A complementary approach that also takes into account the order of magnitude of vari- ables is to simplify the governing equations by defining “characteristic” quantities and then dividing all properties by those to make them “non-dimensional”. This way, the non-dimensional quantities that enter the equation on their own should all be of order unity so that the resulting collection of parameters in some part of the equation measures their relative importance.
A classic example for this is based on the Navier Stokes equation for an incompress- ible, Newtonian fluid. When body forces driving flow are due to temperature T fluctua- tions in (the Earth’s) gravitational field