Abstract
AbstractOscillating grids are frequently used with water and Newtonian fluids to generate controlled turbulence and mixing. Yet, their use with shear thinning fluids still requires experimental characterization. Proper orthogonal decomposition (POD) is applied to PIV measurements of the flow generated by an oscillating grid in water and a shear thinning dilute polymer solution (DPS) of xanthan gum. The aims are to investigate the ability of POD to isolate periodic flow structures, and to use it to describe the effects of the shear thinning property. A dominance of the low order POD modes is evidenced in DPS. The methods applied in blade stirred tanks to identify oscillatory motion fail here. However, a strong mode coupling in the grid swept region is observed, determined by the working fluid and by an underlying chaotic nature of the flow. Possibilities of reconstructing turbulence properties using high order modes are discussed.
Highlights
INTRODUCTION AND BACKGROUNDImproving the understanding of the flow dynamics of non-Newtonian fluids is a current goal for many industrial and environmental applications
Modes 1 and 2 in the swept zone (SZ) region are structures according to the grid design: Six regions of high magnitude corresponding to the six mesh “holes” are visible
Even though the mean velocity field has been subtracted from instantaneous fields prior to Proper orthogonal decomposition (POD) decomposition, modes 1 and 2 in the full tank region (FT) region show a magnitude field somehow similar to the mean flow intensity displayed in Reference 46 in both water and dilute polymer solution (DPS)
Summary
Improving the understanding of the flow dynamics of non-Newtonian fluids is a current goal for many industrial and environmental applications. Oscillating grid apparatus have been commonly used in experimental fluid mechanics since the seminal works of.[20,21] Numerous applications can be found in the literature, among which the study of interactions between turbulence and solid impermeable boundaries,[23,24] of turbulence and gas–liquid mass transfer at a free surface,[19,25,26,27,28] in stratified media,[20,21,29,30] or to study the behavior of bubbles, cells, fibers, and flocculation aggregates suspended in a turbulent liquid phase.[31,32,33,34,35] Oscillating grid stirred tanks are tools that allow to generate and study controlled turbulence They can be used as reference cases to help improve the understanding of conventional (bio)chemical reactors. This article comes as a complement to the specific analysis of turbulence properties in the tank,[46] and to the phase resolved analysis of turbulence in the neighborhood of the grid[40] both presented in Reference 53
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