  In order to address the difficulties of classical fluid kinematics in
describing vorticity and the paradox of linear correlation between viscous
force and vorticity in the Navier-Stokes equations, the study examines the
inherent relationship between momentum conservation and energy conservation and
rederives the fundamental equations of fluid flow from new dynamic hypothesis.
The study reveals that momentum conservation and mechanical energy conservation
are the same concept termed in different descriptions. The material derivative
of velocity is to depict the acceleration of fluid particles in Eulerian
perspective. In the study, it is assumed that fluids only obey the theorem of
momentum, which describes the translational motion of fluids. The centroid of
fluid elements can undergo translational motion under a curl force field. In
the hypothesis, the symmetry of stress tensor is determined by the properties
of stress field rather than the conservation of moment of momentum for fluid
elements. The stress tensor is symmetric when stress field is a curl free
field, and the stress tensor is asymmetric when stress field is a curl field.
New dynamic equilibrium equations for fluids and the constitutive relationship
for Newtonian fluid are proposed under the hypothesis. The Navier-Stokes
equations are obtained. The new model of fluid dynamics does not require Stokes
hypothesis. It is obtained that vorticity reflects the magnitude distribution
of shear flow in flow field.
