Engineering

Reynolds Number

Definition

The Reynolds Number is a dimensionless quantity that predicts whether fluid flow will be laminar or turbulent, defined as Re = ρvL/μ (density × velocity × length / dynamic viscosity).

Explanation

Developed by Osborne Reynolds in 1883, it remains a cornerstone of fluid dynamics. Flow with Re below ~2,300 in a pipe is typically laminar and smooth; above ~4,000, it becomes turbulent and chaotic. Engineers use it to scale wind tunnel models: a small model at high velocity can replicate the same flow behavior as a full-sized aircraft.

02

RELATED ARTICLES

Torque Units: Newton-Meters, Pound-Feet, and Applications

From automotive specs to bolt tightening, torque units explained — N·m, lb·ft, kgf·m, and how torque relates to power.

 Pressure Units Explained: Pascal, PSI, Bar, and ATM

A practical guide to pressure units used in weather, engineering, medicine, and diving — with conversion factors and real-world reference points.

 Density Units: kg/m³, g/cm³, and Specific Gravity

A practical guide to density units — how they relate, what common materials weigh per volume, and why objects float or sink.
03

RELATED TERMS

Torque

Torque is the rotational equivalent of force, equal to the applied force multiplied by the perpendicular distance from the pivot point, measured in newton-meters (N·m) or pound-feet (lb·ft).

Stress

Stress is the internal force per unit area within a material resulting from externally applied forces, measured in pascals (Pa) or pounds per square inch (psi).

Strain

Strain is the dimensionless ratio of a material's deformation (change in length) to its original length under applied stress: ε = ΔL/L₀.

Viscosity

Viscosity is a fluid's resistance to deformation or flow, quantifying internal friction between fluid layers; dynamic viscosity is measured in pascal-seconds (Pa·s) or centipoise (cP).

RPM

RPM (revolutions per minute) is a unit of rotational speed measuring how many complete turns an object makes in one minute.