Browsing by Author "Venayagamoorthy, Subhas Karan, advisor"
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Item Open Access Dynamics and structure of stably stratified turbulence(Colorado State University. Libraries, 2012) Schaad, Simon, author; Venayagamoorthy, Subhas Karan, advisor; Julien, Pierre Y., committee member; Dasi, Lakshmi Prasad, committee memberThe dynamics and turbulent structures of stably stratified turbulence are explored via direct numerical simulations (DNS). The structural features of stratified turbulence and its relationship to the flow dynamics has been the subject of many recent investigations. In strongly stratified turbulent flows, the formation of large-scale quasi-horizontal vortices in layers with strong vertical variability has been observed in laboratory experiments. Enstrophy isosurfaces of strongly stable flows indicate the emergence of randomly distributed 'pancake'-like structures with near horizontal orientation at later times. The strongly stratified simulations are diffusive and dominated by linear internal waves. The results suggest a decoupling between horizontal and vertical dynamics as the vertical dynamics can be described using rapid-distortion theory (RDT) while horizontal dynamics continue to be dominated by non-linear effects not captured by RDT. The integral flux Richardson number for decaying turbulence is the ratio of background potential energy gain to turbulent kinetic energy loss. The traditional flux-based formulation converges upon this ratio only when integrations are performed over an entire event, while the irreversible formulation converges rapidly without error from reversible effects. Mixing efficiency is a property of the flow for energetic flow but becomes a property of the fluid for diffusive flows and subject to Prandtl number effects. RDT models predict the flux Richardson number scales as the inverse Prandtl number at the diffusive limit when the Prandtl number is greater than unity. Mixing efficiency comparisons between DNS and physical grid-tow experiments reveal a large discrepancy for strong stratification, which is attributed in part to the low Reynolds numbers attained in both DNS and grid-tow experiments. Overturns are unstable conditions where heavier fluid resides above lighter fluid. The collapse of these local instabilities produce additional patches of turbulence and mixing making overturns an important mechanism in stratified turbulence. The overturning structures in strongly stratified flow resemble the quasi-horizontal vorticity structures and were found to be correlated with increased horizontal vorticity. The Thorpe scale, a measure of overturning structure height, and the Ozmidov scale equate only at the critical condition where inertial and buoyancy effects are equal (i.e. the turbulent Froude number is unity). The error of inferred dissipation rates from equating the Thorpe and Ozmidov scales was found to be up to two orders of magnitude.Item Open Access Energetics and dynamics of flow through baffle drop shafts using physical and computational model studies(Colorado State University. Libraries, 2023) Aluthwalage, Kasun Prabodha Sahabandu, author; Venayagamoorthy, Subhas Karan, advisor; Loc, Ho Huu, advisor; Nelson, Peter, committee member; Windom, Bret, committee memberA drop shaft is one of the main hydraulic structures that is used to convey water from higher to lower elevations while dissipating potential energy in storm water management systems, water treatment plants, and hydropower stations. Drop shafts need to be adjusted for higher discharges because of the increased urban flooding due to climate change and rapid urbanization. Traditional baffle drop shafts have limited flow capacity and are unstable due to their asymmetric nature. The novel baffle drop shaft is proposed here for larger range of flow discharges. To the author's knowledge, there are no previous studies that have thoroughly investigated the energy dissipation potential of the novel baffle drop shaft. Hence, there is a need to establish a design relationship between key parameters such as the shaft diameter, baffle spacing, and discharge to inform best design practices. A 1:10 physical model study was carried out to investigate the energy dissipation of a novel baffle drop shaft using different discharges. Pressure and velocity were measured at two locations on the baffles using low range pressure sensors (100 mbar) and an electromagnetic velocity meter. Timed averaged pressure and velocity on the baffles increased with discharge. These averaged quantities were considered to calculate global and local energy dissipation through the shaft. The global energy dissipation efficiency was calculated based on the inlet and outlet channel flow data, and was found to range from 89.6% to 91.9%. The flow regime profiles were quite similar on each baffle section of the shaft; hence, we can consider the energy dissipation in each baffle to be equivalent. Under free-flow conditions, the energy dissipation efficiency decreases