Hypolimnetic oxygenation systems, such as linear bubble-plume diffusers, are used to improve raw water quality. Linear bubble-plume diffusers were installed in Spring Hollow Reservoir (SHR) and Carvins Cove Reservoir (CCR). Diffusers induce mixing that aids distribution of oxygen throughout the hypolimnion. The induced mixing also creates an undesirable effect by increasing hypolimnetic oxygen demand (HOD). Nevertheless, oxygenation systems are commonly used and long-term oxygenation is hypothesized to actually decrease HOD. Increased oxygen concentrations in combination with the induced mixing affect the location of the oxic/anoxic boundary relative to the sediment water interface. If the oxic/anoxic boundary is pushed beneath the sediment/water interface, the concentrations of soluble iron and manganese in the bulk water are reduced. This work was performed to further validate a recently published bubble-plume model that predicts oxygen addition rates and the elevation in the reservoir where the majority of the oxygen is added. Also, the first field observations of a theoretically expected secondary plume are presented. Model predicted addition rates were compared to observed accumulation rates to evaluate HOD over a wide range of applied gas flow rates. Observations in both reservoirs showed evidence of horizontal spreading that correlated well with plume-model predictions and of vertical spreading below diffuser elevations, showing oxygen penetration into the sediment. Experimental observations of a theoretically expected secondary plume structure also correlated well with model predictions. Plume-induced mixing was shown to be a function of applied gas flow rates, and was observed to increase HOD. HOD was also observed to be independent of bulk hypolimnion oxygen concentration, indicating that the increase in oxygen concentration is not the cause of the increased HOD. Long-term oxygenation resulted in an overall decrease in background HOD as well as a decrease in induced HOD during diffuser operation. Elevated oxygen concentrations and mixing, which occur naturally during destratification and artificially during oxygenation, were observed to coincide with low dissolved metal concentrations in CCR. Movement of the oxic/anoxic boundary out of the sediment, which is also common during stratified periods, appears to facilitate transport of reduced Mn to the overlying waters. Hypolimnetic oxygenation increased oxygen concentrations throughout the hypolimnion, including down to the SWI, and induced mixing, although not to the extent observed during destratification. Subsequently, elevated Mn concentrations were observed to be restricted to the benthic waters located immediately over the sediments, while bulk (hypolimnion) water Mn concentrations remained low. The good agreement between the model and the experimental data show that the model can be used as a predictive tool when designing and operating bubble-plume diffusers. Linear bubble-plume diffusers provide sufficient horizontal and vertical spreading to enable oxygen to reach the sediments. Hypolimnetic oxygenation, despite the increased HOD, is a viable method to manage the negative consequences of hypolimnetic anoxia in water-supply reservoirs. / Ph. D.

Lakes and reservoirs exhibit a number of mixing and transport mechanisms. Understanding the transport is crucial to understanding and predicting constituent and density structures. Transport in waterbodies can be natural, such as seiche-induced boundary mixing or advectively-driven inflows. Hypolimnetic oxygenation using bubble-plumes also leads to enhanced mixing. Whether natural or plume-induced, increased mixing will alter the waterbody properties. Conversely, the density structure affects the behavior of plumes as well as inflowing and outflowing water. For example, stratification resulting from impounding a river can result in nutrient and suspended solids retention. Similarly, operation of plumes can induce mixing in the hypolimnion, resulting in warming, increased nutrient transport, and resuspension of settled particles. Modeling is extremely useful in determining the effects of dams on water quality constituents, enhanced transport, and the performance of mitigation techniques, such as hypolimnetic oxygenation. In this work, a variety of modeling techniques are used to evaluate natural and man-made mixing mechanisms. These include simple temperature and mass budgets, a two-dimensional lake model, and a two-phase plume model. A bubble-plume and plume-enhanced mixing was studied in Lake Hallwil. It was found that the plume-lake interaction was much more complex then previously expected, and knowledge of the seiche- and plume-enhanced near-field was necessary to accurately model the plume performance. A two-dimensional lake model was then coupled with a linear-plume model to accurately predict not only the plume performance, but also the plume-enhanced mixing in Spring Hollow Reservoir. The same two-dimensional lake model, used in conjunction with data analysis, demonstrated that the Iron Gate I Reservoir was not a significant sink for suspended solids, with only the large, adjacent side bay (Orsova Bay) thought to be the permanent sink. Furthermore, significant stratification did not develop, preventing substantial primary productivity. While the impoundment did change the water quality characteristics, the extent is much less than previously expected. The modeling methods presented here and the coupled plume-reservoir model should be useful tools for the design, modeling and greater understanding of bubble-plumes and other transport-related phenomena in lakes and reservoirs. / Ph. D.

