A new buoyant jet model is presented in this thesis to simulate the trajectory and dilution of a fluid from a single port or line source. The new features include: A generalised derivation of the governing equations so that buoyant jets discharged from a source of any shape can be modelled within the one framework, and the effects of high-frequency internal waves on the motion of the buoyant jet. Past buoyant jet models were constructed for specific cases and their application is necessarily restricted. In this thesis, a new model is developed in a Lagrangian framework that can be applied to buoyant jet discharges at any angle into ambient waters that may be stratified or unstratified, flowing or stagnant. The model is validated using both laboratory and field data. Furthermore, the model is applicable to the continuous discharge of a buoyant jet from line, axisymmetric or elliptic sources and to the instantaneous discharge of a spherical puff. No previously published model is capable of unifying and solving all of these problems within the one framework. Transforming the governing equations to their non-dimensional form shows that the trajectory and dilution of discharges from line or axisymmetric sources or of spherical puffs into a flowing, stratified ambient environment are uniquely specified using three parameters. These are: the non-dimensional size of the outlet port, the relative importance of the initial fluxes of momentum and buoyancy, and the number of orthogonal planes through which entrainment can occur. This is a significant advance in the understanding of the processes affecting buoyant jets. When high-frequency internal waves are present in the receiving waters they can have significant effects on the buoyant jet. These effects are incorporated into the present model. Using data obtained from an experiment conducted off Sydney the effects of internal waves on the height of rise and dilution of the buoyant jet were found to exceed a factor of two. Consequently, it is important that the effects of internal waves (when present) be incorporated into any buoyant jet model.

In the event of a small break loss of coolant accident (SBLOCA) in a nuclear reactor, cold fluid is injected through the reactor system high pressure injector to compensate for the coolant loss. When this flow rate is less than a critical value, however, the hot fluid in the cold leg penetrates into the vertical injection line in a process called buoyant backflow. Because the resulting penetrations induce thermal stresses in the pipe, the presence of backflow in the injection lines is potentially significant. Since these penetrations could potentially damage the pipe, it was the purpose of this study to evaluate the backflow behavior. To this end, both the critical injection conditions and the subcritical penetration depth were experimentally determined through flow simulation in a 1/5 scale model. In addition, the experimental trends wi-re modeled theoretically. By matching the theoretical results to the experimental data, it was determined that backflow began below a critical Froude number of .65 and increased in depth with the negative logarithm of the injection velocity. The agreement between theory and experiment was excellent. For a certain class of reactor systems, the full scale Froude numbers were then compared to the critical value obtained in the analysis. For the systems involved in this comparison, the full scale Froude numbers were shown to be less than .65 for all practical flow rates. As a consequence, buoyant backflow is expected within the injection lines of these reactors, under safety injection conditions. / Graduation date: 1991

The buoyancy (or density = mass volume-1) of marine larval fish is influenced by their nutritional status, starved larvae being less dense than fed larvae of like age. Buoyancy has, therefore, been proposed as an indicator by which to assess the nutritional condition of marine fish larvae in experiments and in situ. It has also been hypothesized that larval fish experiencing advanced starvation will exhibit increased density due to water loss resulting from osmoregulatory breach and failure. The magnitude of osmoregulation-related changes in density has not been examined and its influence on interpretations of nutritional condition and vertical distributions are unknown. Through a series of controlled laboratory experiments performed on larvae of cod (Gadus morhua L.), I developed evidence that osmoregulatory breach and failure has a strong positive effect on the density of larval fish, and that this effect on density is not limited to larvae experiencing advanced stages of starvation. These effects of osmotic breach and failure are sufficient to obscure effects resulting exclusively from nutritionally caused changes in density. Hence, if not identified and controlled these osmotically driven density changes can bias parameterization of buoyancy-related condition indices. I developed methods for isolating nutritional and osmotic effects. This facilitated the calibration of a nutritionally based density index with which to evaluate the nutritional state of cod larvae. Comparison of the calibrated density assay with a suite of widely used indices of condition (morphometric, behavioural and biochemical) showed density to be superior in its capacity to correctly classify larvae with respect to their nutritional state. The density index was also less subject to bias resulting from uncontrolled changes in larval size. A linear regression model based on these findings was developed to assess the relationship between density as an index of condition and near-term larval

