主要区别–动态粘度与运动粘度

粘度对于任何依赖于流体流动的过程都是非常重要的。通常引用两种粘度：动态粘度和运动粘度。动态粘度和运动粘度之间的主要区别在于，动态粘度是一种测量流体流动困难程度的方法，而运动粘度是流体的动态粘度除以其密度。

什么是动态粘度(dynamic viscosity)？

Whenever a fluid flows against a surface, the different layers of fluid exert frictional forces between each other, causing them to flow at different speeds. A force needs to be applied to a layer of fluid in order to make it flow at a c***tant speed relative to any other layer. The force  required to move a layer of fluid in this way is related to the velocity at which the fluid layer will move by the equation:

where  is the area of the layer and  is the distance between the layers.  is a c***tant of proportionality, and it is known as the dynamic viscosity of the fluid. In this sense viscosity is a measurement of how difficult it is to make a fluid flow. Note that the above equation is only valid for so-called Newtonian fluids. Non-Newtonian fluids cannot be described with one value for viscosity.

Honey is more viscous than water, and so it is more difficult to get it flowing.

不同层之间的力产生一种剪切应力，因为力与层平行。因此，动态粘度也称为剪切粘度。在其他情况下，动态粘度也称为绝对粘度。测量动态粘度的国际单位是帕斯卡秒（Pa s）。然而，测量粘度最常用的单位是厘泊斯（cP）。1000 cP=1帕/秒。

什么是运动粘度(kinematic viscosity)？

Kinematic viscosity () of a fluid is the ratio of the fluid’s dynamic viscosity  to its density :

运动粘度的国际单位是m2 s-1。然而，测量运动粘度更常用的单位是厘斯托克（cSt）。106 cSt=1 m2 s-1。注意，由于粘度取决于温度（液体中粘度随温度升高而降低，而气体温度升高时粘度降低）。引用物质的粘度时，应指明温度。

动态(dynamic)和运动粘度(kinematic viscosity)的区别

密度的影响

动态粘度和运动粘度都测量流体流动的困难程度。运动粘度是用密度来衡量的，而动态粘度则不是。

计量单位

动态粘度的国际单位是帕斯。更常见的测量单位是厘泊（cP）。

运动粘度的国际单位是m2 s-1。通常用厘斯托克（cSt）来测量。

Image Courtesy

“Viscosity Manifest” by Beny Shlevich (Own work) [CC BY 2.0], via flickr