as the discharge increases. Physical models are costly and time-consuming for performing parametric studies of flowthrough such structures because each and every geometric configuration needs to be constructed in the lab. Computational Fluid Dynamics (CFD) is a more feasible option to conduct an in-depth investigation of the energetics and dynamics of flow in a baffle drop shaft since it is faster and more cost-effective than a physical model study. The CFD models have been built to simulate the hydraulic behavior of baffle drop shafts using OpenFOAM. This software is adaptable for modeling diverse flow issues due to the variety of models and numerical techniques that it incorporates. A suitable turbulence model that is commonly used in CFD for modeling turbulent flows such as in drop shafts is the RANS-based realizable k- ϵ model. Mesh sensitivity analysis was also performed to establish grid independences of the solution. Benchmark geometry CFD models were calibrated using four locations in the physical model, and velocity and pressure measurements at the edge of the baffle were used for validation with remarkable agreement. A parametric study was conducted using shaft diameters (D) of 0.8 m, 0.9 m, and 1 m, six baffle spacings (h) ranging from 0.23m to 0.48 m, and baffle rotating angles (θ) of 180◦, 250◦, and 270◦. Global energy dissipation efficiency (η) ranged from 92% to 97%. The η value decreased with discharge but was higher under free flow conditions in the baffle drop shaft. The geometric parameters D, h, and θ have little influence on energy dissipation. Considering structural integrity, available space, construction costs, and maintenance costs, the baffle drop shaft needs to be optimized to achieve the desired hydraulic performance. Maximum pressure was observed at the water jet impact location close to the outer shaft wall. Air entrainment is also a significant consideration in designing baffle drop shafts because its impact is critical in applications like hydro power generation. The bulking of the flow due to air entrainment needs to be considered to evaluate the maximum flow carrying capacity of baffle drop shafts. In summary, designing baffle drop shafts requires a multi-criteria approach that is mainly dependent on the design requirements on energy dissipation, structural integrity, construction costs, air entrainment, application, and location.Item Open Access Flow dynamics and scalar transport in drinking water contact tanks(Colorado State University. Libraries, 2013) Barnett, Taylor C., author; Venayagamoorthy, Subhas Karan, advisor; Julien, Pierre Y., committee member; Sakurai, Hiroshi, committee memberThe research and studies presented in this thesis focus on ways to improve the internal hydraulics of chlorine contact tanks used in drinking water disinfection systems. This was accomplished through the use of computational fluid dynamics (CFD) and physical tracer studies of a number of different systems. Three primary tank modifications were investigated in these studies: internal baffling; inlet modifications; and random packing material. The findings from these studies were then applied in a case study of the Jamestown chlorine contact tank. All of the studies presented in this thesis use the baffle factor (BF) designation as defined by the United States Environmental Protection Agency as the primary indicator of a system's disinfection efficiency. The CFD models used for the internal baffling study were first validated using a laboratory scale study of the Embsay chlorine contact tank in Yorkshire, England. This tank footprint was then modified to replicate a precast concrete tank that was installed in the Hydraulics Laboratory located at the Engineering Research Center (ERC) at Colorado State University. This concrete tank was used as the footprint for a parametric study in which the number and length of internal baffles were modeled in various configurations. The internal hydraulics of this baffle tank were optimized using only two dimensionless relationships namely: the baffle opening ratio L* and the baffle opening to channel width ratio Lbo/Wch. The resulting tank geometry from these two relationships yielded a BF of 0.80 and also maximized the length to width ratio of each channel within the concrete tank. The inlet modification study was performed to investigate how the BF of a 400-gallon doorway storage tank could be improved. Three different inlet types with two inlet sizes were modeled and simulated for six different flow rates. Three of these CFD simulations were then physically tested using both saline and lithium tracers to validate the computer models. Key findings from this study show that the size of the inlet and its orientation play a dominant role in the internal hydraulics of the system. For the random packing material study, three different packing material sizes, two tank sizes, and two different flow rates were tested. CFD models were not feasible due to the randomness of how the packing material would settle in these contact