Browsing by Subject "modelling"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Open Access
    A methodology for implementing a water balance of ESKOM power stations using the online condition monitoring software EtaPRO
    (2021) Sewlall, Preetha; Fuls, Wim
    Eskom produces approximately 90% of the electricity used in South Africa of which approximately 90.8% is from fossil fuel power plants. The process of electricity generation requires a significant quantity of raw water; therefore, Eskom is considered a strategic water user in South Africa. Water management is a growing focus area due to the increase in water usage and requires continuous improvement. Water management has been identified as an area lagging behind on the advanced analytics initiatives in Eskom. Excel based tools were used for the development of water balance models and water performance calculations in Eskom. This was attributed to the user-friendly functionality and availability to all users. However, the Excel tool posed challenges in allowing for standardisation and validation of calculations, tracking of model changes, continuous trending and storage of data as well as structured graphical user interfaces for screens and dashboard developments. There was therefore a need to develop a methodology on how to structure a water balance model for coal-fired power plants with standard calculation templates that allowed for customisation by each power plant within Eskom. It was required that the water balance model be implemented on a performance and monitoring tool allowing for comparison of power plant targets to actual online data in real time, enhancing the monitoring capabilities. It should have the ability to generate real time water performance data creating an opportunity for improved water management across the generation fleet. The approach adopted in this dissertation was to learn from existing Eskom Excel water balance tools and develop a standard mathematical model in the form of EtaPRO calculation templates. These templates are be structured such that they function as process components to develop water balances at power plants. The mathematical verification of the Excel calculations were to be conducted using Mathcad. The access to real time data, performance monitoring capabilities and availability at all Eskom Power plants, led to the selection of EtaPRO as the modelling platform. The research conducted led to the development of a methodology for setting up a water balance model for a wet-cooled coal-fired power plant. Calculation templates developed into EtaPRO were validated against the Mathcad mathematical model. The results included a well-documented mathematical model of a water balance in Mathcad and the development of 19 calculation templates that perform the function of standard process components. In addition to calculation templates, multiple Non Volatile (NV) records were created to allow the power plants to capture and track permanent data inputs. NV records also allow for creation of case studies, improving the process monitoring capabilities. A water balance model for a selected power plant was simulated in EtaPRO using the developed calculation templates and user defined formulae. Test screens and dashboards were created to illustrate how the calculation templates and water balance framework would be used to develop a typical water balance model and monitoring system. In conclusion, it is possible to develop process models within the EtaPRO software from well-defined mathematical models to address the performance monitoring concerns on water systems within Eskom.
  • Thumbnail Image
    Item
    Open Access
    A Non-linear Visco-elastic Model for Dynamic Finite Element Simulation of Bovine Cortical Bone
    (2021) Blignaut, Caitlyn; Ismail, Ernesto; Cloete, Trevor
    Modelling and simulation of the human body during an impact situation such as a car accident, can lead to better designed safety features on vehicles. In order to achieve this, investigation into the material properties and the creation of a numerical model of cortical bone is needed. One approach to creating a material model of cortical bone suitable for these situations is to describe the material model as visco-elastic, as reported by Shim et al. [1], Bekker et al. [2] and Cloete et al. [3]. The work by Shim et al. and Bekker et al. developed three-dimensional models, but do not accurately capture the transition in behaviour in the intermediate strain rate region, while Cloete et al. developed a phenomenological model which captures the intermediate strain rate behaviour in one dimension. This work aims to verify and extend these models. The intermediate strain rate regime (1 s−1 to 100 s−1 ) is of particular interest because it is a key characteristic of the behaviour of cortical bone and several studies have been conducted to gather experimental data in this region [3, 4, 5, 6]. The behaviour can be captured using non-linear viscoelastic models. This dissertation focuses on the development and implementation of a material model of cortical bone based on non-linear visco-elastic models to capture the intermediate strain rate regime behaviour. The material model was developed using uni-axial test results from cortical bone. The model by Cloete et al. has been improved and extended, and issues of local and global strain rate with regards to the viscosity have been clarified. A hereditary integral approach was taken in the analysis and implementation of discrete models and was found to be consistent with mathematical models. The model developed was extended to three dimensions in a manner similar to that of Shim et al. and Bekker et al. for implementation in commercial finite element software (LS-Dyna and Abaqus).
