Turbulence vs Viscosity

chalmers

Turbulence is a flow status while viscosity is the property of the fluid, so if there is a flow (surely there must be a fluid to flow), the viscosity is concerned.
Turbulence happens when the viscous stress can not dampen the instability of the fluid particle or control volume. Surely the instabily of particle motion is related with the fluctuation of teh particles, so called the Reynolds stress. So whether the flow is laminar or turbulence depends on the relative value of viscous stress and the Reynolds stress.
I do not know much about vortex dynamics. But as I think the vortexes are generated to dissipate the energy. Large eddies feed the energy to the smaller eddies and it keeps on going until the viscous effect (which is big enough to overcome the Reynolds stress at a certain small scale, Komogorov scale) dissipates the energy into internal energy. The energy dissipation also happens in large scales but mainly at the scales around the Komogorov scale.
Similar situation happens in the turbulent boundary layer. As the distance normal to the wall decreases, the viscous effect becomes dominant comparing to the Reynolds stress, so the flow becomes laminar.
The viscosity is a fluid property. Viscosity times the strain rate tensor (Newtonian fluid) represents the viscous stress, which means a fluid particle with small viscosity can also have the high local viscous stress if the local velocity gradient is high.

海上钢琴师

楼上所说是目前流行的关于流场中能量耗散的一种说法，即大涡从主流获取能量，并通过级联（cascade）逐级向下传递能量，然后在最小的粘度尺度上将机械能耗散为热能。不过从能量损失的机理上看，并非只有分子间的交换过程才是唯一的机械能耗散机制，大的漩涡之间的碰撞也可以发生能量损失，这就是我们教科书上经常提到的局部损失问题。大尺度的旋涡之间的相互碰撞、参混过程中，必然包含着机械能的耗散，而并非一定要逐级传递给小旋涡才能发生耗散。而且即使从机械能传递的机制上看，除了大旋涡向小旋涡的正向传递，必然也有小旋涡向大旋涡的反向级联过程。总之，流体内部的机械能传递和耗散机制，并非仅有一个树状、正向的机制，尽管那可能是一个占主导地位的机制。

guzi1112

希望继续深入，期待............