Stokes flow is the regime of viscous fluid motion in which inertial terms are neglected. For a steady incompressible Newtonian fluid with constant dynamic viscosity 
and no body force, its equations
are
 |  |  |
(1)
|
 |  |  |
(2)
|
where 
is pressure and 
is the velocity field. The
first equation uses the gradient of 
and the componentwise Laplacian
of 
; the second expresses zero divergence.
These linear partial differential equations
are obtained from the Navier-Stokes equations
by omitting the inertial terms (Ishimoto and Yamada 2012).
For fixed geometry, reversing the prescribed surface velocities reverses 
and the pressure difference from its reference value. This
reversibility underlies the scallop theorem.