Extensive experimental data is available from previous research into the behaviour of buoyant jets released into an unstratified ambient. The experimental data has been the basis for theoretical and numerical modelling work, and currently several numerical models exist that are employed in the design of engineering structures built for the disposal of wastewater in the ocean. However there are still flow configurations with limited or no available experimental data, and hence confidence in the use of the models under some circumstances is limited. These circumstances include two-dimensional trajectory flows that are discharged at oblique angles to the ambient and buoyant jet flows with three-dimensional trajectories. As part of the current project an experimental investigation is conducted into the behaviour of discharges that have either two-dimensional or three-dimensional trajectories, focussing particularly on those configurations with currently limited available experimental data. A light attenuation technique is developed for the investigation of such flows, largely because it enables the behaviour of discharges with three-dimensional trajectories to be recorded with relative ease. However, this technique provides integrated views of the flow and hence the interpretation of the integrated concentration data is aided by assumed mean cross-sectional concentration profiles. In the strongly advected region (with the exception of the weak-jet) a double-Gaussian approximation is shown to provide a reasonable representation of mean concentration profiles. In the weakly advected regions and the weak-jet region, it is well- known that a single Gaussian adequately represents the mean flow structure. A new numerical model, the Momentum Model, is developed to assist in the design and to monitor the performance of the experimental investigation. Unlike other models, the behaviour of the flow is determined by the relative magnitudes of the initial excess momentum flux, the buoyancy-generated momentum flux and the entrained ambient momentum flux. It is shown that ratios of these momentum fluxes are equivalent to the length-scales traditionally employed for this task. Predictions from the Momentum Model are compared with data from the current and previous experimental investigations and, in addition, predictions from two representative numerical models, VisJet and CorJet. Predictions from the Momentum Model are shown to be consistent with data for a wide variety of discharge configurations. These predictions are also generally consistent with those of VisJet and CorJet. However, the experimental results from the II buoyant jet discharged in a moving ambient show that the spreading rates of the strongly advected flows (puffs and thermals) differ, and while this difference is incorporated into the Momentum Model, it is not evident in the VisJet and CorJet predictions. Numerical model predictions of negatively buoyant discharges are shown to be inadequate. This discharge configuration is investigated in some detail experimentally and additional analytical solutions of the flow behaviour are developed to aid in the interpretation of the flow behaviour. The experimental results show that buoyancy-induced instabilities on the inner side of the jets, which generate additional vertical mixing, significantly alter the form of the mean concentration profiles in this region. This results in considerably higher integrated dilutions along the flow centreline. Another significant difference between the newly developed Momentum Model and the existing numerical models (VisJet and CorJet), is the approach taken to dealing with oblique discharges in a cross-flow. Experimental results in combination with additional analytical solutions show that for initial discharge angles of 20° and less, an oblique discharge in a cross-flow becomes a weak-jet in the strongly advected region, and for angles of 40° and above, the flow becomes a puff. The strongly advected behaviour predicted by the Momentum Model changes abruptly at the transition angle, and is reasonably consistent with the data. The gradual change in strongly advected behaviour employed by VisJet and CorJet does not appear to be appropriate in the puff region. Finally a preliminary experimental investigation of discharges with three-dimensional trajectories shows that there are significant discrepancies between the predicted behaviour and the experimental data. This is surprising given the numerical models are, for the most part, able to predict the behaviour of flows with two-dimensional paths with reasonable accuracy. It is evident that flows with three-dimensional paths are modified more severely by the different directions of the initial, buoyancy-generated, and entrained ambient momentum fluxes than the current models suggest.

Thesis (M.S.M.E.)--University of Delaware, 2005. / Principal faculty advisor: Ajay K. Prasad, Dept. of Mechanical Engineering. Includes bibliographical references.

In the present thesis turbulent natural convection of air within different challenging test cases are investigated numerically by means of an unstructured finite volume code, Code_Saturne. First, flow within both two-dimensional vertical and inclined differentially heated rectangular cavities at 60° and 15° to the horizontal for an aspect ratio of H/L=28.6 and Rayleigh number of 0.86×10e6 is computed using several high and low-Re models. Here the effectiveness of the RANS models in Code_Saturne is assessed through comparisons with a range of available experimental data. After some tests of thermal field inside vertical cavity, the “two-velocity-scale wall function” is chosen to be used with high-Re models. In both vertical and inclined cases the overall flow pattern appears similar, with a single circulation cell, and a boundary layer at the wall. The levels of turbulence energy are generally slightly lower in the inclined case. Most models give a reasonable prediction of measured Nusselt number, with the two low-Re approaches generally being closer to the data than the schemes employing wall functions. For the 15° inclined cavity, a multi cellular motion is shown by the high-Re models. Nevertheless, all the model predictions disagree with experimental data due to the presence in real flow of 3-D unsteady structures as found in Benard convection problems. These cannot, definitely, be reproduced using a 2-D geometry. Both highly resolved LES and unsteady RANS computations are then conducted, for turbulent natural convection of air inside 15° unstably and stably stratified cavities. In accordance with recent experimental data, the LES computations for both enclosures returned three-dimensional time-averaged flow fields. In the case of the unstably stratified enclosure, the flow is highly unsteady with coherent turbulent structures in the core of the enclosure. Results of LES computations show close agreement with the measured data. Subsequent comparisons of different URANS schemes with the present LES are used in order to explore to what extent these models are able to reproduce the large-scale unsteady flow structures. All URANS schemes have been found to be able to reproduce the 3-D unsteady flow features present in the 15° unstable cavity. However, the low-Re model tested as well as requiring a high resolution near-wall grid, also needed a finer grid in the core region than the high-Re models, thus making it computationally very expensive. Flow within the 15° stable cavity also shows some 3-D features, although it is significantly less unsteady, and the URANS models tested here have been less successful in reproducing this flow pattern. Finally, natural convection of CO2 inside a horizontal annular penetration enclosure, which can be found in AGR's, has been performed using a highly resolved LES and a set of RANS models. The Rayleigh number is 1.5×10e9. RANS models agree with the present LES on the fact that the flow is unsteady and there are large-scale oscillations present which decrease in amplitude as one moves from the open towards the closed end of the annular enclosure. Overall heat transfer and thermal quantitative and dynamic results show that RANS schemes are in close agreement with the current LES data except some discrepancies shown by the high-Re model which can be returned to the limitation of the simple wall function used to predict such complex flow.

