Andres Tejada-Martinez

Temporal large eddy simulations of turbulent viscoelastic drag reduction flows

We report on temporal large eddy simulations (TLES) of the turbulent channel flow of a dilute polymer solution modeled with the FENE-P (finitely extensible nonlinear elastic in the Peterlin approximation) constitutive equation. The large eddy simulations are based upon an approximate temporal deconvolution method [Pruett et al., Phys. of Fluids, 18, 028104–1, (2006)] for residual Newtonian stress modeling and secondary regularization for unresolved subfilter Newtonian stress. The filtered conformation tensor equation involves deconvolution for stretching and for the nonlinear spring force, as well as secondary regularization. Results are shown at a friction Reynolds number 180 for Weissenberg numbers and molecular extensibilities spanning the moderate to high drag reducing regimes. Excellent agreement is obtained between TLES and direct numerical simulations (DNS) in terms of percent drag reduction prediction. TLES is also able to reproduce the high level of anisotropy of turbulence, which confirms recent...

A dynamic Smagorinsky model with dynamic determination of the filter width ratio

Various low-pass, spatial test filters specific to dynamic model large-eddy simulation (LES) on finite element topologies are proposed and analyzed. A number of simulations of decaying isotropic turbulence are performed using the stabilized finite element method of Whiting and Jansen with the purpose of understanding the dependence of the dynamic model on the test filter of choice and the elusive filter width ratio. From these numerical experiments, a key assumption is extracted leading to the derivation of a new dynamic model in which the sole model parameter, the filter width ratio, is computed dynamically. Traditionally, the dynamic model parameter has taken the filter width ratio as a constant. The new dynamic model is tested on LES of decaying isotropic turbulence on hexahedral, tetrahedral, and wedge topologies.

Developments in computational fluid dynamics-based modeling for disinfection technologies over the last two decades: A review

In the last two decades, Computational Fluid Dynamics (CFD) has shown great potential as a powerful and cost-efficient tool to troubleshoot existing disinfection contactors and improve future designs for water treatment industry. However, numerous challenges in the simulation of water disinfection processes still remain. This review summarizes past CFD studies of the hydraulic and associated disinfection efficiency of disinfection contactors. Hydraulic efficiency studies based on flow and tracer transport simulation were found to be the most common and successful. Challenges existing in flow and disinfection simulation are identified and discussed. These challenges can be overcome via advanced turbulent simulation approaches, such as Large Eddy Simulation and multi-phase resolving simulations. Although turbulence-chemistry interaction is found to be the most challenging problem for proper representation of the reaction system and inactivation kinetics, solutions to this challenge can be overshadowed unless errors induced by unresolved unsteady flow and multi-phase flow are reduced sufficiently.

Evaluating hydraulic and disinfection efficiencies of a full-scale ozone contactor using a RANS-based modeling framework.

The capability of predicting hydraulic and disinfection efficiencies of ozone disinfection contactors is essential for evaluating existing contactors and improving future designs. Previous attempts based on ideal and non-ideal models for the hydraulics and simplified mechanisms for chemical reaction modeling have resulted in low accuracy and are restricted to contactors with simple geometries. This manuscript develops a modeling framework for the ozonation process by combining computational fluid dynamics (CFD) with a kinetics-based reaction modeling for the first time. This computational framework has been applied to the full-scale ozone contactor operated by the City of Tampa Water Department. Flow fields, residence time distribution, ozone concentration distribution, and concentration-contact time (CT) distribution within the contactor have been predicted via the computational framework. The predictions of ozone and bromate concentrations at sample points agree well with physical experimental data measured in the contactor. The predicted CT values at the contactor outlet demonstrate that the disinfection performance of the ozone contactor operated by the City of Tampa Water Department is sufficient to meet regulation requirements. The impact of seasonal flow rate change on disinfection performance is found to be significant and deserves attention during the management and operation of a water treatment plant.

Taenia eggs in a stabilization pond system with poor hydraulics: concern for human cysticercosis?

The objective of this study is to compare the removal of Taenia eggs to the removal of Ascaris eggs in a wastewater stabilization pond system consisting of three ponds in series, where the hydraulic residence time distribution has been characterized via a tracer study supported by computational fluid dynamics modeling. Despite a theoretical hydraulic retention time of 30 days, the peak dye concentration was measured in the effluent of the first pond after only 26 hours. The smaller-sized Taenia eggs were detected in higher concentrations than Ascaris eggs in the raw wastewater. Ascaris eggs were not detected in the pond system effluent, but 45 Taenia eggs/L were detected in the system effluent. If some of these eggs were of the species Taenia solium, and if the treated wastewater were used for the irrigation of crops for human consumption, farmers and consumers could potentially be at risk for neurocysticercosis. Thus, limits for Taenia eggs in irrigation water should be established, and precautions should be taken in regions where pig taeniasis is endemic. The results of this study indicate that the theoretical hydraulic retention time (volume/flow) of a pond is not always a good surrogate for helminth egg removal.

Reynolds-Averaged Navier-Stokes Simulation of the Flow and Tracer Transport in a Multichambered Ozone Contactor

AbstractThis paper reports on results from steady-state Reynolds-averaged Navier-Stokes simulation (RANSS) of the flow and passive nonreactive tracer transport inside a multichambered, laboratory-scale ozone contactor using a structured, collocated, finite-volume discretization. Simulations are posed following a physical laboratory experiment. Simulations performed on different grids lead to varying degrees of short-circuiting and dispersion, which ultimately leads to differences in the residence time distribution (RTD) of the tracer (released at the inflow as a pulse) and in particular t10 index. The predicted t10 index depends on grid density and RANS turbulence model. For fine grids that resolve the viscous sublayer, a model with proper behavior within this sublayer has to be applied for accurate prediction of t10. Simulations on all considered grids with proper turbulence model yield cumulative RTD and associated t10 index in good agreement with the experimental data despite the underresolution of the...

