Motion of a Fluid Particle (Kinematics)
It may seem difficult to distinguish between rotation and angular deformation of the infinitesimal fluid particle.
It is important to do so, because pure rotation involves no deformation but angular deformation does, and fluid deformation generates shear stresses
Figure below shows a typical xy plane motion decomposed into the four components
- translation, in which the particle moves from one point to another
- rotation of the particle, which can occur about any or all of the x, y or z axes
- linear deformation, in which the particle’s sides stretch or contract
- angular deformation, in which the angles (which were initially 90° for our particle) between the sides change
1Fox and McDonald's Introduction to Fluid Mechanics, 8th Edition.