  The velocities of the same fluid particle observed in two different reference
systems are two different quantities and they are not equal when the two
reference systems have translational and rotational movements relative to each
other. Thus, the velocity is variant. But, we prove that the divergences of the
two different velocities are always equal, which implies that the divergence of
velocity is invariant. Additionally, the strain rate tensor and the gradient of
temperature are invariant but, the vorticity and gradient of velocity are
variant. Only the invariant quantities are employed to construct the
constitutive equations used to calculate the stress tensor and heat flux
density, which are objective quantities and thus independent of the reference
system. Consequently, the forms of constitutive equations keep unchanged when
the corresponding governing equations are transformed between different
reference systems. Additionally, we prove that the stress is a second-order
tensor since its components in different reference systems satisfy the
transformation relationship.
