 The operator formula_11 is the material derivative.By introducing the tensors (matrices) formula_12, formula_13 and formula_14, which describes crude shear flow, pure shear flow and compression flow, respectively, 

the classic Navier-Stokes equation gets a lucid form.Note that the term in the momentum equation that contains the volume viscosity disappears for an "incompressible fluid" because the divergence of the flow equals 0.