Large-eddy simulation of turbulent non-Newtonian flows: A comparison with state-of-the-art RANS closures

Abstract

In the present research, a comparative study of improved Large-Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) closures are carried out with the focus on the influence of terms accounting for the effect of Non-Newtonian (NN) power-law fluid rheology. Using different available databases for shear-thinning turbulent pipe flows, a new set of calibrated constants is proposed for the four-equation k-ε-ζ-f RANS model which results in improved overall performance compared to the original constants. Analyzing the results, the accuracy of this model in comparison with the conventional RANS models, to the first degree, is attributed to its intrinsic ability to account for the wall damping effect, which resulted in about 50% improvement in turbulent kinetic energy prediction with respect to the other RANS models, and then, to the extra NN terms leading to the more accurate prediction of the drag-reduction phenomenon. However, the sensitivity of this model results to the model constants is high which can question the validity of the constants for more practical high-Reynolds-number problems. On the other hand, LES models provided more reliable results with the best performance shown by the Dynamic Lagrangian (DL) SGS closure, especially in predicting mean apparent viscosity and second-order statistics. Both NN k-ε-ζ-f RANS and DL LES models demonstrated great potential for non-Newtonian turbulent flows and the choice between them would be based on the level of accuracy required and the limitation of computational costs.