Abstract
The study of turbulence in complex fluids is of great interest in many environmental and industrial applications, in which the interactions between liquid phase rheology, turbulence, and other phenomena such as mixing or heat and mass transfer have to be understood. Oscillating grid stirred tanks have been used for many purposes in research involving turbulence. However, the mechanisms of turbulence production by the oscillating grid itself have never been studied, and oscillating grid turbulence (OGT) remained undescribed in non-Newtonian, shear-thinning, dilute polymer solutions until recently (Lacassagne et al., in Phys Fluids 31(8):083,102, 2019). The aim of this paper is to study the influence of the shear-thinning property of dilute polymer solutions (DPS), such as xanthan gum (XG), on mean flow, oscillatory flows, and turbulence around an oscillating grid. Liquid phase velocity is measured by particle image velocimetry (PIV) in a vertical plane above the central grid bar. Mean, oscillatory and turbulent components of the velocity fields are deduced by triple Hussain–Reynolds decomposition based on grid phase-resolved measurements. Outside of the grid swept region, the amplitude of oscillatory fluctuations quickly become negligible compared to that of turbulent fluctuations, and the triple and classical Reynolds decomposition become equivalent. Oscillatory jets and wakes behind the grid and their interactions are visualized. Turbulent (Reynolds) and oscillatory stresses are used to evidence a modification of oscillatory flow and turbulence intensity repartition in and around the grid swept region. Energy transfer terms between mean, oscillatory and turbulent flows are estimated and used to describe turbulence production in the grid swept region. Energy is injected by the grid into the oscillatory component. In water, it is transferred to turbulence mostly inside the grid swept region. In DPS, oscillatory flow persists outside of the grid swept zone. Energy is transferred not only to turbulence , in the grid swept region, and far from the tank’s walls, but also to the mean flow, leading to an enhancement of the latter. Mean flow production and enhancement mechanisms are explainable by oscillatory jet variable symmetry and intensity, and by time- and space-variable viscosity. Backward transfer from turbulence to oscillatory flow is also evidenced in DPS. Finally, using phased root mean square (rms) values of turbulent velocity fluctuations, it is shown that in water, the decay of turbulence intensity behind an oscillating grid can be related to the decay of fixed grid turbulence for specific grid positions, a result no longer valid in DPS.Graphic abstract
Highlights
A multitude of environmental phenomenon and industrial processes involve turbulent flows of complex liquid phases such as suspensions (Cuthbertson et al 2018), multiphase media (Alméras et al 2015), or liquids with complex rheology
The aim of this paper is to study the influence of a shearthinning behavior on mean flow, oscillatory flows, and turbulence around an oscillating grid
For non-time-resolved particle image velocimetry (PIV) measurements of turbulent flows, it is commonly said that a few hundreds of independent fields are sufficient to reach convergence of averaged quantities, and about 1000 is required for secondorder statistical quantities
Summary
A multitude of environmental phenomenon and industrial processes involve turbulent flows of complex liquid phases such as suspensions (Cuthbertson et al 2018), multiphase media (Alméras et al 2015), or liquids with complex rheology. It is commonly said that the jets and wakes behind the grid’s holes and bars interact to generate turbulence (Herlina 2005; Hopfinger and Toly 1976; Thompson and Turner 1975; Voropayev and Fernando 1996), which diffuses away from the grid. It is of great interest for turbulence study since it theoretically creates almost no mean flow and, allows to study the effects of turbulence alone, which is not the case for fixed grid systems. A full characterization of OGT in prismatic tanks came with the pioneer works of Thompson and Turner (1975) and Hopfinger and Toly (1976). Thompson and Turner (1975) studied several types of grids with different mesh sizes M and different bar shapes, and showed that the best homogeneity and intensity of turbulence was achieved with square section bars and a solidity parameter
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