When properly designed, hypolimnetic aeration and oxygenation systems can replenish dissolved oxygen in water bodies while preserving stratification. A comprehensive literature review of design methods for the three primary devices was completed. Using fundamental principles, a discrete-bubble model was first developed to predict plume dynamics and gas transfer for a circular bubble-plume diffuser. This approach has subsequently been validated in a large vertical tank and applied successfully at full-scale to an airlift aerator as well as to both circular and linear bubble-plume diffusers. The unified suite of models, all based on simple discrete-bubble dynamics, represents the current state-of-the-art for designing systems to add oxygen to stratified lakes and reservoirs. An existing linear bubble plume model was improved, and data collected from a full-scale diffuser installed in Spring Hollow Reservoir, Virginia (U.S.A.) were used to validate the model. The depth of maximum plume rise was simulated well for two of the three diffuser tests. Temperature predictions deviated from measured profiles near the maximum plume rise height, but predicted dissolved oxygen profiles compared very well to observations. Oxygen transfer within the hypolimnion was independent of all parameters except initial bubble radius. The results of this work suggest that plume dynamics and oxygen transfer can successfully be predicted for linear bubble plumes using the discrete-bubble approach. To model the complex interaction between a bubble plume used for hypolimnetic oxygenation and the ambient water body, a model for a linear bubble plume was coupled to two reservoir models, CE-QUAL-W2 (W2) and Si3D. In simulations with a rectangular basin, predicted oxygen addition was directly proportional to the update frequency of the plume model. W2 calculated less oxygen input to the basin than Si3D and significantly less mixing within the hypolimnion. The coupled models were then applied to a simplified test of a full-scale linear diffuser. Both the W2 and Si3D coupled models predicted bulk hypolimnetic DO concentrations well. Warming within the hypolimnion was overestimated by both models, but more so by W2. The lower vertical resolution of the reservoir grid in W2 caused the plume rise height to be over-predicted, enhancing erosion of the thermocline. / Ph. D.

[pt] Este trabalho apresenta uma investigação experimental do escoamento induzido pela interação entre uma pluma de bolhas e um escoamento cruzado oscilatório. Condições de escoamento similares podem ser encontrados em processos de aeração artificial utilizados na mitigação da poluição nos rios e na representação de vazamento de gás natural no fundo dos oceanos. No presente trabalho, ondas superficiais controladas foram inseridas em um canal de água para gerar oscilações na corrente do escoamento cruzado. As ondas foram geradas a partir de uma placa móvel na superfície da água e determinadas condições de escoamento instável foram selecionadas para a investigação. O ar foi injetado pelo fundo do canal para formar a pluma de bolhas. A técnica Particle Image Velocimetry (PIV) foi empregada para medir a velocidade do escoamento. Antes da estimativa da velocidade, as imagens foram pré-processadas aplicando-se rotinas desenvolvidas no Matlab a fim de distinguir as partículas traçadoras das bolhas de ar e criar máscaras dinâmicas para as imagens do sistema PIV. Assim, o campo vetorial de velocidade foi estimado utilizando algoritmos padrão do PIV. Além disso, as propriedades das bolhas, como tamanho e velocidade, também foram estimadas a partir das imagens adquiridas. Finalmente, foi analisada a interação entre a pluma de bolhas com o escoamento cruzado instável. / [en] This work presents an experimental investigation of the flow field induced by the interaction between a bubble plume and an oscillating cross flow. Similar flow conditions can be found in artificial aeration processes used for mitigation of pollution contamination in rivers and submarine outfalls in coastal areas. The mixing zone is highly dependent of the flow field near the plume hence the efficiency of aeration processes. In the present work, controlled surface waves were introduced to generate oscillations in streamwise and wall normal components of the cross flow. The waves were excited with a moving paddle and unsteady flow conditions were selected for the investigation. Air was injected in the bottom wall of the water channel to form the bubble plume. Particle Image Velocimetry (PIV) techniques were employed to measure the velocity flow field. Prior to velocity estimation, images were pre-processed using Matlab routines in order to distinguish tracer particles from air bubbles and to create a dynamic mask for the PIV images. Thus, the velocity vector field was estimated using standard PIV algorithms. In addition, properties of the bubbles, such as size and velocity, were also estimated from the acquired images. Finally, the interaction between the bubble plume with the unsteady cross flow was analyzed.

