I am not a mechanical engineer, though I did study fluid mechanics in my biomedical engineering past. In fluid flows, the Reynolds number is a dimensionless parameter that describes the ratio of inertial to viscous forces. This relationship is significant because, among other things, it can be used to predict the nature of the flow--whether laminar or turbulent. In biomechanics, fully developed fluid flows are typically of low Reynolds numbers, certainly below the critical threshold at which flow becomes turbulent.
However, in classical mechanics and hydraulics, flows are typically through long, often rigid, pipes. Flows become "fully developed," and can be of much higher Reynolds numbers. In these regimes, assuming one knows the characteristics of the pipe, one can calculate the Darcy friction factor to determine the (pressure) head loss during flow.
Okay, geeking out a bit. The relevance here is that the interplay of Reynolds numbers and Darcy factors can be difficult to deal with; Moody diagrams allow us to relate the two graphically, and to see at a glance whether, given a Darcy Factor and a Reynolds number, flow will be laminar or turbulent for pipes of a specified roughness.
Creating a Moody diagram is no trivial task, though. Or at least, it wasn't until Tom shared his MATLAB code for creating one. Function 'Moody' allows you to specify units (SI or Imperial) and paper size (A4 or Letter), and the name of an output file, and it will create a beautiful Moody diagram for you!
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