Overview
ISO 80000-11:2019 - Quantities and units - Part 11: Characteristic numbers is the second-edition international standard that defines names, symbols and clear definitions for characteristic numbers used to describe transport and transfer phenomena. Characteristic numbers are physical quantities of unit one (commonly called “dimensionless” numbers) that express ratios of forces, energies or characteristic times and are essential for similarity, scaling and modelling in engineering and science.
This part of the ISO 80000 series groups characteristic numbers by process type (momentum transfer, heat transfer, mass transfer in binary mixtures, constants of matter, magnetohydrodynamics and miscellaneous) and expands the list of items from the previous edition. The document is normative-free and is intended as a terminological and definitional reference.
Key topics covered
- Definitions and symbols for widely used characteristic numbers (now increased to a larger set across Clauses 4–9).
- Grouped clauses for common physical domains:
- Momentum transfer (Reynolds, Euler, Froude, Mach, Knudsen, Strouhal, Weber, Grashof, drag/lift/thrust coefficients, etc.)
- Heat transfer and related characteristic numbers (Rayleigh, Nusselt, Biot, Prandtl are examples associated with heat transfer clauses)
- Mass transfer in binary mixtures and associated dimensionless numbers
- Magnetohydrodynamics and constants of matter
- Clear, worded definitions that explain what each number represents (e.g., Reynolds number as the quotient of inertial and viscous forces).
- Notes on common variants, deprecated forms and typical uses (e.g., when a square root or inverse form appears in literature).
Practical applications and users
ISO 80000-11 is valuable for professionals and organizations that require consistent terminology and definitions for modelling, testing and reporting:
- Aerospace and automotive engineers (aerodynamics, drag/lift, Mach and Reynolds scaling)
- Mechanical and civil engineers (fluid systems, pipe flow, Dean and Euler numbers)
- Chemical and process engineers (mass and heat transfer similarity, binary mixtures)
- Research scientists and educators using similarity analysis, dimensional analysis and experimental scaling
- Standards bodies and document authors who need harmonized definitions across technical documentation
Using ISO 80000-11 ensures consistent communication of characteristic numbers in technical reports, simulations, experimental data and design specifications.
Related standards
- Other parts of the ISO 80000 series (e.g., ISO 80000-3, -4, -5, -8, -9, -12) are cross-referenced for underlying quantities (length, mass density, viscosity, temperature, speed of sound, mean free path, etc.). Consult the ISO Online Browsing Platform or the ISO member body for access and implementation guidance.
Keywords: ISO 80000-11:2019, characteristic numbers, dimensionless quantities, Reynolds number, Mach number, fluid dynamics, transport phenomena, heat transfer, mass transfer, standardization.
