Thorsten Stoesser | Cardiff University (original) (raw)

Papers by Thorsten Stoesser

In this study three-dimensional numerical models were refined to predict reactive processes in di... more In this study three-dimensional numerical models were refined to predict reactive processes in disinfection contact tanks (CTs). The methodology departs from the traditional performance assessment of contact tanks via Hydraulic Efficiency Indicators as it allows direct simulation of transport and decay of the disinfectant, inactivation of pathogens and accumulation of by-products. The method is applied to study the effects of inlet and compartment design on contact tank performance, with special emphasis on turbulent mixing and minimisation of internal recirculation and short-circuiting. In contrast to the conventional approach of maximizing the length-to-width ratio, the proposed design changes are aimed at addressing and mitigating against adverse hydrodynamic structures, which have historically led to poor hydraulic efficiency in many existing contact tanks. The results suggest that water treatment facilities can benefit from in-depth analyses of the flow and kinetic processes through computational fluid dynamics, resulting in up to 38% more efficient pathogen inactivation and 14% less disinfection by-product formation.

Journal of Hydro-envoronment Reseach, Sep 1, 2014

The hydrodynamics and mixing processes in small- and full-scale baffled disinfection tanks are st... more The hydrodynamics and mixing processes in small- and full-scale baffled disinfection tanks are studied experimentally and numerically. Velocity and tracer transport measurements are carried out to quantify the hydrodynamics and to obtain reliable data used to validate a three-dimensional computational fluid dynamics (CFD) model. The flow in the tank under investigation is extensively three-dimensional due to the existing inlet condition of the tank, resulting in short-circuiting and internal recirculation, particularly in the first three compartments. Near the inlet the tracer residence time distribution curve analysis and Hydraulic Efficiency Indicators (HEIs) suggest poor disinfection performance. Further away from the inlet, the flow recovers to a two-dimensional flow and the HEIs improve until the exit of the tank. The computational results demonstrate good agreement between the predicted hydrodynamics and tracer transport with the corresponding experimental data. The numerical model is then employed to investigate the effects of up-scaling of laboratory model findings to a full-scale contact tank. Despite the Froude–Reynolds conflict the full-scale contact tank exhibits similar behaviour to the small-scale tank. The effect of the tank geometry on the disinfection efficiency is demonstrated, highlighting the negative impact of flow three-dimensionality on pathogen inactivation.

A solid understanding of the flow through plant ca nopies is important to better understand river... more A solid understanding of the flow through plant ca nopies is important to better understand river and coastal restoration schemes or the creation of flood retention activities. Large Eddy Simulations (LES) were perfo rmed for open channel flow through emergent vegetation at shallow flow conditions. The vegetation is idealized with rigid cylinders of constant diameter d. The results are c

Water Resources Research, 2011

1] This paper presents numerical simulations of flow through emergent vegetation. Two simulation ... more 1] This paper presents numerical simulations of flow through emergent vegetation. Two simulation strategies are evaluated, (1) Reynolds-averaged Navier-Stokes (RANS)-based simulations employing a vegetation closure model and (2) low-resolution large-eddy simulation (LES). RANS-based models offer efficiency in terms of computational resources, however, it is demonstrated herein that the accuracy of RANS models depends strongly on empirical parameters of the corresponding vegetation closure model. The method of low-resolution LES is an efficient alternative to a fully resolved LES, simulates vegetation drag directly, and does not require empirical parameter input. Predictions of the vegetative flow resistance of emergent vegetation using low-resolution LES are in fairly good agreement with measurements, in particular for low and moderate vegetation densities. This is because prevailing stream-and spanwise-velocity gradients, vertical velocity profiles, and recirculation zones are calculated with reasonable accuracy. Citation: Kim, S. J., and T. Stoesser (2011), Closure modeling and direct simulation of vegetation drag in flow through emergent vegetation, Water Resour. Res., 47, W10511,