tanks. Over 64 saline tracer studies and 6 lithium tracer studies were conducted to complete this study. Key findings show that the initial BF of the system and the volume of the tank filled with the packing material were the dominant variables in the study. The tank size, flow rate, and packing material size had little to no impact on the performance. The Jamestown case study presented in this thesis used findings from both the internal baffle study and the inlet modification study. The BF of the contact tank would fluctuate annually between 0.52 and 0.63 due to a shift in flow regimes caused by a change in the system's flow rate. This turbulent to laminar flow regime change was validated with the use of CFD models coupled with physical tracer studies. Several inlet modifications were investigated using CFD to determine what modifications, if any, the plant operators should implement. Key findings from the CFD models showed that with the proper inlet modification, the BF of the system could be stabilized at 0.63 during both the high flow summer months and low flow winter months.Item Open Access Internal hydraulics of baffled disinfection contact tanks using computational fluid dynamics(Colorado State University. Libraries, 2010) Xu, Qing, author; Venayagamoorthy, Subhas Karan, advisor; Grigg, Neil S., advisor; Gilkey, David P., committee memberThe present study focuses on understanding the internal hydraulics of baffled disinfection contact tanks for small drinking water systems using computational fluid dynamics (CFD). The emphasis of this study is to improve the hydraulic efficiency of disinfection contact tanks. In particular, the answer to the following key question was sought: for a given footprint of a contact tank, how does the hydraulic efficiency of the tank depend on the number and geometry of internal baffles? In an effort to address this question, high resolution two-dimensional (planar) simulations were performed to quantify the efficiency of a laboratory scale tank as a function of the number of baffles. Simulation results of the velocity field highlight dead (stagnant) zones in the tank that occur due to flow separation around the baffles. Simulated longitudinal velocity profiles show good agreement with previous experimental results. Analysis of residence time distribution (RTD) curves obtained for different number of baffles for a given footprint of a tank indicate that there may be an optimum number of baffles for which near plug flow conditions is maximized. This study highlights the increasing role and value of CFD in improving hydraulic design characteristics of water engineering structures. As a precursor to the CFD study, a focused literature review of disinfection systems was done to highlight the basic technologies and related applications. The review presented in this thesis summarizes details of small water treatment plants, disinfection and CT (where C is the concentration of disinfectant at the outlet of the disinfection system, and T is the time taken for the fluid to leave the system.) method, traditional tracer studies, tank design, and the development of numerical simulations. Following the review, the CFD model used for this investigation was validated using results from a previous case study of a large-scale water treatment plant in Canada. This initial CFD study is also used to highlight the uses and abuses of CFD in flow modeling and emphasize the importance of having adequate validation studies to complement the CFD work.Item Open Access Mixing in stably stratified turbulent flows: improved parameterizations of diapycnal mixing in oceanic flows(Colorado State University. Libraries, 2018) Garanaik, Amrapalli, author; Venayagamoorthy, Subhas Karan, advisor; Bienkiewicz, Bogusz, committee member; Barnes, Elizabeth, committee member; Julien, Pierre Y., committee memberMixing of fluid with different properties across a gravitationally stable density interface, due to background turbulence is an ubiquitous phenomenon in both natural and engineered flows. Fundamental understanding and quantitative prediction of turbulent mixing in stratified flows is a challenging problem, with a broad range of applications including (but not limited to) prediction of climate, ocean thermohaline circulation, global heat and mass budget, pollutant and nutrients transport, etc. Large scale geophysical flows such as in the ocean and atmosphere are usually stably stratified i.e. the density increases in the direction of gravitational force. The stabilizing nature of the density layers has a tendency to inhibit the vertical motion. In such flows, diapycnal mixing, i.e. mixing of fluid across the isopycnal surfaces of constant density, plays a crucial role in the flow dynamics. In numerical models of large scale flows, turbulent mixing is inherently a small scale phenomenon that is difficult to resolve and is therefore generally parameterized using known bulk parameters of the flow. In oceans, the mixing of water masses is typically represented through a turbulent (eddy) diffusivity of mass Kρ. A widely used formulation for Kρ in oceanic flows is given as Kρ = Γϵ/N2, ϵ is the rate of dissipation of turbulent kinetic energy, N = √(-g/ρ)(∂ρ/∂z) is the buoyancy frequency of the background stratification, ρ is the density, Γ = Rƒ/(1 - Rƒ) is a mixing coefficient and Rƒ is the mixing efficiency, that is widely (but questionably) assumed to be constant or sometimes parameterized. However, a robust and universal parameterization for the mixing efficiency remains elusive to date despite numerous studies on this topic. This research focuses on improved parameterizations of diapycnal mixing through an integration of theoretical knowledge with observational and high resolution numerical simulation data. The main objectives are: (1) to provide a better assessment of field microstructure data and methodology for data analysis in order to develop/test appropriate parameterization of the mixing efficiency, (2) to determine the relevant length and velocity scales for diapycnal mixing, (3) to provide improved parameterization(s) of diapycnal mixing grounded on physical reasoning and scaling analysis, (4) to provide a practical field method to identify the dynamic state of turbulence in stably stratified flows from measurable length scales in the ocean. First, an analysis of field microstructure data collected from different locations in the ocean was performed to verify existing parameterizations. A key finding is that the mixing efficiency, Rƒ does not scale with buoyancy Reynolds number, Rℓb, as been proposed previously by others. Rather, Rƒ depends on the strength of background stratification. In a strongly stratified thermocline, a constant value for the mixing efficiency is found to be reasonable while for weakly stratified conditions (e.g. near boundaries) a parameterization is required. A discussion on different methods to estimate the background shear and stratification from field data is provided. Furthermore, the present state-of-the-art microstructure instruments measure the small scale dissipation rate of turbulent kinetic energy ϵ from one dimensional components by invoking the small scale isotropy assumption that is strictly valid for high Reynolds number flows. A quantitative assessment of the departure from isotropy in stably stratified flows is performed and a pragmatic method is proposed to estimate the true three dimensional dissipation (ϵ3D) from one dimensional dissipation (ϵ1D) obtained from microstructure profilers in the ocean. Next, a scaling analysis for strongly stratified flow is presented to show that, the true diapycnal length scale Ld and diapycnal velocity scale wd can be estimated from the measurable Ellison length scale, LE and a measurable root mean square vertical velocity, w´, using a turbulent Froude number defined as Fr = ϵ/Nk, where k is the turbulent kinetic energy. It is shown that the eddy diffusivity Kρ can be then directly inferred from LE and w´. For weakly stratified flow regimes, Fr > O(1), Kρ ~ w´LE and for strongly stratified flow regimes, Fr < O(1), Kρ ~ w´LE x Fr. This finding is confirmed with direct numerical simulation (DNS) data for decaying as well as sheared stratified turbulence. This result indicates that Fr is a relevant non-dimensional parameter to identify strength of stratification in stably stratified turbulent flows. DNS with particle tracking is performed to separate isopycnal and diapycnal displacements of fluid particles, an analysis that is not possible from an Eulerian approach or from standard field measurements. The Lagrangian analysis show that LE is indeed an isopycnal length scale. Furthermore, having established that Fr is the signature parameter which can describe the state of stratified turbulence, a parameterization of mixing coefficient, Γ (or Rƒ) as a function of turbulent Froude number Fr is developed using scaling arguments of energetics of the flow. Proposed parameterization is then verified using DNS data of decaying, sheared and forced stratified turbulence. It is shown that for Fr << O(1), Γ ~ Fr0, for Fr ~ O(1), Γ ~ Fr-1 and for Fr >> O(1), Γ ~ Fr-2. Finally, a practically useful method to identify the dynamic state of turbulence in stably stratified flows is developed. Two commonly measurable length scales in the ocean are the Thorpe overturning length scale, LT and the dimensionally constructed Ozmidov length scale, LO. From scaling analysis and DNS data of decaying, sheared and forced stratified turbulence a new relation between Fr and the ratio of the length scales, LT/LO is derived. The new scaling is, for LT/LO > O(1), Fr ~ (LT/LO)-2 and for LT/LO < O(1), Fr ~ (LT/LO)-2/3.Item Open Access New insights into flow over sharp-crested and pivot weirs using computational fluid dynamics(Colorado State University. Libraries, 2021) Sinclair, Joseph, author; Venayagamoorthy, Subhas Karan, advisor; Gates, Timothy K., advisor; Gao, Xinfeng, committee memberIrrigation for agriculture is the highest use of fresh water in the world. Efficient and equitable access and distribution