The Swedish defence agency (FOI) has developed a particle model (called Pello) that simulates the dispersion of aerosols and gases. At the boundaries, such as the ground, the particles can either reflect back into the domain (the atmosphere) or be absorbed. Which of the events that occurs is decided by a certain probability, which in the present model depends on mere physical properties. In this thesis we have investigated a newly proposed boundary behaviour which also depends on the time step used in the numerical simulations. We verified the accuracy of the new model by using a dispersion model with an explicit solution. To gain a better understanding of how important parameters at the boundary influence each other, we performed a sensitivity analysis. Simulations showed an overall improving concentration profile as the time step became smaller and the new model working well. The convergence order of the simulations was found to be close to 0.5. In this thesis we have shown that there exist an upper limit for the time step, which depends on the specific model. The present used time step at FOI does not have this versatile property. But having this upper limit for the time step close to the boundary, and a uniform time step can be time demanding. This lead us to the conclusion that an adaptive time step should be implemented.

Fluid flow network simulation codes are commonly used as a design and analysis tool for many engineering problems such as gas distribution networks, power plants and heat pumps. Two formulations of conservation of momentum have been widely applied in fluid flow network simulation models namely those based on static pressure and those based on total pressure. The total pressure formulations are convenient in that they eliminate the difficulties associated with the calculation of the convective terms and components such as pipe junctions are treated in a straightforward manner based on total pressure losses. However, the different formulations of total pressure for compressible and incompressible flow require different formulations of the momentum conservation equation, which is inconvenient for implementation in a generic network simulation code. In this thesis a united total pressure formulation is first derived which is valid for all fluids and therefore eliminates the inconvenience of switching between the compressible and incompressible formulations. A non-iterative method for the solution of the non-isothermal discretised equations based on the total pressure formulation is then introduced and consistency is illustrated. The method appears to be very stable for subsonic flows, while rapid steady state convergence is observed. A systematic comparison is also done with traditional static pressure based methods and the similarities and differences between the two formulations are illuminated. The different time scales involved in the simulation of transient flow in fluid networks are problematic when conventional fixed time step methods are used for time-wise integration. The time scales associated with acoustic and kinematic wave phenomena as well as storage effects can differ by orders in magnitude. This thesis also presents a simple adaptive time step algorithm which can be readily used in conjunction with all the commonly used first order methods for fluid flow networks. Two test problems are selected to demonstrate the efficiency and savings obtained with this procedure. The adaptive time step algorithm correctly selects appropriate time steps for all phenomena and significant computational savings are observed for accurate integration. In addition, a procedure is implemented which automatically selects the appropriate integration method. The resulting algorithm is a fully adaptive algorithm which switches between a fully implicit method and a semi-implicit method. Two test problems are once again used to demonstrate the efficiency and savings. The fully adaptive algorithm correctly selects appropriate methods for all phenomena and significant additional computational savings are observed. / Thesis (PhD (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013

Fluid flow network simulation codes are commonly used as a design and analysis tool for many engineering problems such as gas distribution networks, power plants and heat pumps. Two formulations of conservation of momentum have been widely applied in fluid flow network simulation models namely those based on static pressure and those based on total pressure. The total pressure formulations are convenient in that they eliminate the difficulties associated with the calculation of the convective terms and components such as pipe junctions are treated in a straightforward manner based on total pressure losses. However, the different formulations of total pressure for compressible and incompressible flow require different formulations of the momentum conservation equation, which is inconvenient for implementation in a generic network simulation code. In this thesis a united total pressure formulation is first derived which is valid for all fluids and therefore eliminates the inconvenience of switching between the compressible and incompressible formulations. A non-iterative method for the solution of the non-isothermal discretised equations based on the total pressure formulation is then introduced and consistency is illustrated. The method appears to be very stable for subsonic flows, while rapid steady state convergence is observed. A systematic comparison is also done with traditional static pressure based methods and the similarities and