High Performance Computing in Science and Engineering ’06, 2007

The paper presents results of a Large Eddy Simulation (LES) of the flow in an open channel where ... more The paper presents results of a Large Eddy Simulation (LES) of the flow in an open channel where the channel bed is roughened with one layer of spheres. The roughness height k, which corresponds to the sphere diameter d is 0.23 of the channe l depth. The Reynolds number Re τ , based on the average friction velocity u τ and the channel depth h is approximately 2820. The flow configuration was selected to correspond to laboratory experiments of Detert (2005), which are currently in progress. Mean streamwise velocities from the LES are compared with the measured data and the distributions of the calculated turbulence intensities are evaluated by comparing them with empirical relationships for flow over rough walls suggested by Nezu (1977). The occurrence of low-and high-speed streaks is examined and their spanwise spacing is quantified. Moreover, sweeps and ejections are shown to occur as well as the amalgamation process i.e. ejection of fluid into the outer layer associated with vortex growth.

Applications in Environmental Hydraulics, 2005

... few models within CFD codes that conceptually or physically represent the hydrodynamic impact... more ... few models within CFD codes that conceptually or physically represent the hydrodynamic impact of a ... numerical concepts for the representation of vegetation are illustrated within a 3D Finite Volume ... show significant variation in the mean velocity and indicate the generation of a ...

Springer Proceedings Physics, 2007

Journal of Hydro-environment Research, 2015

The hydrodynamics and mixing processes in small-and full-scale baffled disinfection tanks are stu... more The hydrodynamics and mixing processes in small-and full-scale baffled disinfection tanks are studied experimentally and numerically. Velocity and tracer transport measurements are carried out to quantify the hydrodynamics and to obtain reliable data used to validate a threedimensional computational fluid dynamics (CFD) model. The flow in the tank under investigation is extensively three-dimensional due to the existing inlet condition of the tank, resulting in short-circuiting and internal recirculation, particularly in the first three compartments. Near the inlet the tracer residence time distribution curve analysis and Hydraulic Efficiency Indicators (HEIs) suggest poor disinfection performance. Further away from the inlet, the flow recovers to a two-dimensional flow and the HEIs improve until the exit of the tank. The computational results demonstrate good agreement between the predicted hydrodynamics and tracer transport with the corresponding experimental data. The numerical model is then employed to investigate the effects of up-scaling of laboratory model findings to a full-scale contact tank. Despite the FroudeeReynolds conflict the full-scale contact tank exhibits similar behaviour to the small-scale tank. The effect of the tank geometry on the disinfection efficiency is demonstrated, highlighting the negative impact of flow three-dimensionality on pathogen inactivation.

Proceedings of the Water Environment Federation, 2009

... 1 School of Civil and Environmental Engineering, Georgia Institute of Technology, 200 Bobby D... more ... 1 School of Civil and Environmental Engineering, Georgia Institute of Technology, 200 Bobby Dodd Way, Atlanta, GA 30332-0373 2 Department of Civil Engineering, Chosun University 3 Carollo Engineers 4 Korea Water Resources Corporation (Kwater) ABSTRACT ...

A vertical axis turbine is designed and tested experimentally in the hydraulics laboratory of the... more A vertical axis turbine is designed and tested experimentally in the hydraulics laboratory of the School of Civil and Environmental Engineering at Georgia Tech. Experimental parameters investigated are solidity, blade number, blade shape for different ambient current speeds. A test rig is constructed which allows for fairly accurate measurements of torque an turbine rotational speed. The experimental program aims at

Water disinfection tanks such as chlorine and ozone contactors typically consist of multiple comp... more Water disinfection tanks such as chlorine and ozone contactors typically consist of multiple compartments featuring a serpentine flow pattern. Due to the complex hydrodynamics, the design and optimization of these tanks are often carried out by employing computational fluid dynamics (CFD) simulations. The present study demonstrates the influence of certain modeling aspects when the widely used Reynolds-Averaged Navier Stokes (RANS) based CFD approach to predict hydrodynamics and disinfection performance is employed. Three different contact tank geometries are examined numerically using RANS. The time-averaged velocity predictions are reasonably accurate when compared to validation data. However, in baffled contact tanks, the time-averaged flow differs quite significantly from the instantaneous flow, with the consequence that RANS-based models require careful calibration of the turbulent mixing parameter, i.e., the turbulent Schmidt number, when computing transport of solutes.