of this water is vital to survival of the Earth's population. Open channels are the most common means of conveying water for agricultural irrigation and hydraulic structures are often used in these open channels to regulate and measure flow to achieve desired conditions. Sharp-crested weirs are one of the most popular of these structures and pivot weirs are quickly becoming a more widely used hydraulic structure. The purpose of this study was to reexamine both types of weirs to better understand how they operate for flow regulation and measurement and to provide insights into the flow structure around the weir. Computational fluid dynamics, or CFD, was the primary tool used, with a commercial code called FLOW-3D being the specific software selected. Prior to investigating the weirs, preliminary studies were carried out to identify the best-practices in building an open-channel and hydraulic flow simulation in FLOW-3D. It was found that because FLOW-3D has no method of specifying developed flow prior to entering the model domain, additional care had to be taken to develop flow within the computational domain. The upstream length in the models was often extended to give the simulated flow more time and distance to develop. Additionally, the first-cell height had to be within a certain dimension to produce accurate velocity profiles due to the use of the logarithmic law of the wall boundary condition to solve for velocity in the first cell. Finally, a study analyzing the effects of the simulated downstream distance after a free-flowing sharp-crested weir revealed that the downstream distance has no effect on upstream flow. The sharp-crested weir parametric study analyzed velocity and pressure profiles over the crest, several calculated discharge coefficients, and turbulence flow structures upstream of the weir using high-resolution two-dimensional simulations. Three distinct operating regimes were identified based on the profiles over the crest as well as plots of the discharge coefficient against h/P where h is the upstream potentiometric head above the weir crest and P is the height of the crest above the channel bed. The first regime, the high-acceleration regime, occurs when h/P < 0.6. Flow accelerates greatly near the weir crest which results in negative pressure. The discharge coefficient has a negative linear trend with h/P in this regime. The next region occurs where 0.6 < h/P < 2.0 and is called the ideal-operating regime. In this regime, flow is not experiencing acceleration or inundation and better maintains the assumptions used in deriving the classical rating equation. The discharge coefficient is relatively constant in this case and a single value can be used with minimal error for all flow rates within this range. The final regime, the weir-inundated regime, is where h/P > 2.0. The weir is often submerged here and the effect of the weir on the flow is diminished due to the high depth of flow. Turbulence patterns upstream of the weir appear to have a relationship to the Reynolds number, Re, of the flow with eddies reaching a minimum size at a Re = 70,000. The region of smallest eddy size correlated to the ideal-operating regime, again lending to the hypothesis that flow is more efficiently controlled within this regime. Six flow rates at five different gate angles (27°, 47°, 57°, 72°, and 90°) were tested for the pivot weir study. After analysis of the h/P values and discharge coefficients, it was found that the flow rates bounding the ideal-operating regime shift lower in magnitude as the gate angle decreases. Each angle also has an associated relatively constant discharge coefficient in its ideal-operating regime, meaning a single coefficient value may be used with minimal error. Comparison of the average discharge coefficient for each angle revealed a minimum value at 72° and a maximum value at 27°. The fraction of the total upstream mechanical energy head comprised of the velocity head was found to increase as gate angle decreased. Visual contours of velocity and pressure depicted how the flow changes as it approaches weirs of varying angles, with the recirculation zone moving from upstream of the weir to solely downstream of the weir for angles below 47°. Plots of the non-dimensional pressure and velocity profiles over the weir crest revealed that velocity over the crest increases as the inclination angle decreases. At the 47° weir, the flow acceleration created a region of negative relative pressure close to the weir. These results highlight how flow over both the sharp-crested weir and pivot weir varies considerably. Thus, caution must be exercised in using empirical discharge coefficients for a broad range of h/P value.Item Open Access Nonlinear internal wave - topographic interaction and turbulent mixing using numerical simulations(Colorado State University. Libraries, 2021) Klema, Matthew Roy, author; Venayagamoorthy, Subhas Karan, advisor; Nelson, Peter, committee member; Rathburn, Sarah, committee member; Thornton, Christopher, committee memberTo