Either way, the strain rate tensor expresses the rate at which the mean velocity changes in the medium as one moves away from the point except for the changes due to rotation of the medium about as a rigid body, which do not change the relative distances of the particles and only contribute to the rotational part of the viscous stress via the rotation of the individual particles themselves . ( These changes comprise the vorticity of the flow, which is the curl ( rotational ) of the velocity; which is also the antisymmetric part of the velocity gradient .)
