Assessing openfoam for DNS of turbulent non-Newtonian flow in a pipe

Transition and turbulence in shear-thinning non-Newtonian fluids are not fully understood yet they can have important application in industrial processes in the chemical, polymer and mineral processing industries. Most existing DNS studies on shear-thinning fluids apply only to simple geometries such as pipes and channels and use bespoke computer codes that are not generally applicable to complex geometries. Therefore, a more generalised DNS approach is needed for modelling turbulence and transition in shear-thinning fluids. The widely used open source CFD library OpenFOAM has recently shown great potential in DNS of turbulent flow. In this study, we conduct benchmark testing on OpenFOAM for DNS of turbulent Newtonian and shear-thinning non-Newtonian flow in a pipe. Results predicted by OpenFOAM for DNS of shear-thinning fluids are assessed by comparing with those obtained by a validated spectral element DNS code Semtex. For Newtonian fluids, we find the mean flow profiles predicted by OpenFOAM correspond very well with Semtex results and experimental data. However, for shear-thinning fluids, OpenFOAM predicts the flow being more transitional with both lower radial and azimuthal turbulence intensities than Semtex results. OpenFOAM does not capture the near wall structure as well as Semtex due to its comparatively lower second-order accuracy and the maximum difference likely to be encountered is in the peak value of turbulence intensities which can be as high as 15%. Despite the lower second order spatial and temporal discretisation schemes applied in OpenFOAM, the study reveals OpenFOAM is still capable to provide reasonably good DNS results in modelling turbulent canonical flow for engineering grade simulations.