This thesis presents an experimental study which investigates the behaviour of gas bubbles rising in a liquid metal and the related bubble-driven flow under the influence of external DC magnetic fields. The experimental configuration considered here concerns a cylindrical container filled with the eutectic alloy GaInSn. Argon gas bubbles are injected through a single orifice located at the container bottom in the centre of the circular cross-section. A homogeneous magnetic field was generated by a Helmholtz configuration of a pair of water-cooled copper coils. The magnetic field has been imposed either in vertical direction parallel to the main bubble motion or in horizontal direction, respectively. A vertical magnetic field stabilizes and damps the liquid metal flow effectively. The temporal variations of the fluid velocity with time become smaller with increasing magnetic induction. The velocity magnitudes are decreased, and the velocity distributions along the magnetic field lines are smoothed. The flow field keeps the axisymmetric distribution. A horizontal magnetic field destabilizes and enhances the flow within a range of moderate Hartmann numbers (100 < Ha < 400). The flow becomes non-axisymmetric due to the non-isotropic influence of the magnetic field. In the meridional plane parallel to the field lines, the flow changes its direction from a downward to an upward motion. Enhanced downward flows were observed in the meridional plane perpendicular to the field lines. The liquid velocity in both planes shows strong, periodic oscillations. The fluid motion is dominated by large-scale structures elongated along the magnetic field lines over the entire chord lengths of the circular cross-section.

This thesis presents an experimental study which investigates the behaviour of gas bubbles rising in a liquid metal and the related bubble-driven flow under the influence of external DC magnetic fields. The experimental configuration considered here concerns a cylindrical container filled with the eutectic alloy GaInSn. Argon gas bubbles are injected through a single orifice located at the container bottom in the centre of the circular cross-section. A homogeneous magnetic field was generated by a Helmholtz configuration of a pair of water-cooled copper coils. The magnetic field has been imposed either in vertical direction parallel to the main bubble motion or in horizontal direction, respectively. A vertical magnetic field stabilizes and damps the liquid metal flow effectively. The temporal variations of the fluid velocity with time become smaller with increasing magnetic induction. The velocity magnitudes are decreased, and the velocity distributions along the magnetic field lines are smoothed. The flow field keeps the axisymmetric distribution. A horizontal magnetic field destabilizes and enhances the flow within a range of moderate Hartmann numbers (100 < Ha < 400). The flow becomes non-axisymmetric due to the non-isotropic influence of the magnetic field. In the meridional plane parallel to the field lines, the flow changes its direction from a downward to an upward motion. Enhanced downward flows were observed in the meridional plane perpendicular to the field lines. The liquid velocity in both planes shows strong, periodic oscillations. The fluid motion is dominated by large-scale structures elongated along the magnetic field lines over the entire chord lengths of the circular cross-section.

The phenomenon of gas entrainment, as a result of impinging liquid jets, was experimentally investigated. The purpose of these investigations was to create a solid experimental database necessary for the development and validation of computational fluid dynamics (CFD) codes. In this work, various experimental setups were built to allow employing various imaging measurement techniques with high spatial and temporal resolution. High-speed imaging was applied for characterizing the flow structure that develops under the free surface. It was found that gas entrainment takes place as soon as the jet impact velocity overcomes the value of 1.2 m/s. The bubble plume, formed as a result of impingement, consists of two distinct regions: an inner region with high turbulence and fine freely dispersed bubbles and an outer region, where larger bubbles move towards the free surface. Two mechanisms are responsible for the occurrence of gas entrainment. High-speed camera observations were validated by means of ultrafast x-ray computed tomography, an innovative non-intrusive measurement technique. Also, quantitative information regarding the bubble plume was acquired from the high-speed observations, in terms of: penetration depth, width and spreading angle of the bubble plume. Measurements, based on two wire-mesh sensors, were performed to assess the gas entrainment rate. In these measurements, void fraction distributions and gas velocities were quantified. The entrainment rate was calculated as an integral over the entrained volumetric gas fraction. It was found to be a function of the jet velocity and length. Results were validated using dual-plane x-ray computed tomography. Results were in agreement with the ones obtained from the wire-mesh sensors and approximately four to six times smaller than predictions found in related publications. Instantaneous as well as time-averaged velocity fields of the continuous phase were gained by means of particle image velocimetry (PIV). Axial time-averaged velocities followed a power law profile, typical for fully developed flow conditions. Two recirculating vortices were found in the flow: one occurs as a result of the water adhering to the lateral wall of the tank and the flow being confined by the bottom wall, while the second one is generated in the wake of rising bubbles. Bubble entrainment was found to reduce liquid phase mean velocities and to enhance fluctuations in the streamwise direction. This is reflected in the distribution of the turbulence kinetic energy. Last but not least, several examples of comparisons between experimental data and CFD results stand to demonstrate the importance of the experimental observations gathered in the frame of this work. It is shown that the experimental data provides a good basis not only for qualitative comparisons, but also for quantitative correlations.