  • No Thumbnail Available
    Item
    Open Access
    An investigation of spatial-temporal diel changes in Loligo reynaudii catch rates in the commercial squid jig fishery of South Africa
    (2025) Botes, Peet; Githaiga-Mwicigi, Jean; Ghebrehiwet, Dawit Yemane
    Loligo reynaudii (chokka) squid has been jigged commercially on the South African South-southeast coast since 1982. Starting in 2019, a chokka-directed hydroacoustic survey programme has formed part of scientific efforts to best inform the management of this fishery, mainly focussed on surveying at daytime during October-November, on the main fishing grounds between Plettenberg Bay and Port Alfred. However, it has so far remained unclear whether this programme may be improved in terms of spatial-temporal focus, to better survey acoustically detectable aggregations of adult squid throughout the year. Considering that commercial jig catches similarly depend on the prevalence, intensity and duration of aggregations; and assuming standardised Catch Per Unit Effort (CPUE) to be a representative measure of local abundance and proportional representation of regional population distribution: this study employed statistical modelling to specifically investigate whether, on the commercial squid jig fishing grounds of South Africa, there is: (1) an effect of diel period on chokka CPUE; (2) seasonal variation in any effects of diel period on CPUE; and (3) spatial variation in any effects of diel period on CPUE between seasons and years. The data used were DFFE (South African Department of Forestry, Fisheries and the Environment) commercial session-level squid jig catch logbook records. A tweedie generalized additive model was applied following an iterative semi-inductive modelling approach, for an optimal account of data non-linearity and non-normality when estimating CPUE as a function of relevant space-time variables. The data were limited, inter alia, to 2006-2022 sessions conducted purely at day/night between 20-29°E. The final model explained 24.4 % deviance and all terms were statistically significant in their effect on the response (p<0.05). There were three main findings when estimating relevant terms' marginal effects. Firstly, chokka concentrations are greater at day than night. Secondly, there is clear seasonality in the aggregated proportion with a single greater daytime and lesser nighttime peak in annual spawning concentrations, around October-December. Thirdly, CPUEs involved varying spatial-temporal complexity while being greater at day than night across most of the fishing grounds for most of the average year. The main takeaway was a dismissal of any notions that acoustically surveyable, large aggregations of squid may likely be found consistently in areas and at times other than when-where acoustic efforts have typically been expended in late spring so far, on inshore spawning grounds targeted by the commercial jigging fleet. Recommendations for future research include accounting for session-level variations in relevant oceanographic parameters when modelling session-level jig CPUE.
  • Thumbnail Image
    Item
    Restricted
    Cross-shelf biogeochemical characteristics of sediments in the central Benguela and their relationship to overlying water column hypoxia
    (2007) Van der Plas, A K; Monteiro, P M S; Pascall, A
    Data from two cross-shelf sediment sampling cruises were used to explain reasons for the sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions. The spatial extent of this benthic–pelagic flux link is limited to the innershelf mud belt system on the Namibian shelf. The inshore mud belt is the primary deposition area of the carbon and nitrogen new production export flux. The offshore organic-rich zones are thought to be relict particulate organic matter originating from the inshore mud belt rather than from an overlying pelagic source. These data were used to set up a multi-layer sediment model that was used through sensitivity analyses to elucidate the input characteristics that result in the most significant feedbacks on hypoxia in the overlying water. The analyses showed that, although the new production flux is a requirement to drive an oxygen demand in the sediments, the onset and persistence of anoxia may depend critically on a low-oxygen boundary condition threshold. This is thought to be a key differentiating factor between systems that, despite comparable carbon export fluxes, are characterised by a persistent hypoxia/anoxia signal and those that are characterised by episodic hypoxia events. It was concluded that sediment oxygen demand and methane and 'sulphide' emissions from the central Benguela sediments are responses to external hypoxia boundary conditions rather than the local drivers of oxygen variability.
  • Thumbnail Image
    Item
    Open Access
    Modelling organic material in activated sludge systems
    (2004) Ekama, G A; Wentzel, M C
    A simple predictive model for the activated sludge reactor inorganic suspended solids (ISS) concentration is presented. It is based on the accumulation of influent ISS in the reactor and an ordinary heterotrophic organism (OHO) ISS content (fiOHO) of 0.15 mg ISS/mgOHOVSS and a variable phosphate accumulating organism (PAO) ISS content (fiPAO) proportional to their P content (fXBGP). The model is validated with data from 21 investigations conducted over the past 15 years on 30 aerobic and anoxic-aerobic nitrification denitrification (ND) systems and 18 anaerobic-anoxic-aerobic ND biological excess P removal (BEPR) systems variously fed artificial and real wastewater and operated from 3 to 20 d sludge age. The predicted reactor VSS/TSS ratio reflects the observed relative sensitivity to sludge age, which is low, and to BEPR, which is high. For effective use of the model for design, two significant issues require attention: measurement of the influent ISS concentration, which is not commonly done in wastewater characterisation analyses; and estimating a priori the P content of PAOs (fXBGP), which can vary considerably depending on the extent of anoxic P uptake BEPR that takes place in the system. Some guidance on selection of the mixed liquor VSS/TSS ratio for design is given.
  • Thumbnail Image
    Item
    Open Access
    PHY1023H - Principles of Physics A
    (2014-09-18) Buffler, Andy; Morrison, Angus
    PHY1023H is a a first-year, calculus-based introductory course primarily for students on the General Entry for Programmes in Science (GEPS). The first half of this course provides students with the essential tools and skills that are required for dealing successfully with physics at first-year university level. The three broad areas that are covered are (a) mathematical techniques and their relationship with physical phenomena, (b) experimental procedures and (c) communication skills, in particular report writing. The second half of the course covers material similar to that of the first half of PHY1004W. Second semester: Mechanics: vectors, kinematics, dynamics, work, energy power, conservative and non-conservative forces, friction, impulse, momentum, collisions, rotation, rotational dynamics, torque, rotational inertia, rotational energy, angular momentum, static equilibrium, gravitation. Properties of matter: elasticity, elastic moduli, hydrostatics, hydrodynamics. Thermodynamics: temperature, heat, kinetic theory of gases, thermodynamic laws, entropy.