https://repositorio.ufu.br/handle/123456789/5467 2026-08-23T03:12:58Z 2026-08-23T03:12:58Z https://repositorio.ufu.br/handle/123456789/49405 2026-08-11T06:29:10Z 2026-08-04T00:00:00Z

Title: Modelagem matemática e computacional de escoamentos turbulentos usando a metodologia mista pseudoespectral de fourier e fronteira imersa Abstract: Turbulence in fluid flows is one of the most complex and relevant physical phenomena in engineering, and its computational modeling demands high-fidelity and computationally efficient methodologies. In this context, the present thesis proposes the development, verification, and validation of two novel three-dimensional computational codes based on the IMERSPEC methodology, a hybridization of the Fourier Pseudospectral Method (MPF) and the Immersed Boundary Method (MFI), for the simulation of laminar and turbulent complex flows in the presence of walls. The developed codes, referred to as IMERSPEC MPI and IMERSPEC CUDA, are distinguished by their parallelization strategies: the former employs the MPI library with parallelization along two of the three flow directions, while the latter exploits heterogeneous CPU/GPU parallelism through the CUDA API, performing all computations entirely on the graphics processing unit. Both codes were computationally verified using the Method of Manufactured Solutions (MSM) for the isolated MPF and for the MPF coupled with the MFI in coincident and non-coincident grid configurations, achieving spectral accuracy and fourth-order convergence, respectively. Additionally, the Spectral Direct Imposition Method (MIED) was proposed and investigated as a fully spectral alternative to the MDFM for non-coincident grids, whose results indicated lower accuracy than the MDFM at comparable computational cost. The physical validation of both codes was carried out through the simulation of laminar flow over a backward-facing step, a classical complex flow benchmark, with results consistent with experimental reference data. Comparing the two codes under identical simulation conditions on different hardware configurations, the IMERSPEC CUDA outperformed the IMERSPEC MPI across all evaluated mesh sizes, achieving a computational cost reduction of approximately twenty-nine times for the laminar case. It is emphasized that this result is obtained when comparing the IMERSPEC MPI executed on a cluster with 64 processes and the IMERSPEC CUDA running on the NVIDIA A100 GPU. On the other hand, it is noted that optimal execution parameters for the IMERSPEC CUDA code must be determined according to the GPU to be used, i.e., the optimal execution parameters are hardware-dependent. Based on the selection of the higher-performance code, subsequent developments for Large Eddy Simulation (LES) modeling were conducted exclusively with the IMERSPEC CUDA. In the turbulent flow simulations, the interaction between the IMERSPEC methodology and the LES approach was investigated, identifying and correcting implementation aspects that prevented the correct reproduction of turbulent flow, such as the restructuring of the complementary domain, the adoption of an exponential filter for the dealiasing procedure, and the definition of the top boundary conditions of the computational domain. Three functional subgrid-scale (SGS) closure models were implemented and compared for turbulent flow over a backward-facing step at Reynolds number Re_H = 5100: the classical Smagorinsky model with Van Driest damping functions, the WALE model (Wall-Adapting Local Eddy-viscosity), and the Germano-Lilly dynamic model. Results obtained with meshes of $32\times128\times256$ and $64\times256\times512$ collocation points were compared against computational and experimental reference data. The WALE model demonstrated the best overall performance, with an error of only 0,5% in the reattachment length estimation and the lowest computational cost among the three evaluated models, making it the most suitable choice for the present simulations, although this subgrid-scale model presents mean velocity profiles and Boussinesq-Reynolds mean stress tensor components with significant discrepancies relative to reference data. The Germano-Lilly dynamic model yielded results slightly superior to those of the classical Smagorinsky model, with reattachment length errors of 11% and 13%, respectively, at a higher computational cost. The IMERSPEC CUDA with the LES approach and the Germano-Lilly dynamic model, the most computationally demanding configuration, reproduced 450 s of physical time in less than 72 hours on the NVIDIA A100 GPU, demonstrating the viability of the methodology for high-order simulations within competitive computational time.

2026-08-04T00:00:00Z https://repositorio.ufu.br/handle/123456789/49241 2026-08-01T06:28:49Z 2026-03-31T00:00:00Z

Title: Experimental study on interlayer temperatures in HSLA steel wire arc additive manufacturing aiming for self-tempering Abstract: Directed Energy Additive Manufacturing (GMA-DED) of hardened, high-strength, low-alloy (HSLA) steel components can produce hard, potentially brittle deposits that require subsequent heat treatment to achieve the desired properties. In this thesis, a thermal management strategy for HSLA thin walls manufactured by GMA-DED was developed and validated, in which progressively increasing interlayer (IT) temperatures were actively controlled to keep the entire thin wall at the same temperature and intensify an in situ heat treatment (self-tempering), preserving the wall geometry and avoiding productivity losses due to excessive waiting times between layers. The basic criterion of analysis was to maintain always the same wall width, keeping the wire feed speed constant and adjusting the deposition speed, even at the expense of variations in the heat source's energy per unit length of the deposited layers. Using the hardened ER90S-B3 consumable, the shielding gas for thin-wall deposition under short-circuit transfer was initially selected by combining operational stability metrics with geometry-based criteria, resulting in the choice of the Ar + 8% CO₂ mixture. This gas provided the selected parameters with the highest operational stability and the most consistent wall quality, and was adopted as a reference condition for investigating the influence of IT on the microstructural and geometric evolution of the wall. The Near-Immersed Active Radiation Heating (NIARH) approach was developed, which allowed control of the IT up to 550 °C during thin-walled deposition while maintaining stable weld pool dynamics and short-circuit transfer. An original pyrometry-based system was designed and evaluated to monitor the IT and the cooling rates of the layers. A graphical methodology was developed to estimate the dilution between layers and to predict the architecture of heat treatments applied sequentially to the deposited layers during multiple thermal cycles. The results showed that the thermal history was mainly governed by the IT, counterbalancing the effects of preheating with those of lower deposition energy per unit of layer length (the cooling rates were independent of the IT, although the permanence time of each layer in the tempering temperature range increased). Porosity was measured using Archimedes' principle and showed no significant effect of the TI. However, the harshness showed clear dependence on IT: it remained relatively high and stable at low ITs and decreased as IT increased. The microstructural observations of the walls indicated the predominant presence of bainite, except in the condition with the highest TI, which showed a tendency toward increased carbide formation. Overall, the results demonstrated the feasibility of promoting in situ self-tempering through high, controlled interlayer temperatures in thin walls produced by GMA-DED, offering a practical way to adjust properties while maintaining the process's geometric integrity and stability.

