[pt] Compreender a resposta mecânica de materiais dependentes do tempo / histórico de deformação é essencial para várias aplicações industriais, especialmente no transporte por dutos. Esses materiais exibem um comportamento reológico complexo, que influencia enormemente suas características de escoamento. Apesar de décadas de pesquisa, os modelos existentes muitas vezes são incapazes de prever com precisão respostas complexas além do escoamento simples de cisalhamento. Consequentemente, há uma necessidade urgente de investigação para refinar a compreensão e a capacidade preditiva nesse campo, a fim de garantir operações eficientes e seguras de processos industriais envolvendo o escoamento desses materiais. Este trabalho tem como objetivo investigar o recomeço de escoamento de fluidos tixotrópicos por meio de métodos numéricos e experimentais. Especificamente, este estudo envolve a modelagem de uma suspensão de Laponite tixotrópica 1,25 por cento wt usando uma equação constitutiva que representa fielmente o comportamento real do material durante processos que induzem quebra ou reconstrução microestrutural. As simulações numéricas foram utilizadas para analizar processos de reinício de escoamento em dutos usando a suspensão modelada neste trabalho, comparando-a com uma suspensão presente na literatura modelada com o mesmo modelo, objetivando compreender o papel da tixotropia no reinício do escoamento. Além disso, foi realizada uma tentativa de validar a equação constitutiva obtida. A análise numérica indicou que ambos os fluidos exibiram comportamento qualitativo semelhante para a evolução temporal da fluidez; enquanto uma alta tixotropia causou uma transição gradual do repouso para o escoamento, baixa tixotropia resultou em transições súbitas. Altos gradientes de pressão impostos foram associados a menores tempos de reinício e regiões cisalhadas maiores ao longo do domínio espacial para ambos os fluidos. Adicionalmente, para ambos os fluidos, o regime permanente do escoamento foi unicamente definido pelo gradiente de pressão imposto. Apesar das limitações do aparato experimental e das notáveis diferenças entre as previsões numéricas e os resultados experimentais para a suspensão de Laponita formulada neste trabalho, o modelo matemático empregado mostrou-se uma ferramenta útil para estudar o reinício de escoamento de fluidos tixotrópicos. / [en] Understanding the mechanical response of time-/strain history-dependent materials is essential for various industrial applications, notably in pipeline transportation. These materials exhibit complex rheological behavior, that hugely influence their flow characteristics. Despite decades of research, accurately modeling the mechanical behavior of these materials remains a challenge, with existing models often constrained by their inability to predict complex responses beyond simple shear flow configurations. Consequently, there is a pressing need for further investigation to refine our understanding and predictive capabilities in this field, ensuring the efficient and safe operation of industrial processes involving such materials. This work aims to investigate the flow restart of thixotropic fluids through both numerical and experimental methods. Specifically, this study involves modeling a thixotropic Laponite suspension 1.25 por cento wt using a constitutive equation that faithfully accounts the real material s behavior during processes that induce microstructural breakage or construction. Numerical simulations were used to analyze the restart processes in a pipe using the modelled suspension and compare it with a suspension present in the literature modelled with the same mathematical model to understand the role of thixotropy in the flow restart. Additionally, an attempt to validate the constitutive equation obtained was made. The numerical analysis indicated that both fluids exhibited similar qualitative behavior for the temporal fluidity evolution. While high thixotropy caused a gradual transition from rest to flow, low thixotropy resulted in sudden transitions. Higher imposed pressure gradients were associated with earlier resumption of flow and larger sheared regions across the spatial domain for both fluids. Additionally, for both fluids, the steady-state regime was solely defined by the imposed pressure gradient. Despite the limitations of the experimental apparatus and the notable differences between the numerical predictions and experimental results for the Laponite suspension formulated in this work, the mathematical model employed showed to be a useful tool to study the restart flow of thixotropic fluids.

