Turbulence is a phenomenon that we experience in our every day life. Understanding turbulent flow remains a challenge for both theory and experiment. Turbulence occurs when the viscous forces in a fluid become smaller when compared with the inertial forces. Therefore, the perturbations that are generated in the fluid from the boundary are not dumped out. Instead, they grow and the flow beaks up into eddies of various sizes which are carried out by the flow.
The question that I would like to ask is:
For which reason is the mixing process enhanced by turbulence? And why does the drag force present the same feature? Thank you!
Simply because the mixing process, the main characteristic of turbulence, not only mixing mass , but also heat and momentum.
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When there is of momentum transfer in energy regions with greater momentum to regions with lower momentum this process transfers energy from large eddies to smaller, as large vortices have more energy and less dissipation rate, the transfer increases dissipation of the flow (ie sure you already know).
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With increasing the dissipation rate increase drag forces, because the transfer of energy from more remote areas of the edges closest to induce higher gradient areas both in small vortices in the flow as in global higher average velocity gradients along the wall increasing stress and consequently the viscous drag.
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Attention: When talking that the inertial forces are dominant in relation viscous, does not mean that the latter decrease, on the contrary, when the turbulence the viscous dissipation also increases (the only possible way to dissipate energy, transforming into momentum heat).
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It's a somewhat circular reasoning, but it is the classical interpretation.
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If you look in some book that describes the mixing length theory of Prandtl (the oldest books) will see a more accurate description.
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