Proceedings of the ICE - Water and Maritime Engineering, 2003

... UNI Résumé / Abstract. This paper attempts to compare the accuracy of a two-dimensional (2D) ... more ... UNI Résumé / Abstract. This paper attempts to compare the accuracy of a two-dimensional (2D) depth-averaged code (TELEMAC-2D) and a three-dimensional (3D) code (SSIIM) in the numerical simulation of free surface flows. ...

Engineering Applications of Computational Fluid Mechanics, 2012

The paper introduces a three-dimensional numerical model that solves the Reynolds Averaged Navier... more The paper introduces a three-dimensional numerical model that solves the Reynolds Averaged Navier-Stokes (RANS) equations on a curvilinear grid system using a novel nested grid approach. The main benefit of the model is the possibility to model locally complex hydraulic features in large rivers like the flow field at hydraulic structures. The entire study domain in such a case can be discretized with a coarser resolution, whereas a much finer resolution can be applied to a defined zone of the obstructions, where a detailed description of the flow field is needed. The model is tested on a laboratory experiment carried out at the Georgia Institute of Technology, where the flow field around a single and two double circular cylinders in a flatbed flume was studied. Simulated flow velocity, turbulent kinetic energy and bed shear stress distributions are in good agreement with measurements. However, deviations downstream of the piers indicate the limitation of the steady state description of the flow in the unstable wake zone. Nevertheless, the nested grid approach presented herein is a promising step towards the modeling of the local scouring phenomenon due to the relatively low computational demand.

Journal of Hydraulic Engineering, 2015

ABSTRACT

Water Research, 2014

In this study three-dimensional computational fluid dynamics (CFD) models, incorporating appropri... more In this study three-dimensional computational fluid dynamics (CFD) models, incorporating appropriately selected kinetic models, were developed to simulate the processes of chlorine decay, pathogen inactivation and the formation of potentially carcinogenic byproducts in disinfection contact tanks (CTs). Currently, the performance of CT facilities largely relies on Hydraulic Efficiency Indicators (HEIs), extracted from experimentally derived Residence Time Distribution (RTD) curves. This approach has more recently been aided with the application of CFD models, which can be calibrated to predict accurately RTDs, enabling the assessment of disinfection facilities prior to their construction. However, as long as it depends on HEIs, the CT design process does not directly take into consideration the disinfection biochemistry which needs to be optimized. The main objective of this study is to address this issue by refining the modelling practices to simulate some reactive processes of interest, while acknowledging the uneven contact time stemming from the RTD curves. Initially, the hydraulic performances of seven CT design variations were reviewed through available experimental and computational data.

Water Research, 2011

Three-dimensional laser-induced fluorescence (3DLIF) was applied to visualize and quantitatively ... more Three-dimensional laser-induced fluorescence (3DLIF) was applied to visualize and quantitatively analyze mixing in a lab-scale UV reactor consisting of one lamp sleeve placed perpendicular to flow. The recirculation zone and the von Karman vortex shedding that commonly occur in flows around bluff bodies were successfully visualized. Multiple flow paths were analyzed by injecting the dye at various heights with respect to the lamp sleeve. A major difference in these pathways was the amount of dye that traveled close to the sleeve, i.e., a zone of higher residence time and higher UV exposure. Paths away from the center height had higher velocities and hence minimal influence by the presence of sleeve. Approach length was also characterized in order to increase the probability of microbes entering the region around the UV lamp. The 3DLIF technique developed in this study is expected to provide new insight on UV dose delivery useful for the design and optimization of UV reactors.