view the abstract, please see the full text of the document.Item Open Access The innovative application of random packing material to enhance the hydraulic disinfection efficiency of small scale water systems(Colorado State University. Libraries, 2021) Baker, Jessica L., author; Venayagamoorthy, Subhas Karan, advisor; De Long, Susan K., advisor; Niemann, Jeffrey D., committee member; Leisz, Stephen J., committee memberIn a world where the quality of our water supplies is declining and our infrastructure is deteriorating, let alone the lack of available water in arid regions, the treatment of drinking water is becoming ever more challenging – especially for small scale systems that lack technical and financial support. The innovative application of random packing material (RPM) has been proposed as a possible tool to aid small water treatment systems (SWTSs) improve their disinfection contact systems in order to meet the Safe Drinking Water Act (SDWA) standards and provide the communities they serve with safe drinking water. While it has been demonstrated at the laboratory–scale that RPM can significantly improve the hydraulic disinfection efficiency of a contact basin in terms of baffling factor (BF) there was a lack of fundamental understanding of why RPM is so effective. Conceptually, the RPM slows and spreads the jet flow from a sharp inlet. Yet the mechanics of a jet flow through a highly porous material such as RPM is not well understood. Insight into the dynamics of such a flow is important in order to be able to use RPM in a manner that maximizes the benefits and minimizes the (unintended) drawbacks. The main aim of this dissertation is to use laboratory-scale experiments to study the mechanics of a turbulent jet flow from a long pipe through RPM and the impact on the hydraulic disinfection efficiency and final water quality for a disinfection contactor. There are three main objectives in this work: (1) To gain fundamental insights regarding turbulent jet flow through a highly porous media (such as RPM); (2) To address practical concerns for the application of the use of RPM in disinfection contactors; and (3) To provide guidance in terms of best practice for the innovative use of RPM to enhance hydraulic disinfection efficiency in SWTSs. The first part of this dissertation focuses on the resulting flow fields of a turbulent jet flow (5-20 gpm) through a wall of RPM of various thicknesses (L). An experiment was conducted in a flume using a Particle Image Velocimetry (PIV) system to map the flow fields downstream of the jet up to x⁄dj ≈ 30 (where dj is the diameter of the jet, i.e. inlet pipe). Once the PIV data were verified using a Laser-Doppler Anemometry (LDA) system and validated for a jet into an ambient (provided as a baseline), the velocity fields of the jet flow downstream of the walls of RPM were analyzed. A second order relationship was observed between the thickness of RPM and the spread of the flow. It was also observed that the jet velocities decay exponentially through RPM. With respect to flow rate, the spreading rate increased slightly, but there was a slight decrease in the decay of the jet as the flow rate increased. While the maximum velocities were reduced by over 90% after L ≈ 5dj, it was only after L ≈ 15dj that the flow downstream of the RPM was nearly uniform. Furthermore, the coefficients of drag showed a non-monotonic relationship with respect to the particle Reynolds number (Redp) that followed the well-established trend of a uniform flow around an infinitely long cylinder. This relationship provides valuable insight into the different regimes of the highly complex flow within and/or downstream of a highly porous material. Next, the potential improvement in the hydraulic disinfection efficiency and the possible energy loss as a result of the presence of random packing material in a laboratory-scale chlorine contactor were investigated. Tracer tests were conducted on a 55-gal drum tank filled with RPM in varying amounts in different configurations to measure the efficiency of each setup in terms of baffling factor. The bulk pressure drop was measured to determine the energy loss for each configuration. The results of this study show that securing RPM near the inlet, in any amount, improves the BF by 300% to more than 900%. The amount of RPM begins to have an impact at or above an inlet jet Reynolds number of 27,700. Also, changes in head loss due to the presence of RPM (in any amount, configuration, and/or flow rate) were generally considered to be negligible. Finally, a concern surrounding the potential for excessive biofilm growth is addressed through a long-term study. The inflow, outflow, and RPM were monitored for heterotrophic bacteria (via heterotrophic plate counts) and Pseudomonas aeruginosa as indicators of bacteriological water quality and the presence of biofilm. The results of this study show that there was no substantial biofilm growth in a lab-scale chlorine contactor and no substantial increase in bacterial counts for the bulk