differences between the two formulations are illuminated. The different time scales involved in the simulation of transient flow in fluid networks are problematic when conventional fixed time step methods are used for time-wise integration. The time scales associated with acoustic and kinematic wave phenomena as well as storage effects can differ by orders in magnitude. This thesis also presents a simple adaptive time step algorithm which can be readily used in conjunction with all the commonly used first order methods for fluid flow networks. Two test problems are selected to demonstrate the efficiency and savings obtained with this procedure. The adaptive time step algorithm correctly selects appropriate time steps for all phenomena and significant computational savings are observed for accurate integration. In addition, a procedure is implemented which automatically selects the appropriate integration method. The resulting algorithm is a fully adaptive algorithm which switches between a fully implicit method and a semi-implicit method. Two test problems are once again used to demonstrate the efficiency and savings. The fully adaptive algorithm correctly selects appropriate methods for all phenomena and significant additional computational savings are observed. / Thesis (PhD (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013

This thesis introduces a comprehensive methodology to improve electromagnetic transient (EMT) modelling of power cables systems. Several improved modelling and validation techniques are proposed at the parameter estimation, time domain simulation and validation stages of the EMT modelling of transmission lines. A novel approach is developed to model sector-shaped cables in electromagnetic transient type programs. First, the applicability of elemental sub-conductor technique is extended to accurately calculate the frequency dependent impedances of sector-shaped cables. The derived admittance and propagation characteristics of the sector-shaped cable are fitted with rational functions using the method of vector fitting in an EMT-type program. The time domain simulations are validated with the numerical inverse Laplace transform method. A novel frequency domain approach is presented to model cascaded transmission systems. The procedure is based on obtaining four composite propagation functions representing the cascaded system. The performance of the technique does not diminish with increased number of cascaded segments and it preserves the intrinsic details of each line segment. This method is capable of modelling cascaded overhead lines or cables with different characteristic admittances and line lengths. This method can be used to validate EMT models of cascaded transmission systems. An improved generalized transmission line model is introduced which is capable of accommodating time steps greater than the travel time of the line. The time step of the conventional EMT models of transmission lines is constrained by the smallest travel time of the line. When the high frequency transients at the line terminations are not of interest, inaccurate nominal π equivalents are used with large time steps to reduce the computational burden. The proposed model not only is more accurate than the π equivalents, but also degenerates to the conventional frequency dependent EMT model when used with time steps smaller than the travel time. Therefore, the proposed model is highly convenient as it can be used for all types of EMT simulations without resorting to nominal π equivalents when the large simulation time steps must be used to reduce computational burden. / February 2017

Com os grandes avanços ocorridos na disponibilização de computadores, existe uma tendência contínua para a utilização de técnicas computacionais auxiliando no projeto de equipamentos de engenharia. Cada vez mais estão se obtendo resultados bastante próximos às condições reais, incluindo a simulação de motores de combustão interna. Neste sentido o presente trabalho tem o objetivo de analisar o escoamento turbulento no processo de admissão de ar em um motor operando em ciclo Diesel. A investigação é focada na determinação da influência do passo de tempo no cálculo do coeficiente de descarga e razão de swirl. Adicionalmente, o campo de velocidades, pressão, energia cinética turbulenta e outros parâmetros são apresentados e analisados, com o objetivo de auxiliar no entendimento da dinâmica envolvida. Essencialmente, dois modelos de turbulência são empregados, juntamente com dois tratamentos de parede. Seus resultados também são confrontados e discutidos. A geometria considerada é de um motor Fiat 1.9 L quatro tempos com duas válvulas. A análise é concentrada em um único cilindro. O pacote computacional utilizado é o Star-cd, e seu aplicativo es-ice. A independência de malha foi obtida, chegando a 1.672.056 volumes. Os resultados são apresentados de duas formas. A primeira delas refere-se a resultados de simulações em regime permanente, realizadas em boa parte por outros autores, com ênfase na determinação do coeficiente de descarga e razão de swirl, estes confrontados com valores experimentais, visando à validação da metodologia. Fica evidente a importância da escolha do modelo de turbulência na simulação de motores de combustão interna, assim como das funções de interpolação utilizadas. Na segunda parte os resultados referem-se a uma análise transiente, considerando o movimento do pistão e válvulas, a 1500 RPM. Observa-se a grande exigência quanto ao passo de tempo requerido no transiente real, ficando demonstrado que para esta velocidade