Another example is Fourier's law of diffusion of thermal energy given as

where is the diffusive heat flux vector, the temperature gradient and the thermal conductivity. The usefulness of this relationship depends on the availability of the value of of the material being studied. This property in turn must be determined from laboratory experiments.

Besides material properties, certain flows admit universal behaviour independent of the choice of the fluid. For example, in a fully developed turbulent boundary-layer the mixing length scale as

where is the distance from the wall, is boundary-layer thickness and and are universal constants that represent the state of the flow. These constants must be determined from laboratory experiments.