(399d) Effects of Shear Flow Structure on Turbulent Transport and Mixing

One of the main turbulent flow characteristics is its ability to disperse mass particles and heat in a very efficient manner [1]. In general, the multiple length and time scales that characterize the structure of the flow field drive this dispersion. For anisotropic flow fields, such as those in pipes and channels, the effects of viscosity and advection compete with each other as different flow structures contribute to dispersion depending on the distance from the wall and on the properties of the dispersing material. Robert Brodkey and his students designed elegant experiments that allowed the visualization of flow structures as they convected downstream in a turbulent boundary layer [2-4]. They realized that these structures were associated with high Reynolds stresses that were several times larger than the mean around them, revealing that turbulence is produced and sustained through such flow structures. Computations and experiments over several decades cemented the concept of coherent structures and how they produce turbulence [5], including the development of models for the prediction of turbulence based on very large scale motions in the outer flow [6]. While these coherent flow structures produce turbulence, they play a critical role for the transport of matter and energy in shear flows and they enhance mixing, but not all of them have equal importance [7]. Brodkey’s passion to design and conduct careful experiments for understanding the fundamentals of turbulence generation and the analogies between momentum, heat and mass transport has been an inspiration. We have used direct numerical simulations of Poiseuille and Couette flows and Lagrangian computations for the transport of passive scalars in turbulent channel flows to explore the effects of coherent flow structures on turbulent transport [8-11]. In this talk, we will visit the flow effects on the dispersion of passive scalars in view of our simulation results, and will discuss the importance of these concepts in mixing applications [12] and in other cases of turbulence, such as blood flow in medical devices [13].

ACKNOWLEDGEMENTS

The support of NSF under grant 1803014 is gratefully acknowledged as are the use of computing facilities at the University of Oklahoma Supercomputing Center for Education and Research (OSCER) and at XSEDE (under allocation CTS-090025).

REFERENCES

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11. Nguyen, Q., and D.V. Papavassiliou, 2020, “Using helicity to investigate scalar transport in wall turbulence,” Physical Review Fluids, 5(6), 062605m.

12. Nguyen, Q., and D.V. Papavassiliou, 2018, “Scalar mixing in anisotropic turbulent flow,” AIChE Journal, 64(7), pp. 2803-2815.

13. Pham O.L., Feher, S.E., Nguyen, Q.T., and D.V. Papavassiliou, 2022, “Distribution and history of extensional stresses on vWF surrogate molecules in turbulent flow,” Scientific Reports, 12, 171.