o menor passo de tempo utilizado, 0,05° (5.5555E-6 s), ainda é insuficiente para alguns momentos do ciclo. É possível notar maior influência no coeficiente de descarga do que na razão de swirl, em relação aos passos de tempo utilizados. A forte dependência do modelo de turbulência nos resultados obtidos é mais uma vez confirmada, conforme o esperado, já que as hipóteses sobre a física do fenômeno são diferentes em cada modelo. Os resultados quanto ao tratamento na parede não apresentaram significantes diferenças, quando aplicados junto ao modelo de turbulência k-ω SST. / Considering the increase in the availability of computers, there is a continuing trend toward the use of computational simulation aiding in the design of engineering equipments. Reasonable results, close to the real conditions, are obtained, including the simulation of internal combustion engines. In this way, the present work has the objective of analyzing the turbulent flow in the air intake process of an engine operating in Diesel cycle. The investigation focuses on the determination of the time step in the calculation of the air discharge coefficient and swirl ratio. Additionally, the turbulent kinetic energy, pressure and velocity fields, besides other parameters, are presented and analyzed, with the objective of aiding in the understanding of the involved dynamics. Essentially, two turbulence models are employed, together with two wall treatments. Their results are also confronted and discussed. The considered geometry is a four-stroke, 1.9-L FIAT engine, with two valves. The analysis is concentrated on a single cylinder. The software package used is the Star-cd, and its application es-ice. The mesh independence is carried out, arriving in 1.672.056 volumes. The results are presented in two ways. The first one refers to simulation results of the steady state, also accomplished by other authors, with emphasis in the determination of the discharge coefficient and swirl ratio. These data are confronted with experimental values, aiming to validate the applied methodology. The importance of the choice of the turbulent model becomes evident in the simulation of internal combustion engines, as well as the interpolation functions used. In the second part the results refer to a transient analysis, considering the valves and piston movement, at 1500 rpm. It is observed the great demand on time step required is observed for the real transient, demonstrating that, for this speed, the smallest time step used, 0.05º (5.5555E-6 s), is still insufficient for some moments of the cycle. Also regarding the time step, it is possible to notice a greater influence in the discharge coefficient than in the swirl ratio. The strong dependence of the turbulence model on the results is once again confirmed, as expected, since the hypotheses about the physics of the phenomenon are different in each model. The results, regarding the wall treatment, presented no significant differences, when applied together with the SST k-ω turbulence model.

Interest in converting portions of the large volumes of flood control storage capacity in federal multiple-purpose reservoirs in Texas and elsewhere to water supply and other conservation purposes has been growing for some time. Evaluation of storage reallocations involving tradeoffs between flood control and conservation purposes in multiple-purpose, multiple-reservoir systems represents a new area for applying the Water Rights Analysis Package (WRAP) and Texas Water Availability Modeling (WAM) System. A system of 12 multiple-purpose reservoirs operated by the U.S. Army Corps of Engineers (USACE) and Brazos River Authority (BRA) was adopted as a case study in this research to develop and test expanded WRAP/WAM-based methods for analyzing modifications in reservoir storage allocations and related system operations. The research consisted of the following tasks: ? The Brazos River Basin WRAP input dataset from the Texas WAM System (Brazos WAM) has a 1940-1997 hydrologic period-of-analysis. The research included developing and applying methods to extend the period-of-analysis to 1900-2007 providing a better representation of river basin hydrology. The methodology developed could potentially be used to update the other river basin datasets in the statewide WAM System. ? The Brazos WAM has 3,830 control points, 670 reservoirs, and hundreds of water rights. The research included developing and applying methods to create a much easier-to-apply condensed dataset focused on the USACE/BRA reservoir system and associated water rights that have only 48 control points and 14 reservoirs. ? The WRAP/WAM System was developed based on a monthly computational time step. The research included applying developmental methodologies for converting a monthly model to a daily time step that includes disaggregation of monthly naturalized flows to daily flows, calibration of flow routing coefficients, and incorporation of forecasting in the simulation. ? The WRAP/WAM System is designed for assessing water supply reliabilities and stream flow and storage frequencies from the perspective of conservative purposes. The research added flood risk indices to the WRAP modeling system in order to address tradeoffs between flood control and conservation purposes. ? The WRAP/WAM-based simulation study performed with the modified WAM dataset developed in this research demonstrates the improvements in water supply capabilities and tradeoffs with flood control associated with various reservoir storage reallocation strategies and other modifications in reservoir system operations.

