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Computational fluid dynamics (CFD) has been widely used to simulate turbulent flows. Although an increased availability of computational resources has enabled high-fidelity simulations (e.g. large eddy simulation and direct numerical simulation) of turbulent flows, the Reynolds-Averaged Navier-Stokes (RANS) equations based models are still the dominant tools for industrial applications. However, the predictive capability of RANS models is limited by potential inaccuracies driven by hypotheses in the Reynolds stress closure. With the ever-increasing use of RANS simulations in mission-critical applications, the estimation and reduction of model-form uncertainties in RANS models have attracted attention in the turbulence modeling community. In this work, I focus on estimating uncertainties stemming from the RANS turbulence closure and calibrating discrepancies in the modeled Reynolds stresses to improve the predictive capability of RANS models. Both on-line and off-line data are utilized to achieve this goal. The main contributions of this dissertation can be summarized as follows: First, a physics-based, data-driven Bayesian framework is developed for estimating and reducing model-form uncertainties in RANS simulations. An iterative ensemble Kalman method is employed to assimilate sparse on-line measurement data and empirical prior knowledge for a full-field inversion. The merits of incorporating prior knowledge and physical constraints in calibrating RANS model discrepancies are demonstrated and discussed. Second, a random matrix theoretic framework is proposed for estimating model-form uncertainties in RANS simulations. Maximum entropy principle is employed to identify the probability distribution that satisfies given constraints but without introducing artificial information. Objective prior perturbations of RANS-predicted Reynolds stresses in physical projections are provided based on comparisons between physics-based and random matrix theoretic approaches. Finally, a physics-informed, machine learning framework towards predictive RANS turbulence modeling is proposed. The functional forms of model discrepancies with respect to mean flow features are extracted from the off-line database of closely related flows based on machine learning algorithms. The RANS-modeled Reynolds stresses of prediction flows can be significantly improved by the trained discrepancy function, which is an important step towards the predictive turbulence modeling. / Ph. D.

Em função de recentes desenvolvimentos no processo de cálculo de sistemas térmicos de proteção contra gelo em aerofólios, foi identificada a importância da camada limite transicional para a correta previsão do coeficiente de troca de calor externo. O presente trabalho discute os métodos utilizados na modelagem numérica de turbulência e transição, testa a capacidade de um modelo k-epsilon de baixo Reynolds avaliar corretamente a transição para o escoamento em uma placa plana bidimensional e mostra que o acoplamento de uma função de transporte de intermitência ao modelo de turbulência de duas equações é uma alternativa eficiente para se aumentar a qualidade da previsão da transição calculada. A filtragem pela variável de intermitência atua corrigindo os níveis de turbulência fornecidos pelo modelo de duas equações durante a transição. Entre os atrativos da função de transporte de intermitência estão a simplicidade das condições de contorno e o fato de serem expansíveis para escoamentos tridimensionais. Uma equação de transporte de intermitência é implementada e acoplada com um modelo de duas equações do programa Fluent. O modelo é testado para três níveis de turbulência do escoamento livre. Os resultados obtidos são comparados com experimentos e com aqueles fornecidos pelo modelo original sem modificações. As distribuições do coeficiente de atrito e de fator de forma ao longo da placa com a função de intermitência mostram boa concordância com os valores verificados experimentalmente, apresentando significativa melhora em relação àqueles obtidos com o modelo \\ke\\ baixo Reynolds. / As a result of recent developments in the field of thermal anti-ice protection in airfoils, was ideintified the relevance of accurately modeling the transitional boundary layer in order to predict the external heat transfer coefficient. The present work reviews the existing alternatives to turbulence and transition modeling, tests the ability of a low-Reynolds \\ke\\ model in the evaluation of transition for a bidimensional flat plate and shows that the coupling of an intemittency transport function with a two equation turbulence model improves the prediction of transition. The filtering with the intermittency function acts correcting the turbulence intensity levels provided by the two equation model during transition. The simple boundary conditions and the expandability to tridimensional flows are other important characteristics of the intermittency functions. An intermittency transport equation is implemented and tested with a two equation turbulence model of the software Fluent. The model is tested for three freestream turbulence levels and the results are compared with experiments and with those obtained with the original model without modifications. The heat transfer coefficient and boundary layer shape factor evolution calculated with the coupled intermittency transport function show good agreement with experiments, improving the quality of the results of the original model.

