# Computational studies of flow in Couette Rheometer and particle interaction in non-Newtonian fluids

Sammanfattning: This work aims to study the flow of non-Newtonian fluids and has two main goals. First, the power-law and Herschel Bulkley fluids are studied in a Couette rheometer. Second, the static and dynamic interaction of dual particles in power-law fluid are investigated.For the first goal, Computational Fluid Dynamics (CFD) is employed to understand the flow behavior of powe-rlaw fluid in the Couette rheometer, and to find more accurate predictor-corrector methods to enhance the accuracy of fluid properties estimation by the measurement. Unlike the previous numerical-based method, the current correction factors take into account the effect of wide gap into a single coefficient besides the effect of the end parts. This novel method, including the effect of the wide gap, can enhance the accuracy up to 16% depending on the fluid behavior and the gap distance. For Herschel Bulkley fluid, Finite Element Method (FEM) is also employed in addition to CFD. It is shown that compared to the analytical solution, the presented method can enhance the accuracy of the yield stress value estimation about 11% and the consistency index about 23%. In this section, the effect of the plug flow on the inverse problem solution of the Couette flow has also been evaluated. By the Couette inverse problem solution and the superposition assumption of the fluid and solid part of the Herschel Bulkley model, it is shown that except Bingham fluid, the superposition assumption introduces significant error to the calculations especially for power-law index less than 0.7 and ratio of yield stress to consistency index less than 50. For shear thickening fluid, one should takes into account the discrepancy introduced by the assumption as well. The discrepancy introduced is in the range of 0 – 6%.The second goal is to study both static and dynamic interaction of the particles in non-Newtonian media for spherical and cubical particle shapes, respectively. To study the static interaction of the particles, the variation of drag and lift coefficients of two fixed spherical particles for various dual sphere configurations in a power-law fluid has been investigated. The results show that the influence of the shear thinning cannot be overlooked even at the higher Reynolds number. The drag and lift coefficients are compared in different states, and the wake strength is assessed in the tandem cases. Repulsion and attraction forces are observed when the direction of the lift forces is changed due to the high and low pressure regions between the spheres. Regarding the dynamic interaction of cubical particles, the results indicate that there is a horizontal separation distance in which the particles behaves close to that of a singular particle. Moreover, increasing the vertical distance would have the result in decrease of the effect of the horizontal separation distance. Increasing the vertical separation distance greater than 3.0D makes the interaction more complex. Kissing, drafting and tumbling are also observed in these simulations.

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