed in a flow linearly sheared in the cross-flow direction. The impact of the symmetry breaking induced by the shear, on the flow-structure system behavior, is explored. Different interaction regimes are uncovered in the shear rate - reduced velocity domain. Some of them are associated with a profound reconfiguration of the wake and a major alteration of the fluid forces.

This dissertation presents results from a thorough study of the tip-gap turbulent flow structure in a low-speed linear compressor cascade wind tunnel at Virginia Tech that includes a moving belt system to simulate the relative motion between the tip and the casing. The endwall pressure measurements and the surface oil flow visualizations were made on a stationary endwall to obtain the flow features and to determine the measurement profiles of interest. A custom-made miniature 3-orthogonal-velocity-component fiber-optic laser-Doppler velocimetry (LDV) system was used to measure all three components of velocity within a 50 mm spherical measurement volume within the gap between the endwall and the blade tip, mainly for the stationary wall with 1.65% and 3.30% tip gaps as well as some initial experiments for the moving wall. Since all of the vorticity in a flow originates from the surfaces under the action of strong pressure gradient, it was very important to measure the nearest-wall flow on the endwall and around the blade tip. The surface skin friction velocity was measured by using viscous sublayer velocity profiles, which verified the presence of an intense lateral shear layer that was observed from surface oil flow visualizations. All second- and third-order turbulence quantities were measured to provide detailed data for any parallel CFD efforts. The most complete data sets were acquired for 1.65% and 3.30% tip gap/chord ratios in a low-speed linear compressor cascade. This study found that tip gap flows are complex pressure-driven, unsteady three-dimensional turbulent flows. The crossflow velocity normal to the blade chord is nearly uniform in the mid tip-gap and changes substantially from the pressure to suction side. The crossflow velocity relies on the local tip pressure loading that is different from the mid-span pressure loading because of tip leakage vortex influence. The tip gap flow is highly skewed three-dimensional flow throughout the full gap. Normalized circulation within the tip gap is independent of the gap size. The tip gap flow interacts with the primary flow, separates from the endwall, and rolls up on the suction side to form the tip leakage vortex. The tip leakage vortex is unsteady from the observation of the TKE transport vector and oil flow visualizations. The reattachment of tip separation vortex on the pressure side strongly depends on the blade thickness-to-gap height ratio after the origin of tip leakage vortex but is weakly related to it before the origin of tip leakage vortex for a moderate tip gap. Other than the nearest endwall and blade tip regions, the TKE does not vary much in tip gap. The tip leakage vortex produces high turbulence intensities. The tip gap flow correlations of streamwise and wall normal velocity fluctuations decrease significantly from the leading edge to the trailing edge of the blade due to flow skewing. The tip gap flow is a strongly anisotropic turbulent flow. Rapid distortion ideas can not apply to it. A turbulence model based on stress transport equations and experimental data is necessary to reflect the tip gap flow physics. For the moving endwall, relative motion skews the inner region flow and is decorrelated with the outer layer flow. Hence, the TKE and correlations of streamwise and wall normal velocity fluctuations decrease. / Ph. D.

Modeling the effects of surface roughness is an area of concern in many practical engineering applications. Many current roughness models to this point have involved the use of empirical 'constants' and equivalent sand grain roughness. These underdeveloped concepts have little direct relationship to realistic roughness and cannot predict accurately and consistently the flow characteristics for different roughness shapes. In order to aid in the development of turbulence models, the present research is centered around the experimental investigation of seven various shaped single roughness elements and their effects on turbulence quantities in a two-dimensional turbulent boundary layer. The elements under scrutiny are as follows: cone, cone with spatial variations equal to the smallest sublayer structure length scale, cone with spatial variations equal to 2.5 times the smallest sublayer structure length scale, Gaussian-shaped element, hemisphere, cube aligned perpendicular to the flow (cube at 90°), and a cube rotated 45° relative to the flow. The roughness element heights, k+, non-dimensionalized by the friction velocity (U_tau) of the approaching turbulent boundary layer, are 145, 145, 145, 145, 80, 98, and 98 respectively. Analysis of a three-dimensional fetch of the same Gaussian-shaped elements described previously was also undertaken. In order to analyze the complex flow fields, detailed measurements were obtained using a fine-measurement-volume (50 micron diameter) three-velocity component laser-Doppler velocimetry (LDV) system. The data reveals the formation of a horseshoe vortex in front of the element, which induces the downwash of higher momentum fluid toward the wall. This 'sweep' motion not only creates high Reynolds stresses (v^2, w^2, -uv) downstream of the element, but also leads to higher skin-friction drag. Triple products were also found to be very significant near the height of the element. These parameters are important in regards to the contribution of the production and diffusion of the turbulent kinetic energy in the flow. The 'peakiness' of the roughness element was found to have a direct correlation to the production of circulation, whereas the spatial smoothing does not have an immense effect on this parameter. The peaked elements were found to have a similar trend in the decay of circulation in the streamwise direction. These elements tend to show a decay proportional to (x/d)^-1.12, whereas the cube elements and the hemisphere do not have a common trend. A model equation is proposed for a drag correlation common to all roughness elements. This equation takes into account the viscous drag and pressure drag terms in the calculation of the actual drag due to the roughness elements presence in the boundary layer. The size, shape, frontal and wetted surface areas of the roughness elements are related to one another via this model equation. Flow drawings related to each element are presented which gives rise to a deeper understanding of the physics of the flow associated with each roughness element. / Master of Science