Introduction

Non-Cartesian coordinates are often employed in fluid mechanics to exploit the symmetry of particular fluid systems. For example, it is convenient to employ cylindrical coordinates to describe systems possessing axial symmetry. In this appendix, we investigate a particularly useful class of non-Cartesian coordinates known as orthogonal curvilinear coordinates. The two most commonly occurring examples of this class in fluid mechanics are cylindrical and spherical coordinates. (Incidentally, the Einstein summation convention is not used in this appendix.) For more information on orthogonal curvilinear coordinates see Milne-Thomson 2011.