2026-03-31T00:00:00Z https://repositorio.ufu.br/handle/123456789/48919 2026-07-21T06:27:51Z 2026-07-09T00:00:00Z

Title: Proposta de uma metodologia para avaliação de desempenho de fazendas de turbinas hidrocinéticas por métodos de ordem reduzida Abstract: The growing demand for alternative renewable energy sources has increased interest in hydrokinetic turbines as a means of harnessing the high energy density available in riverine and marine currents. To improve energy extraction efficiency and power generation capacity, these devices are commonly deployed in arrays, also known as farms. In this context, the main objective of this work was to develop a methodology for evaluating the performance of hydrokinetic turbine arrays, with an emphasis on the application of low-order models. To this end, the Blade Element Momentum (BEM) method was assessed for the prediction of turbine power and thrust coefficients. This methodology requires lift and drag coefficient curves as functions of the angle of attack for the airfoil composing the turbine blades. Initially, the procedures used to obtain these aerodynamic coefficients were investigated, focusing on their predictive capability. The conventional BEM methodology was validated, and an improvement was proposed. The proposed enhancement consisted of employing the local Reynolds number of each blade section to calculate the lift and drag coefficients, rather than using a single representative Reynolds number evaluated at 70 % of the blade span for the entire blade. This modification was implemented in Fortran, validated, and registered as a new software named HydroBEM. Experimental validation of HydroBEM was performed using two different hydrokinetic turbines with experimental data available in the literature. The first one was a three-bladed turbine of 1.6 m diameter and blades based on a thickened NACA 63-415 airfoil, which was evaluated under two different turbulence intensity conditions. The second turbine had a diameter of 0.444 m and two blades employing the NACA 4415 airfoil. The study of turbine arrays requires accurate knowledge of wake velocity profiles, as these constitute the inflow conditions for downstream turbines. For this purpose, the conventional Jensen wake model and its cosine-modulated formulation were evaluated. Although widely adopted in the literature, these models present important limitations. To overcome these shortcomings, a new hybrid Jensen–Gaussian wake model was proposed. The model was experimentally validated and demonstrated superior performance compared with the standard Jensen formulation. Validation was conducted using literature data for a 0.7 m diameter turbine with three blades based on the NACA 63418 airfoil, as well as experimental data from the 0.444 m diameter turbine with two NACA 4415 blades previously employed in the HydroBEM validation. Finally, the latter turbine was also used to validate the coupled application of HydroBEM and the proposed wake model. The combined methodology provided satisfactory predictions of the performance of coaxial hydrokinetic turbines while maintaining a low computational cost.

2026-07-09T00:00:00Z https://repositorio.ufu.br/handle/123456789/48786 2026-06-27T06:20:15Z 2026-02-23T00:00:00Z

Title: Degradação de revestimentos poliméricos aplicados em tubulações de esgoto Abstract: Urban sewer pipelines operate under severe conditions, including chemical corrosion, microbiological degradation, and abrasive wear. Metallic materials such as ASTM A36 carbon steel are highly susceptible to corrosion, especially in acidic, sulfide-rich environments, leading to material loss and reduced infrastructure service life. To mitigate these effects, the use of polymeric coatings has been widely adopted, although their performance under combined corrosion-abrasion conditions is not yet fully understood. This research proposes an integrated experimental methodology to evaluate the degradation of polymeric coatings applied to ASTM A36 steel, using a synthetic medium representative of urban sewage from Uberlândia (MG). Coal tar-epoxy and Zebron® coatings were tested, with nominal thicknesses of approximately 0.25, 0.50, and 1.00 mm. Electrochemical characterization was performed through open circuit potential (OCP) measurements, potentiodynamic polarization curves, and electrochemical impedance spectroscopy (EIS). Uncoated steel exhibited corrosion potentials below −0.50 V, high corrosion current densities on the order of 10⁻⁴ A·cm⁻², and charge transfer resistances (Rct) below 10² Ω·cm², indicating a high degradation rate. In contrast, polymeric coatings exhibited coating resistance (Rcoating) values of 10⁴ Ω·cm² at higher thicknesses (0.5 mm and 1.0 mm), demonstrating effectiveness in protecting the steel. The Zebron® coating maintained Rcoating values between 10³ and 10⁴ Ω·cm² after 168 h of immersion, especially near 1.00 mm thickness. The coal tar-epoxy coating showed progressive degradation over time, particularly at thicknesses below 0.50 mm. Abrasive wear tests using a rubber wheel in a wet medium revealed greater thickness loss for coal tar-epoxy, reaching approximately 0.05 mm after 8 hours, while Zebron® showed losses below 0.015 mm under similar conditions. The analysis revealed that reduced coating thickness decreases corrosion resistance due to the interaction between abrasion and corrosion, affecting long-term coating durability.

2026-02-23T00:00:00Z