Surface water availability models often use monthly simulation time steps for reasons of data availability, model parameter parsimony, and reduced computational time. Representing realistic streamflow variability, however, requires modeling time steps with sub-monthly or daily temporal resolution. Adding daily time step simulation capability to the Water Rights Analysis Package (WRAP) and the Texas Water Availability Modeling (WAM) System is a growing area of need and interest in water rights permitting, water supply planning, and environmental protection. This research consisted of the following tasks: 1. Key modeling issues are identified that are relevant to daily time step modeling, but are otherwise not considered with monthly simulations. These key modeling issues include disaggregating monthly naturalized flows into daily flows, routing changes to flow through the stream network, reducing impacts to water availability in a priority order based water right system through the use of streamflow forecasting, distributing water right targets from monthly to daily amounts, and integrating flood control reservoir operations into the existing conservation reservoir modeling framework. 2. Two new programs for WRAP are developed to address the key daily time step modeling issues. The new programs include a pre-processor program, DAY, and a daily simulation program, SIMD. 3. A case study of the Brazos River Basin WAM is presented using daily time steps with SIMD. The purpose of the case study is to present an implementation of the daily modeling capabilities. 4. The case study simulation results are used as a basis to draw conclusions regarding monthly versus daily simulation outcomes. The research, as presented through the Brazos River Basin WAM case study, illustrated that incorporating realistic daily streamflow variability into the simulation of a priority order based water allocation system can substantially affect the results obtained for time series of critical period reservoir storage contents, the determination of long-term water right reliability, and the distribution of unappropriated and regulated flows. The modeling capabilities developed by this research advance the state of water availability modeling with sub-monthly time steps by addressing the key modeling issues related to streamflow variability and routing.

There are two main types of logistical models used for long-term performance prediction of autonomous power systems: time series and probabilistic. Time series models are more common and are more accurate for sizing storage systems because they are able to track the state of charge. However, the computational time is usually greater than for probabilistic models. It is common for time series models to perform 1-year simulations with a 1-hour time step. This is likely because of the limited availability of high resolution data and the increase in computation time with a shorter time step. Computation time is particularly important because these types of models are often used for component size optimization which requires many model runs. This thesis includes a sensitivity analysis examining the effect of the time step on these simulations. The results show that it can be significant, though it depends on the system configuration and site characteristics. Two probabilistic models are developed to estimate the temporal resolution error of a 1-hour simulation: a time series/probabilistic model and a fully probabilistic model. To demonstrate the application of and evaluate the performance of these models, two case studies are analyzed. One is for a typical residential system and one is for a system designed to provide on-site power at an aquaculture site. The results show that the time series/probabilistic model would be a useful tool if accurate distributions of the sub-hour data can be determined. Additionally, the method of cumulant arithmetic is demonstrated to be a useful technique for incorporating multiple non-Gaussian random variables into a probabilistic model, a feature other models such as Hybrid2 currently do not have. The results from the fully probabilistic model showed that some form of autocorrelation is required to account for seasonal and diurnal trends. / Graduate