In this thesis study, Detached Eddy Simulation turbulence model is studied in two dimension mainly for flow over single element airfoils in high Reynolds numbers to gain experience with model before applying it to a three dimensional simulations. For this aim, Spalart-Allmaras and standard DES ,DES97, turbulence models are implemented to parallel, viscous, hybrid grid flow solver. The flow solver ,Set2d, is written in FORTRAN language. The Navier-Stokes equations are discretized by first order accurately cell centered finite volume method and solved explicitly by using Runge-Kutta dual time integration technique. Inviscid fluxes are computed using Roe flux difference splitting method. The numerical simulations are performed in parallel environment using domain decomposition and PVM library routines for inter-process communications. To take into account the effect of unsteadyness after the convergence is ensured by local time stepping technique for four order magnitude drop in density residual, global time stepping is applied for 20000 iterations. The solution algorithm is validated aganist the numerical and experimental studies for single element airfoils in subsonic and transonic flows. It is seen that Spalart-Allmaras and DES97 turbulence models give the same results in the non-seperated flows. Grey area is investigated by changing $C_{DES}$ coefficient. Modeled Stress Depletion which cause reduction of eddy viscosity is observed.

Em função de recentes desenvolvimentos no processo de cálculo de sistemas térmicos de proteção contra gelo em aerofólios, foi identificada a importância da camada limite transicional para a correta previsão do coeficiente de troca de calor externo. O presente trabalho discute os métodos utilizados na modelagem numérica de turbulência e transição, testa a capacidade de um modelo k-epsilon de baixo Reynolds avaliar corretamente a transição para o escoamento em uma placa plana bidimensional e mostra que o acoplamento de uma função de transporte de intermitência ao modelo de turbulência de duas equações é uma alternativa eficiente para se aumentar a qualidade da previsão da transição calculada. A filtragem pela variável de intermitência atua corrigindo os níveis de turbulência fornecidos pelo modelo de duas equações durante a transição. Entre os atrativos da função de transporte de intermitência estão a simplicidade das condições de contorno e o fato de serem expansíveis para escoamentos tridimensionais. Uma equação de transporte de intermitência é implementada e acoplada com um modelo de duas equações do programa Fluent. O modelo é testado para três níveis de turbulência do escoamento livre. Os resultados obtidos são comparados com experimentos e com aqueles fornecidos pelo modelo original sem modificações. As distribuições do coeficiente de atrito e de fator de forma ao longo da placa com a função de intermitência mostram boa concordância com os valores verificados experimentalmente, apresentando significativa melhora em relação àqueles obtidos com o modelo \\ke\\ baixo Reynolds. / As a result of recent developments in the field of thermal anti-ice protection in airfoils, was ideintified the relevance of accurately modeling the transitional boundary layer in order to predict the external heat transfer coefficient. The present work reviews the existing alternatives to turbulence and transition modeling, tests the ability of a low-Reynolds \\ke\\ model in the evaluation of transition for a bidimensional flat plate and shows that the coupling of an intemittency transport function with a two equation turbulence model improves the prediction of transition. The filtering with the intermittency function acts correcting the turbulence intensity levels provided by the two equation model during transition. The simple boundary conditions and the expandability to tridimensional flows are other important characteristics of the intermittency functions. An intermittency transport equation is implemented and tested with a two equation turbulence model of the software Fluent. The model is tested for three freestream turbulence levels and the results are compared with experiments and with those obtained with the original model without modifications. The heat transfer coefficient and boundary layer shape factor evolution calculated with the coupled intermittency transport function show good agreement with experiments, improving the quality of the results of the original model.

No description available.

No description available.

With the continuous growth of the space industry and the introduction of reusable rockets, the number of rocket launches is expected to increase significantly in the following years. During rocket launching, the engine exhaust impinges on the launch structure producing a complex flow field. The rocket jet induces large thermoaerodynamic and acoustic loads on the launch structures and the rocket. This thesis aims to study the physics and numerical considerations behind supersonic flows exhausted from rocket engines. First, the treatment of turbulent compressible flows through the Favre-averaged equations and the SST k-ω model are studied. Next, the numerical modeling of the problem, including solver and meshing theory is presented. Then, a model of a nozzle is explained along with how the performance is assessed to finally design a M=3 two-dimensional nozzle using the method of characteristics (MOC). The two-dimensional results are validated using Ansys Fluent, and the same geometry is used for the following axisymmetrical problems, which include the study of a free and impinging jet. The free jet problem serves to study how the nozzle behaves in a two-dimensional axisymmetric problem and to validate the impinging jet results. To obtain the results, RANS-based simulations of a cold, over-expanded jet with adiabatic walls are performed. Empirical formulas were used to verify the results. Lastly, the impinging jet problem is simulated using the same inlet boundary conditions as for the free jet. The impact that the plate distance to the exit of the nozzle has on the position of the shock waves when the jet impinges on the flat surface is assessed. Finally, an optimization of the shape of a wedge to minimize the maximum turbulence kinetic energy produced during steady-state simulation is carried out. As an appendix to the work, an aeroacoustic study of the impinging jet at 4De distance is presented. The results show the direction of propagation of the acoustic waves but due to the lack of acoustic quality of the mesh, the predicted sound pressure levels do not match the expected behavior.

