So far we have dealt with algebra of tensors at a point. The PDEs we wish to
solve involve derivatives of tensors with respect to time and space. We
therefore need to extend our description to a tensor field, i.e. a tensor
that varies across time and spatial domains. In this Chapter we will first
present a mathematical description of all the differential operators we may
encounter. We will then show how a tensor field is constructed in OpenFOAM and
how the derivatives of these fields are discretised into a set of algebraic
equations.