Hydraulic Efficiency in RANS of the Flow in Multichambered Contactors

AbstractAccurate and practical models are in demand for optimization of ozone contactor designs. This paper reports results from Reynolds-averaged Navier-Stokes simulation (RANS) of the flow and passive nonreactive tracer transport inside a multichambered, laboratory-scale ozone contactor using a structured, collocated, finite volume discretization. Simulations are posed following previously published laboratory experiments. Results are presented in terms of velocity distributions, tracer concentration distributions, and tracer residence times. The flow is characterized by short-circuiting and dead zone regions that reduce the hydraulic (disinfection) efficiency or baffling performance of the contactor. RANS-predicted cumulative residence time distribution (RTD) of the tracer (released at the inflow as a pulse) is shown to be in excellent agreement with published experimental data despite the under resolution of the RANS methodology compared with better-resolved methodologies, such as large-eddy simulatio...

Large-Eddy Simulation of Shallow Water Langmuir Turbulence Using Isogeometric Analysis and the Residual-Based Variational Multiscale Method

Abstract : We develop a residual-based variational multiscale (RBVMS) method based on isogeometric analysis for large-eddy simulation (LES) of wind-driven shear flow with Langmuir circulation (LC). Isogeometric analysis refers to our use of NURBS (Non-Uniform Rational B-splines) basis functions which have been proven to be highly accurate in LES of turbulent flows (Bazilevs, Y., et al. 2007, Comput. Methods Appl. Mech. Eng., 197, pp. 173 201). LC consists of stream-wise vortices in the direction of the wind acting as a secondary flow structure to the primary, mean component of the flow driven by the wind. LC results from surface wave-current interaction and often occurs within the upper ocean mixed layer over deep water and in coastal shelf regions under wind speeds greater than 3m s 1. Our LES of wind-driven shallow water flow with LC is representative of a coastal shelf flow where LC extends to the bottom and interacts with the sea bed boundary layer. The governing LES equations are the Craik-Leivobich equations (Tejada-Mart nez, A. E., and Grosch, C. E., 2007, J. Fluid Mech., 576, pp. 63 108; Gargett, A. E., 2004, Science, 306, pp. 1925 1928), consisting of the time-filtered Navier-Stokes equations. These equations possess the same structure as the Navier-Stokes equations with an extra vortex force term accounting for wave-current interaction giving rise to LC. The RBVMS method with quadratic NURBS is shown to possess good convergence characteristics in wind-driven flow with LC. Furthermore, the method yields LC structures in good agreement with those computed with the spectral method in (Thorpe, S. A., 2004, Annu. Rev. Fluids Mech., 36, pp. 584 55 79) and measured during field observations in (D Alessio, S. J., et al., 1998, J. Phys. Oceanogr., 28, pp. 1624 1641; Kantha, L., and Clayson, C. A., 2004, Ocean Modelling, 6, pp. 101 124). [DOI: 10.1115/1.4005059]

Impact of Sludge Layer Geometry on the Hydraulic Performance of a Waste Stabilization Pond

Improving the hydraulic performance of waste stabilization ponds (WSPs) is an important management strategy to not only ensure protection of public health and the environment, but also to maximize the potential reuse of valuable resources found in the treated effluent. To reuse effluent from WSPs, a better understanding of the factors that impact the hydraulic performance of the system is needed. One major factor determining the hydraulic performance of a WSP is sludge accumulation, which alters the volume of the pond. In this study, computational fluid dynamics (CFD) analysis was applied to investigate the impact of sludge layer geometry on hydraulic performance of a facultative pond, typically used in many small communities throughout the developing world. Four waste stabilization pond cases with different sludge volumes and distributions were investigated. Results indicate that sludge distribution and volume have a significant impact on wastewater treatment efficiency and capacity. Although treatment capacity is reduced with accumulation of sludge, the latter may induce a baffling effect which causes the flow to behave closer to that of plug flow reactor and thus increase treatment efficiency. In addition to sludge accumulation and distribution, the impact of water surface level is also investigated through two additional cases. Findings show that an increase in water level while keeping a constant flow rate can result in a significant decrease in the hydraulic performance by reducing the sludge baffling effect, suggesting a careful monitoring of sludge accumulation and water surface level in WSP systems.

Indicators for technological, environmental and economic sustainability of ozone contactors

Various studies have attempted to improve disinfection efficiency as a way to improve the sustainability of ozone disinfection which is a critical unit process for water treatment. Baffling factor, CT10, and log-inactivation are commonly used indicators for quantifying disinfection credits. However the applicability of these indicators and the relationship between these indicators have not been investigated in depth. This study simulated flow, tracer transport, and chemical species transport in a full-scale ozone contactor operated by the City of Tampa Water Department and six other modified designs using computational fluid dynamics (CFD). Through analysis of the simulation results, we found that baffling factor and CT10 are not optimal indicators of disinfection performance. We also found that the relationship between effluent CT obtained from CT transport simulation and baffling factor depends on the location of ozone release. In addition, we analyzed the environmental and economic impacts of ozone contactor designs and upgrades and developed a composite indicator to quantify the sustainability in technological, environmental and economic dimensions.