Internal combustion engines, electric motors and batteries generate a significant amount of heat during operation that needs to be extracted by cooling systems. A cooling system is designed and installed to extract the generated heat and maintain the system temperature in an optimal range. Overheating has several unfavorable outcomes such as less durability and lower energy efficiency. The cooling system consists of several components such as hoses, flow splitters, valves, heat exchangers, coolant, pump, etc. The coolant, as the working fluid, is pumped to different heat generator component to enable the cooling down process. Computational Fluid Dynamics (CFD) is a powerful and cost efficient tool to simulate the cooling processes, design, and evaluate the performance of a cooling system. Generally, one dimensional CFD is a common approach to interpret and explain the cooling processes in the automotive industry due to its high flexibility and computational cost efficiency. Also, three dimensional CFD is employed whenever it is required to study complex physical phenomena and provide detailed information. Additionally, it is possible to couple one dimensional and three dimensional CFD approaches to simulate cooling processes. Not only is the coupled 1D-3D CFD approach able to capture complicated physical processes but also is flexible and cost efficient. The objective of this master thesis is to implement 1D-3D CFD coupled simulation on internal combustion engines’ cooling system and evaluate the advantages and disadvantages of this method. The performance of this method is examined in different case studies with different flow and geometrical characteristics. The effect of various turbulence models and numerical settings are investigated on the quality of the coupled simulations’ results. The coupled simulations are carried out using GT-SUITE and STAR-CCM+ software. The performed simulations show that the coupling method is a convenient approach which is able to capture detailed physics with high precision requiring reasonable computational costs. The results of the coupled simulations depict agreement with the uncoupled 1D CFD simulations, although some discrepancies are observed in complex case studies. Also, it is shown that the coupled simulations are sensitive to numerical settings and physical models, consequently, the case setup should be optimized carefully. / Förbränningsmotorer, elmotorer och batterier genererar en betydande mängd värme under drift som behöver extraheras av kylsystem. Ett kylsystem är utformat och installerat för att extrahera den genererade värmen och hålla systemtemperaturen i ett optimalt intervall. Överhettning har flera ogynnsamma följder, som mindre hållbarhet och lägre energieffektivitet. Kylsystemet består av flera komponenter, till exempel slangar, flödesdelare, ventiler, värmeväxlare, kylvätska, pump etc. Kylvätskan, som arbetsvätska pumpas till olika värmegenerator-komponenter för att möjliggöra nedkylningsprocessen.Computational Fluid Dynamics (CFD) är ett kraftfullt och kostnadseffektivt verktyg för att simulera kylprocesserna, utforma och utvärdera prestanda för ett kylsystem. I allmänhet är endimensionell CFD en vanlig metod för att tolka och förklara kylningsprocesserna i bilindustrin på grund av dess höga flexibilitet och beräkningseffektivitet. Dessutom används tredimensionell CFD när det krävs, för att studera komplexa fysiska fenomen och tillhandahålla detaljerad information. Dessutom är det möjligt att koppla ihop en- och tredimensionell CFD-metod för att simulera kylningsprocesser. Inte bara är den kopplade 1D-3D CFD-metoden möjlig för att betrakta komplicerade fysiska processer, utan är även flexibel och kostnadseffektiv.Syftet med detta examensarbete är att implementera 1D-3D CFD kopplad simulering på förbränningsmotorns kylsystem och utvärdera fördelarna och nackdelarna med denna metod. Uppträdandet av denna metod undersöks i olika fallstudier med olika flöde och geometriska egenskaper. Effekterna av olika turbulensmodeller och numeriska inställningar undersöks genom kvaliteten på resultaten hos kopplingens simuleringar. De kopplade simuleringarna utförs med hjälp av mjukvaran GT-SUITE och STAR CCM +.De utförda simuleringarna visar att kopplingsmetoden är ett bekvämt tillvägagångssätt som kan fånga detaljerad fysik med hög precision till rimliga beräkningskostnader. Resultaten av de kopplade simuleringarna visar överensstämmelse med de frikopplade 1D CFD-simuleringarna, även om vissa avvikelser observeras i komplexa fallstudier. Det visas också att de kopplade simuleringarna är känsliga för numeriska inställningar och fysiska modeller, därför bör fallinställningen optimeras noggrant.