Since motion or flow of a fluid is produced by shearing forces, viscosity is associated with fluid motion. The viscosity of a fluid (liquid or gas) is that property which tends to resist a shearing force. Unitless numbers occur very frequently in fluid mechanics.Viscosity Definitions and Methods of Measurement So when you calculate $Re = s L / \nu$, you come out with a pure number that has no units. Note that is the same units as kinematic viscosity. We could have just kept the original two quantities.Īs for the units, the other two quantities in the reynold's number equation are length and velocity. Now it's 3 equations, exactly 1 of which has to do with the type of fluid, and 2 that have to do with the particular situation. Because it comes up so often, and because both density and viscosity are intrinsic to the type of fluid, we simplify the Reynold's number equation to just $Re = s L / \nu$, where $\nu$ is kinematic viscosity. The reynold's number is super important and it comes up all the time in fluid mechanics. Of these four quantities, 2 are intrinsic to a particular fluid (density and dynamic viscosity) and 2 are more to do with the situation (the length and the velocity). The Reynold's number is defined as $Re = \rho s L / \mu$, where $\rho$ is density, $s$ is velocity, L is a length, and $\mu$ is dynamic viscosity. Flows with high reynolds number will behave completely differently than flows with low reynolds number. One of the fundamental distinctions has to do with the ratio of inertia forces to viscous forces, which is called the Reynold's number. To understand what it is, you have to understand that there are many different types of fluid flow, and they behave very differently. Thinking of it as so many $m^2$ of fluid flowing per second is not what it is about. Don't try to think of it as resistance of fluid to flowing, because that's not what it is.
Kinematic viscosity is something totally different. honey has much higher dynamic viscosity than water, or cold motor oil has higher dynamic viscosity than warm motor oil. High dynamic viscosity = more resistance to flow. This is what lay people think of when they think viscosity. Dynamic viscosity represents the resistance of fluid to shear forces as you said.
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