Com os grandes avanços ocorridos na disponibilização de computadores, existe uma tendência contínua para a utilização de técnicas computacionais auxiliando no projeto de equipamentos de engenharia. Cada vez mais estão se obtendo resultados bastante próximos às condições reais, incluindo a simulação de motores de combustão interna. Neste sentido o presente trabalho tem o objetivo de analisar o escoamento turbulento no processo de admissão de ar em um motor operando em ciclo Diesel. A investigação é focada na determinação da influência do passo de tempo no cálculo do coeficiente de descarga e razão de swirl. Adicionalmente, o campo de velocidades, pressão, energia cinética turbulenta e outros parâmetros são apresentados e analisados, com o objetivo de auxiliar no entendimento da dinâmica envolvida. Essencialmente, dois modelos de turbulência são empregados, juntamente com dois tratamentos de parede. Seus resultados também são confrontados e discutidos. A geometria considerada é de um motor Fiat 1.9 L quatro tempos com duas válvulas. A análise é concentrada em um único cilindro. O pacote computacional utilizado é o Star-cd, e seu aplicativo es-ice. A independência de malha foi obtida, chegando a 1.672.056 volumes. Os resultados são apresentados de duas formas. A primeira delas refere-se a resultados de simulações em regime permanente, realizadas em boa parte por outros autores, com ênfase na determinação do coeficiente de descarga e razão de swirl, estes confrontados com valores experimentais, visando à validação da metodologia. Fica evidente a importância da escolha do modelo de turbulência na simulação de motores de combustão interna, assim como das funções de interpolação utilizadas. Na segunda parte os resultados referem-se a uma análise transiente, considerando o movimento do pistão e válvulas, a 1500 RPM. Observa-se a grande exigência quanto ao passo de tempo requerido no transiente real, ficando demonstrado que para esta velocidade o menor passo de tempo utilizado, 0,05° (5.5555E-6 s), ainda é insuficiente para alguns momentos do ciclo. É possível notar maior influência no coeficiente de descarga do que na razão de swirl, em relação aos passos de tempo utilizados. A forte dependência do modelo de turbulência nos resultados obtidos é mais uma vez confirmada, conforme o esperado, já que as hipóteses sobre a física do fenômeno são diferentes em cada modelo. Os resultados quanto ao tratamento na parede não apresentaram significantes diferenças, quando aplicados junto ao modelo de turbulência k-ω SST. / Considering the increase in the availability of computers, there is a continuing trend toward the use of computational simulation aiding in the design of engineering equipments. Reasonable results, close to the real conditions, are obtained, including the simulation of internal combustion engines. In this way, the present work has the objective of analyzing the turbulent flow in the air intake process of an engine operating in Diesel cycle. The investigation focuses on the determination of the time step in the calculation of the air discharge coefficient and swirl ratio. Additionally, the turbulent kinetic energy, pressure and velocity fields, besides other parameters, are presented and analyzed, with the objective of aiding in the understanding of the involved dynamics. Essentially, two turbulence models are employed, together with two wall treatments. Their results are also confronted and discussed. The considered geometry is a four-stroke, 1.9-L FIAT engine, with two valves. The analysis is concentrated on a single cylinder. The software package used is the Star-cd, and its application es-ice. The mesh independence is carried out, arriving in 1.672.056 volumes. The results are presented in two ways. The first one refers to simulation results of the steady state, also accomplished by other authors, with emphasis in the determination of the discharge coefficient and swirl ratio. These data are confronted with experimental values, aiming to validate the applied methodology. The importance of the choice of the turbulent model becomes evident in the simulation of internal combustion engines, as well as the interpolation functions used. In the second part the results refer to a transient analysis, considering the valves and piston movement, at 1500 rpm. It is observed the great demand on time step required is observed for the real transient, demonstrating that, for this speed, the smallest time step used, 0.05º (5.5555E-6 s), is still insufficient for some moments of the cycle. Also regarding the time step, it is possible to notice a greater influence in the discharge coefficient than in the swirl ratio. The strong dependence of the turbulence model on the results is once again confirmed, as expected, since the hypotheses about the physics of the phenomenon are different in each model. The results, regarding the wall treatment, presented no significant differences, when applied together with the SST k-ω turbulence model.