Treated and untreated liquid that is discharged from industrial and desalination plants is one of the main factors that break the ecological balance and destroys aquatic habitat in lakes, rivers and coastal areas where the effluent is discharged. Positively and negatively buoyant jets are two categories of outfalls which are generated because of the destiny difference between the effluent and ambient fluid. In order to ensure minimal impact of the effluent on the environment, it is necessary to estimate the dilution of the jet and compare it with environmental regulations on the level of required dilution to ensure that the concentration of the effluent is diluted quickly enough and the concentration of the effluent at different points does not exceed the allowed concentrations. This study investigated the positively buoyant jet, which happens near the coastal and near water area. For instance, cooling water that flows out from a power plant or factory, wasted water that is discharged from an industrial plant near river, submerged drainage from civil municipal sewer systems and treated water from desalination plant in coastal area. Density difference, velocity and inclined angle of the jet were considered as the main factors that contribute to the jet spreading and were compared to develop the best solution for its dilution. The jet was discharged inclined downward to allow for more mixing and dilution of the effluent with the ambient water. In order to simulate a positive jet, tap water was injected in saline ambient. A large number of experiments were conducted in the laboratory and using camera imaging. The jet trajectory was estimated from the images using image processing and the impact of various parameters such as Froude number and jet velocity were investigated. The opensource software OpenFOAM, was employed for numerical simulations which is a finite volume model ensures mass conservation and allows for flexible mesh size for further accuracy and optimization of computational cost. Using this Computational Fluid Dynamics (CFD) model, the numerical simulations were performed, and the results were compared with laboratory experiments. A Reynold-Averaged Navier-Stokes (RANS) approach was employed in the numerical simulations which offers a good balance between accuracy and computational cost. It was found that the numerical model in conjunction with the second order turbulence model called Launder-Reece- Rodi model (LRR) had a reasonable agreement with the experimental data.

>Magister Scientiae - MSc / In this thesis, I explore the subjects of fluid dynamics and the Hot-Wire Chemical Vapour Deposition (HWCVD) process. HWCVD, in its simplicity, is one of the more powerful and elegant deposition techniques available in thin film research which allows for both the growth and post deposition treatments of functional thin films. In the HWCVD process, the quality of the final films is determined by a fixed set of deposition parameters namely: temperature, pressure and the gas flow rate. Finding the optimal combination of these parameters is key to obtaining the desired film specifications during every deposition. Conducting multiple trial experiments to determine said parameters can be expensive and time consuming, this is where simulation methods come into play. One such simulation method is Computational Fluid Dynamics (CFD) modelling

Computational fluid dynamics (CFD) is commonly used to visualize and understand complicated fluid flow and heat transfer in many industries. It is imperative to validate the CFD computer models in order to avoid costly design choices where experimentation cannot be used to ratify the predictions of computer models. Assessments of CFD computer models in the literature conclude that significant errors occur in computer model predictions of fluid flow influenced by buoyancy forces. The Experimental Fluid Dynamics Laboratory at Utah State University constructed a wind tunnel with which to perform experiments on buoyancy induced fluid flow. The experiments measured the heat transfer and fluid velocity occurring in the buoyant flows to be used to validate computer models. Additional experimental measurements at the inlet and around the walls from each experiment allowed the computer models to simulate the fluid flow with realistic boundary conditions.For this study, four experiments were performed, including two cases where the buoy- ancy influence was significant, and two where it was not. For each set of two cases, one experiment was performed where the heat transfer occurred from a wall of the wind tunnel held at constant temperature and in the other experiment the wall temperature fluctuated axially. This study used the experimental data to validate computer models available in the general purpose CFD software STAR-CCM+, including the k − ε models: realizable two- layer, standard two-layer, standard low-Re, v2 − f, the k − ω models from Wilcox and Menter, and the Reynolds stress transport and Spalart–Allmaras models. The k − ε stan- dard low-Re model was found most capable overall of predicting the fluid flow and heat transfer that occurred in the flows where the buoyancy influence was significant. For the experimental cases where the buoyancy influence was less significant, the validation results were inconsistent.

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