A. Stamou

Improving the numerical modeling of river water quality by using high order difference schemes

Abstract A one-dimensional numerical model is presented for the description of water quality in rivers. The model is an improvement over existing models, since it involves two differential schemes of third-order truncation error (QUICK and QUICKEST), which avoid the stability problems of central difference, while remaining relatively free of the inaccuracies of numerical dispersion associated with the upwind difference scheme. The model is successfully applied to four simple, steady-state and transient water quality problems. Results show that the commonly used upwind scheme performs very poorly. The central difference scheme performs satisfactorily in steady-state implicit calculations, but not in transient problems, where it shows numerical dispersion and significant oscillations. Calculations with QUICK and QUICKEST are stable and accurate involving the minimum error and minimum level of numerical dispersion in both steady-state and transient problems.

Two-phase modeling of batch sedimentation

A two-phase model for the simulation of sedimentation processes is presented. The model solves the continuity and momentum equations for the pure-clear liquid and the sludge phases, and it is verified against a well-known benchmark problem, for which analytical solutions exist. Numerical simulations of a typical 1-D batch sedimentation process for mono-dispersed particles are carried out and results are found to be in satisfactory agreement with experimental data and model predictions of other researchers. A further expansion of the model to two-dimensions leads to predictions of the dynamic behavior of settling tanks and the eAect of the inclination angle on the sedimentation process. ” 1999 Elsevier Science Inc. All rights reserved.

3-D numerical modeling of supercritical flow in gradual expansions

A 3-D CFD model based on the Volume of Fluid method to calculate the free surface was used to study supercritical flow in gradual open channel expansions. The model was first applied to a modified Rouse expansion, for which extensive experimental data are available. A detailed comparison between the numerical and the experimental results concerning surface profiles, velocities, free surface uniformity, and shock front location showed satisfactory agreement, especially for lower supercritical Froude numbers. The model was subsequently applied to optimize and finalize the design of a compound expansion structure with variable bottom elevation, piers, and inner walls. CFD calculations showed that flow conditions in this structure are satisfactory, provided that bottom ramps are used to ensure a uniform variation of the bottom elevation

Estimation of the effect of the degree of sewage treatment on the status of pollution along the coastline of the Mediterranean Sea using broad scale modelling.

A preliminary investigation was performed to estimate the effect of the degree of treatment in Sewage Treatment Plants (STPs) on the status of pollution along the coastlines of the Mediterranean Sea. Data from questionnaires and the literature were collected and processed (a) to identify 18 approximate 1D surface coastal currents, (b) to estimate their prevailing direction and average flow velocity and (c) to estimate the water pollution loads and identify their locations of discharge. Then, a simplified 1D water quality model was formulated and applied for the existing conditions and two hypothetical scenarios: (1) all coastal cities have STPs with secondary treatment and (2) all coastal cities have STPs with tertiary treatment to determine BOD(5), TN and TP concentrations in the 18 surface coastal currents. Calculated concentrations were compared and discussed. A main conclusion is that, to reduce pollution in the Mediterranean Sea measures should be adopted for upgrading the water quality of the rivers discharging into the Mediterranean Sea, along with the construction of STPs for all the coastal cities.

On the prediction of flow and mixing in settling tanks using a curvature-modified k-ϵ model

Abstract The k-ϵ model, which has been modified to include the effects of mean streamline curvature, is used to predict the flow field and the flow-through curve (FTC) in two model settling tanks. Predictions are compared with measurements and with calculations using the standard k-ϵ model. The results show the superiority of the modified turbulence model over the standard model to predict the flow field and mixing characteristics. All calculations were carried out on a personal computer.

Integrated modelling of single port, steady-state thermal discharges in unstratified coastal waters

An integrated model is presented for the calculation of the discharge of thermal effluents from power plants into coastal waters; the model consists of the near field model CorJet and the far field model FLOW-3DL that are interconnected via an active coupling algorithm. Firstly, the model is validated using experimental data; moreover, calculations are compared with passive coupling simulations to identify the dominant differences among these methods. Then, the model is applied to simulate the single-port thermal discharge originating from a thermal power plant to the non-stratified coastal waters in the region of Mantoudi in Evia, Greece. Model predictions are compared with CORMIX far field estimations and calculations employing passive coupling. Calculations verify the need for the application of an integrated active model. The detailed information for the coupling algorithm that is contained in this paper, including its difficulties and their resolution, permits its implementation to any active coupling between practically any near field with any far field model.

