For the special case of an incompressible flow, the pressure constrains the flow so that the volume of fluid elements is constant: isochoric flow resulting in a solenoidal velocity field with   The incompressible momentum Navier–Stokes equation results from the following assumptions on the Cauchy stress tensor:   Dynamic viscosity need not be constant – in incompressible flows it can depend on density and on pressure. Any equation expliciting one of these transport coefficient in the conservative variables is called an equation of state. The divergence of the deviatoric stress is given by:  because formula_29 for an incompressible fluid. Incompressibility rules out density and pressure waves like sound or shock waves, so this simplification is not useful if these phenomena are of interest. The incompressible flow assumption typically holds well with all fluids at low Mach numbers (say up to about Mach 0.3), such as for modelling air winds at normal temperatures. For incompressible (uniform density ρ0) flows the following identity holds:  where is the specific (with the sense of "per unit mass") thermodynamic work, the internal source term. Then the incompressible Navier–Stokes equations are best visualised by dividing for the density:  where is called the kinematic viscosity. It is well worth observing the meaning of each term (compare to the Cauchy momentum equation):  The higher-order term, namely the shear stress divergence , has simply reduced to the vector Laplacian term . This Laplacian term can be interpreted as the difference between the velocity at a point and the mean velocity in a small surrounding volume. This implies that – for a Newtonian fluid – viscosity operates as a "diffusion of momentum", in much the same way as the heat conduction. In fact neglecting the convection term, incompressible Navier–Stokes equations lead to a vector diffusion equation (namely Stokes equations), but in general the convection term is present, so incompressible Navier–Stokes equations belong to the class of convection–diffusion equations. In the usual case of an external field being a conservative field:  by defining the hydraulic head:  one can finally condense the whole source in one term, arriving to the incompressible Navier–Stokes equation with conservative external field:  The incompressible Navier–Stokes equations with conservative external field is the fundamental equation of hydraulics.