Com os grandes avanços ocorridos na disponibilização de computadores, existe uma tendência contínua para a utilização de técnicas computacionais auxiliando no projeto de equipamentos de engenharia. Cada vez mais estão se obtendo resultados bastante próximos às condições reais, incluindo a simulação de motores de combustão interna. Neste sentido o presente trabalho tem o objetivo de analisar o escoamento turbulento no processo de admissão de ar em um motor operando em ciclo Diesel. A investigação é focada na determinação da influência do passo de tempo no cálculo do coeficiente de descarga e razão de swirl. Adicionalmente, o campo de velocidades, pressão, energia cinética turbulenta e outros parâmetros são apresentados e analisados, com o objetivo de auxiliar no entendimento da dinâmica envolvida. Essencialmente, dois modelos de turbulência são empregados, juntamente com dois tratamentos de parede. Seus resultados também são confrontados e discutidos. A geometria considerada é de um motor Fiat 1.9 L quatro tempos com duas válvulas. A análise é concentrada em um único cilindro. O pacote computacional utilizado é o Star-cd, e seu aplicativo es-ice. A independência de malha foi obtida, chegando a 1.672.056 volumes. Os resultados são apresentados de duas formas. A primeira delas refere-se a resultados de simulações em regime permanente, realizadas em boa parte por outros autores, com ênfase na determinação do coeficiente de descarga e razão de swirl, estes confrontados com valores experimentais, visando à validação da metodologia. Fica evidente a importância da escolha do modelo de turbulência na simulação de motores de combustão interna, assim como das funções de interpolação utilizadas. Na segunda parte os resultados referem-se a uma análise transiente, considerando o movimento do pistão e válvulas, a 1500 RPM. Observa-se a grande exigência quanto ao passo de tempo requerido no transiente real, ficando demonstrado que para esta velocidade o menor passo de tempo utilizado, 0,05° (5.5555E-6 s), ainda é insuficiente para alguns momentos do ciclo. É possível notar maior influência no coeficiente de descarga do que na razão de swirl, em relação aos passos de tempo utilizados. A forte dependência do modelo de turbulência nos resultados obtidos é mais uma vez confirmada, conforme o esperado, já que as hipóteses sobre a física do fenômeno são diferentes em cada modelo. Os resultados quanto ao tratamento na parede não apresentaram significantes diferenças, quando aplicados junto ao modelo de turbulência k-ω SST. / Considering the increase in the availability of computers, there is a continuing trend toward the use of computational simulation aiding in the design of engineering equipments. Reasonable results, close to the real conditions, are obtained, including the simulation of internal combustion engines. In this way, the present work has the objective of analyzing the turbulent flow in the air intake process of an engine operating in Diesel cycle. The investigation focuses on the determination of the time step in the calculation of the air discharge coefficient and swirl ratio. Additionally, the turbulent kinetic energy, pressure and velocity fields, besides other parameters, are presented and analyzed, with the objective of aiding in the understanding of the involved dynamics. Essentially, two turbulence models are employed, together with two wall treatments. Their results are also confronted and discussed. The considered geometry is a four-stroke, 1.9-L FIAT engine, with two valves. The analysis is concentrated on a single cylinder. The software package used is the Star-cd, and its application es-ice. The mesh independence is carried out, arriving in 1.672.056 volumes. The results are presented in two ways. The first one refers to simulation results of the steady state, also accomplished by other authors, with emphasis in the determination of the discharge coefficient and swirl ratio. These data are confronted with experimental values, aiming to validate the applied methodology. The importance of the choice of the turbulent model becomes evident in the simulation of internal combustion engines, as well as the interpolation functions used. In the second part the results refer to a transient analysis, considering the valves and piston movement, at 1500 rpm. It is observed the great demand on time step required is observed for the real transient, demonstrating that, for this speed, the smallest time step used, 0.05º (5.5555E-6 s), is still insufficient for some moments of the cycle. Also regarding the time step, it is possible to notice a greater influence in the discharge coefficient than in the swirl ratio. The strong dependence of the turbulence model on the results is once again confirmed, as expected, since the hypotheses about the physics of the phenomenon are different in each model. The results, regarding the wall treatment, presented no significant differences, when applied together with the SST k-ω turbulence model.