主要區別–動態粘度與運動粘度

粘度對於任何依賴於流體流動的過程都是非常重要的。通常引用兩種粘度：動態粘度和運動粘度。動態粘度和運動粘度之間的主要區別在於，動態粘度是一種測量流體流動困難程度的方法，而運動粘度是流體的動態粘度除以其密度。

什麼是動態粘度(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.

不同層之間的力產生一種剪下應力，因為力與層平行。因此，動態粘度也稱為剪下粘度。在其他情況下，動態粘度也稱為絕對粘度。測量動態粘度的國際單位是帕斯卡秒（Pa s）。然而，測量粘度最常用的單位是釐泊斯（cP）。1000 cP=1帕/秒。

什麼是運動粘度(kinematic viscosity)？

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

運動粘度的國際單位是m2 s-1。然而，測量運動粘度更常用的單位是釐斯托克（cSt）。106 cSt=1 m2 s-1。註意，由於粘度取決於溫度（液體中粘度隨溫度升高而降低，而氣體溫度升高時粘度降低）。引用物質的粘度時，應指明溫度。

動態(dynamic)和運動粘度(kinematic viscosity)的區別

密度的影響

動態粘度和運動粘度都測量流體流動的困難程度。運動粘度是用密度來衡量的，而動態粘度則不是。

計量單位

動態粘度的國際單位是帕斯。更常見的測量單位是釐泊（cP）。

運動粘度的國際單位是m2 s-1。通常用釐斯托克（cSt）來測量。

Image Courtesy

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