Abstract
Turbulent channel flow simulation of a shear-thinning fluid is considered – see Arosemena et al. (J. Fluid Mech., vol. 908, 2021, p. A43) – and compared with a Newtonian base case to reveal the effects of the shear-dependent rheology on the near-wall structures. Analyses of different flow statistics revealed that, for the shear-thinning fluid case, the streamwise vortices appear to grow in size, depart from the wall and present a lessening in their intensity. Information regarding variations in the quasi-longitudinal vortices is also obtained from three-dimensional structures identified through a normalized $Q$-criterion. With shear-thinning rheology, it is shown that the structures are comprised of wall-attached and -detached families which are taller than for a Newtonian fluid. Also, for a given height, the structures appear to be longer, with approximately the same width and overall larger volume for the shear-thinning fluid case; albeit their fractal dimension remains the same when compared to the Newtonian base case. Moreover, it is observed that the number density of vortical structures decreases with shear-thinning fluid behaviour. These observations, in conjunction with the known changes to the longitudinal velocity structures which appear to be less streaky, more spanwise separated and thickened with shear-thinning rheology, strongly suggest that the near-wall self-sustaining process has been disrupted. As we move slightly away from the wall and with shear-thinning behaviour, the local increase in viscosity seems to lead to less energetic vortices whereas the streaks are provided with an additional source of energy due to fluctuations in viscosity.
Highlights
Turbulence is ever present in nature and in numerous man-made processes
Data of statistically converged turbulent channel flow simulations of Generalized Newtonian (GN) fluids at Reτ = 180 (Arosemena et al 2021) are considered to analyse how the near-wall quasi-streamwise vortices change with shear-thinning rheology compared with the Newtonian base case
The shear-dependent rheology leads to drag reduction amounting to approximately 10 % and the study comprises statistics compiled from flow field variables and from identified three-dimensional vortical structures
Summary
Wall-bounded turbulent flows, in particular, are extremely important for technological applications. Close to 25 % of the energy used in industry is destined to transport fluids in pipes and channels, or to propel vehicles through air or water, and approximately a quarter of that energy is irreversibly dissipated by turbulence in regions close to walls (Jiménez 2013). In several industrial settings, the transported fluid is non-Newtonian. Considering the previous context, this work focuses on purely viscous fluids and is an extension of the turbulent channel flow study of Arosemena, Andersson & Solsvik (2021). Our aim is to explore the changes on the near-wall turbulent structures, in particular the quasi-streamwise vortices, due to shear-thinning fluid behaviour; a non-Newtonian rheology exhibiting drag-reducing features
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