Characteristic numbers are
dimensionless numbers
A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no physical dimension is assigned, with a corresponding SI unit of measurement of one (or 1) ...
used in
fluid dynamics to describe a character of the flow. To compare a real situation (e.g. an
aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engine ...
) with a small-scale model it is necessary to keep the important characteristic numbers the same. Names of these numbers were standardized in
ISO 31
ISO 31 ( Quantities and units, International Organization for Standardization, 1992) is a superseded international standard concerning physical quantities, units of measurement, their interrrelationships and their presentation. It was revised and ...
,
part 12.
Dimensionless numbers
Fluid dynamics
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