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Geometric similarity means similarity of linear size, e. g.
length L and diameter d for object and its model.
The ratio of similar sizes of an object and its model is
called constants of similarity, for example:

н  с  соnst
  (3.1)
м
Constant of similarity с is scale (transitional) multiplier,

which shows how many times sizes need to be changed, in order
to make model similar to the object.
If the ratio between similar intervals of process, that
occurs in two systems, is constant, then the process follows the
temporal similarity.
Physical similarity of processes at similar moment of
time requires of providing permanence of proportion of physical
values, which characterize the process. It can be expressed
through constant and invariant of similarities. Invariants of
similarities, which are dimensionless complex of heterogeneous
variables, called criteria or numbers of similarities.
Hydrodynamic criteria of similarity include criteria of
Reynolds, Euler, Darcy.
The most common are such criteria of similarity:
Reynolds criteria
 d
Re  , (3.2)

Froude criteria
 2
Fr  , (3.3)
g d 
Archimedes criteria

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