Reynolds Numbers

Reynolds Numbers help us distinguish between laminar and turbulent flows. Laminar flows bear low Reynolds Numbers while turbulent flows have higher Reynolds Numbers. At its core, the Reynolds number of a moving fluid is the dimensionless ratio of its inertial forces to its internal viscous forces. However, the earlier conceptualization of the Reynolds Number is often easier as it simply states that this number dictates how laminar or how turbulent a given flow is.

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In a pipe, the general equation for the Reynolds Number is:

Where Q is the volumetric flow rate, D is the pipe's diameter, nu is the dynamic viscosity, and A is the cross-sectional area of the pipe. In the second equation, since Q = Av, we can quantify the Reynolds Number in terms of fluid flow velocity. This has a very important implication for fluids. If the fluid's velocity is high, the fluid will become extremely turbulent and since D and nu are pretty much constant, the Reynolds Number of a fluid flow is dependent on fluid flow velocity. The general rule is that if the Reynolds Number is greater than 2900, the flow is definitely turbulent whereas if the number is less than 2300, the flow is laminar.