R

Reynolds number:
The Reynolds number is a measure for the importance of viscous friction in a flow, related to the speed of the flow. For low Reynolds numbers, the flow speed is low and/or the fluid is very viscous, and friction is important. For high Reynolds numbers, the speed is high, and friction is unimportant. Small Reynolds numbers play a role in describing small particles in slow flows, such as microorganisms and blood cells. Flows with high Reynolds numbers are important in aerodynamics. To give an example: a person on a bike, pedaling away at a leisurely speed of 6 meters per second (13 miles per hour), already gives a Reynolds number of about 100,000.