Edited sentence: As a result, in the Navier–Stokes equations result from the following form for deviatoric stress tensor:.The Navier–Stokes equations result from the following assumptions on the deviatoric stress tensor :.the deviatoric stress can be expressed as the product of the tensor gradient  of the flow velocity with a viscosity tensor , i.the deviatoric stress tensor has zero trace, so for a three-dimensional flow 2μ + 3μ” = 0.the deviatoric stress vanishes for a fluid at rest, and – by Galilean invariance – also does not depend directly on the flow velocity itself, but only on spatial derivatives of the flow velocity.the fluid is assumed to be isotropic, as valid for gases and simple liquids, and consequently  is an isotropic tensor; furthermore, since the deviatoric stress tensor is symmetric, it turns out that it can be expressed in terms of two scalar dynamic viscosities μ and μ”:  where  is the rate-of-strain tensor and  is the rate of expansion of the flow.with the quantity between brackets the non-isotropic part of the rate-of-strain tensor  The dynamic viscosity μ does not need to be constant – in general it depends on conditions like temperature and pressure, and in turbulence modelling the concept of eddy viscosity is used to approximate the average deviatoric stress