Le transport des bruts paraffiniques, et tout particulièrement leur remise en écoulement après un arrêt, dans de longues conduites sous-marines soumises à de basses températures, peut être difficile du fait de l'augmentation de leur viscosité. Le comportement rhéologique d´une huile paraffinique modèle, possédant des propriétés macroscopiques d'écoulement analogues à celles des bruts paraffiniques, est d'abord analysé en utilisant la vélocimétrie par imagerie par résonance magnétique associée à des mesures de contrainte de cisaillement au sein d'une géométrie Couette. Nous montrons que lors d'un écoulement forcé à température constante le matériau subit une déstructuration irréversible qui dépend de l'intensité du cisaillement. Ainsi la contrainte apparente critique permettant l'écoulement du matériau dépend de l'histoire thermique et d'écoulement subie par le matériau. Nous étudions ensuite le comportement rhéologique complet de deux bruts réels à partir de différents types de tests rhéométriques (fluages, redémarrage, régime permanent, changement brusque de vitesse) pour différentes histoires d'écoulement, notamment pendant la période de refroidissement. Le comportement détaillé du matériau en régime transitoire ainsi observé peut alors être modélisé. De plus les variations du seuil de contrainte en fonction de l'histoire thermique et de l'écoulement sont aussi décrites, ce qui nous donne le champ de contrainte seuil dans la conduite à l'état initial. Le modèle dans son ensemble est finalement implémenté dans un code de calcul pour simuler le redémarrage de l´écoulement d'un brut paraffinique dans une conduite réelle / Transporting waxy crude oils through long pipelines at low temperatures may be challenging, particularly its flow restart after a pipeline shut-in, due to its viscosity increase. The rheological behavior of a model waxy oil with macroscopic flow properties analogous to waxy crude oils is first analyzed using Magnetic Resonance Imaging velocimetry associated to stress measurements in a Couette geometry. While flowing at constant temperature, major irreversible structure break depending on shear intensity are observed. Thus, the critical apparent shear stress beyond which the material flows depends on the thermal and flow histories of the oil. Next, the rheological behavior of two waxy crude oils is studied using rheometrical tests (creep tests, flow restarts, abrupt changes of shear rate and steady flow) after different flow histories, notably during the cooling process. Then, those experimentally observed trends are modeled. Additionally, a comprehensive study of the yield stress in function of flow and temperature histories is presented. It provides an approach for describing the yield stress field inside the pipeline at the flow restart moment. Finally, the entire rheological model is implemented in the computational code for simulating waxy crude oils flow restart of a real scale pipeline

Le transport des bruts paraffiniques, et tout particulièrement leur remise en écoulement après un arrêt, dans de longues conduites sous-marines soumises à de basses températures, peut être difficile du fait de l'augmentation de leur viscosité. Le comportement rhéologique d´une huile paraffinique modèle, possédant des propriétés macroscopiques d'écoulement analogues à celles des bruts paraffiniques, est d'abord analysé en utilisant la vélocimétrie par imagerie par résonance magnétique associée à des mesures de contrainte de cisaillement au sein d'une géométrie Couette. Nous montrons que lors d'un écoulement forcé à température constante le matériau subit une déstructuration irréversible qui dépend de l'intensité du cisaillement. Ainsi la contrainte apparente critique permettant l'écoulement du matériau dépend de l'histoire thermique et d'écoulement subie par le matériau. Nous étudions ensuite le comportement rhéologique complet de deux bruts réels à partir de différents types de tests rhéométriques (fluages, redémarrage, régime permanent, changement brusque de vitesse) pour différentes histoires d'écoulement, notamment pendant la période de refroidissement. Le comportement détaillé du matériau en régime transitoire ainsi observé peut alors être modélisé. De plus les variations du seuil de contrainte en fonction de l'histoire thermique et de l'écoulement sont aussi décrites, ce qui nous donne le champ de contrainte seuil dans la conduite à l'état initial. Le modèle dans son ensemble est finalement implémenté dans un code de calcul pour simuler le redémarrage de l´écoulement d'un brut paraffinique dans une conduite réelle / Transporting waxy crude oils through long pipelines at low temperatures may be challenging, particularly its flow restart after a pipeline shut-in, due to its viscosity increase. The rheological behavior of a model waxy oil with macroscopic flow properties analogous to waxy crude oils is first analyzed using Magnetic Resonance Imaging velocimetry associated to stress measurements in a Couette geometry. While flowing at constant temperature, major irreversible structure break depending on shear intensity are observed. Thus, the critical apparent shear stress beyond which the material flows depends on the thermal and flow histories of the oil. Next, the rheological behavior of two waxy crude oils is studied using rheometrical tests (creep tests, flow restarts, abrupt changes of shear rate and steady flow) after different flow histories, notably during the cooling process. Then, those experimentally observed trends are modeled. Additionally, a comprehensive study of the yield stress in function of flow and temperature histories is presented. It provides an approach for describing the yield stress field inside the pipeline at the flow restart moment. Finally, the entire rheological model is implemented in the computational code for simulating waxy crude oils flow restart of a real scale pipeline