Water quality of the belilom river

Abstract A general methodology for the study of water quality in rivers is presented. The paper consists of four parts. In the first part the general characteristics of the area of study, which is the Beli Lom river, and its major pollution sources are presented. The effluent of the Razgrad Waste warer Treatment Plant (RWWTP) has been identified as the most significant pollution point source, due to the inadequate performance of the plant. The second part deals with data collection and processing. Four series of data have been collected, including physical, flow and water quality characteristics. In the third part a I-D, finite-difference, second-order model is presented. In the fourth part, the model is calibrated, for the determination of its main coefficients, and is successfully verified by predicting the BOD and DO concentrations in the Beli lom river for all series of data. Finally, the model has been applied to determine the maximum BOD and minimum DO effluent concentrations of the RWWTP, so that a minimum DO concentration is maintained throughout the river.

Habfuzz: A tool to calculate the instream hydraulic habitat suitability using fuzzy logic and fuzzy Bayesian inference

Habfuzz is a Fortran tool (J.C. et al. 1997), which implements the Mamdani Assilian fuzzy inference process (E.H. and S. 1975) and the Bayesian joint probability inference method as described in Brookes et al. (C.J., V., and S.N. 2010), but with fuzzified inputs in order to calculate the instream habitat suitability, given the flow velocity, the water depth and the substrate type of a hydraulically simulated river reach. While it is specifically structured to quickly calculate the fuzzy-logic(or fuzzy-Bayesian)-based instream habitat suitability for fish or freshwater macroinvertebrates, it can be applied to wider research topics requiring fuzzy logic to be addressed.

Prediction of hydrodynamic characteristics of oxidation ditches using the k-ε turbulence model

A numerical model predicting the velocity field in oxidation ditches is presented. A finite difference method is used to solve the partial differential equations governing the steady, two-dimensional flow in the horizontal plane of an oxidation ditch. The turbulent shear stresses are calculated with the aid of the eddy viscosity concept, the distribution of eddy viscosity (vt) being determined via the k-e model. This relates vt to the turbulent kinetic energy (k) and its dissipation rate (e), and determines these quantities from modelled transport equations. The model is applied to a typical oxidation ditch consisting of an inlet, an outlet and an aeration device (rotor), to examine the effect of various parameters, such as the use of guiding walls and the boundary conditions at the rotor. Calculations have shown that the flow field depends mainly on the use of guiding walls. Without guiding walls, the flow is similar to the flow in a strongly curved open channel and exhibits relatively large recirculation regions. Guiding walls create a more unidirectional flow throughout the ditch and reduce drastically the extend of the recirculation regions. The rotor was modelled as a uniform momentum source for the velocity component perpendicular to its shaft. By increasing the magnitude of this momentum source (i.e. by increasing the power supplied by the rotor), the flow velocities in the ditch increase. However, the normalised by the average velocity flow field is not affected noticeably, neither by the boundary conditions at the rotor, nor by the number and position of rotors.

Evaluating the performance of habitat models for predicting the environmental flow requirements of benthic macroinvertebrates

ABSTRACT Although various methods are currently available for modelling the habitat preferences of aquatic biota, studies comparing the performance of data-driven habitat models are limited. In this study, we assembled a benthic-macroinvertebrate microhabitat-preference dataset and used it to evaluate the predictive accuracy of regression-based univariate Habitat Suitability Curves (HSC), Boosted Regression Trees (BRT), Random Forests (RF), fuzzy-logic-based models using the weighted average (FLWA), maximum membership (FLMM), mean of maximum (FLM) and centroid (FLC) defuzzification algorithms and fuzzy rule-based Bayesian inference (FRB). The results show that the BRT model was the most accurate, closely followed by RF, FRB, FLM and FLMM while the FLC and FLWA algorithms had the lowest performance. However, due to the imbalanced nature of the dataset and in contrast to the fuzzy rule-based algorithms, the HSC, BRT and RF models failed to accurately predict the habitat suitability in low-scored microhabitats. We conclude that, given balanced datasets, BRT and RF can be effectively used in habitat suitability modelling. For imbalanced datasets, a properly adjusted RF model can be applied but when the input dataset is large enough to provide sufficient data-driven IF–THEN rules to train an FRB, FLMM or FLM algorithm, these models will produce the most accurate predictions.