outflow over a 10-week period. Thus, the potential for excessive biofilm growth should not be considered a barrier concerning the use of RPM to improve the hydraulic disinfection efficiency of chlorine contactors in small drinking water treatment systems. Overall, this dissertation work aims to contribute a foundational understanding of turbulent jet flow through a highly porous material such as RPM as well as address some practical concerns for the innovative application of RPM to improve the hydraulic disinfection efficiency. From the results of the studies conducted, best practice guidelines have been developed to maximize the potential benefit of using RPM in disinfection contactors. Ultimately, the hope of this work is to promote the use of RPM to help SWTSs that are struggling to meet SDWA standards and to provide the communities they serve with safe drinking water.Item Open Access Turbulence parameterizations for numerical simulations of stably stratified environmental flows(Colorado State University. Libraries, 2011) Elliott, Zachary, author; Venayagamoorthy, Subhas Karan, advisor; Julien, Pierre Y., committee member; Dasi, Lakshmi Prasad, committee memberAlmost all environmental and geophysical flows such as lakes, reservoirs, estuaries, and the atmosphere are turbulent and are also often characterized by stable density stratification. The presence of buoyancy forces due to stratification has a substantial effect on the flow development and turbulent mixing processes, influencing the distribution of pollutants and suspended matter in these flows. Mathematical and computer models can be used to simulate and produce numerical solutions to these flows, providing results that would otherwise not be feasibly attainable in a laboratory setting and that can be used for engineering prediction, design, and analysis purposes. Turbulence models use computational procedures to close the system of mean flow equations and account for the effects of turbulence and stratification through the specification of parameters that characterize the behavior of the flow. In this research, an attempt is made to assess and improve turbulence parameterizations for stably stratified environmental flows. An important parameter describing the transfer of momentum and scalar fluxes in stratified turbulent flows is the turbulent Prandtl number Prt. Specifically, four different formulations of the turbulent Prandtl number Prt are evaluated for stably stratified flows. All four formulations of Prt are strictly functions of the gradient Richardson number Ri, a parameter that provides a measure of the strength of the stratification. A zero-equation turbulence model for the turbulent viscosity νt in a one-dimensional turbulent channel flow is considered to assess the behavior of the different formulations of Prt. Both uni-directional and oscillatory flows are considered to simulate conditions representative of practical flow problems, such as atmospheric boundary layer flows and tidally-driven estuarine flows, to quantify the behavior of each of the four formulations of Prt. It is discussed as to which of the models of Prt allow for a higher rate of turbulent mixing and which models significantly inhibit turbulent mixing in the presence of buoyancy forces resulting from fixed continuous stratification as well as fixed two-layer stratification. The basis underlying the formulation of each model in conjunction with the simulation results are used to highlight the importance of choosing an appropriate parameterization of Prt, given a model for νt in stably stratified flows. Other more complete and dynamic models rely on additional parameters that allow stratified turbulent flow to be modeled as a function of local turbulence quantities rather than mean global properties of the flow. This research also focuses on implementing and testing proposed changes that explicitly account for buoyancy effects in two-equation Reynolds-averaged Navier-Stokes (RANS) turbulence models. Direct numerical simulation (DNS) data of stably stratified homogeneous turbulence are used to study the parameters in two-equation RANS turbulence models such as the buoyancy parameter Cε3 and the turbulent Prandtl number Prt in the k-ε model. Both the gradient Richardson number Ri and the turbulent Froude number Frk are used as correlating parameters to characterize stratification in the k-ε model. It is shown that it may be more appropriate to use Frk as the parameter of choice for the stratification parameter in the k-ε model since it is based on the local properties of the turbulence as opposed to Ri, which is a mean property of the flow. The proposed modifications and alterations to Cε3 and Prt as functions of Ri and Frk are implemented in a one-dimensional water column model called General Ocean Turbulence Model (GOTM) and used to simulate stably stratified channel flows. The results from numerical simulations using the modified versions of the k-ε model are compared to stably stratified channel flow DNS data to assess their efficacy.