in which
is the special symmetrical tensor. This term will also be present in the case of a viscous fluid. But in this case there will also be pressure terms, which depend upon the space derivatives of the
. We shall assume that this dependence is a linear one. Since these terms must be symmetrical tensors, the only ones which enter will be
(for
is a scalar). For physical reasons (no slipping) it is assumed that for symmetrical dilatations in all directions, i.e. when