A simple notation for differential vector expressions in orthogonal curvilinear coordinates

We present a simple notation for performing differential vector operations in orthogonal curvilinear coordinates, and for easily obtaining partial differential expressions in terms of the physical components. We express nth-order tensors as the summed products of the physical components and nth-order polyads of unit vectors (an extension of Gibbs dyadic notation convenient for a summation convention). By defining a gradient operator with partial derivatives balanced by the inverse scale factors, differential vector (or tensor) operations in orthogonal coordinates do not require the covariant/contravariant notation. Our primary focus is on spherical-polar coordinates, but we also derive formulae which may be applied to arbitrary orthogonal coordinate systems. The simpler case of cylindrical-polar coordinates is briefly discussed. We also offer a compact form for the gradient and divergence of general second-order tensors in orthogonal curvilinear coordinates, which are generally unavailable in standard handbooks. We show how our notation relates to that of tensor analysis/differential geometry. As the analysis is not restricted to Euclidean geometry, our notation may be extended to Riemannian surfaces, such as spherical surfaces, so long as an orthogonal coordinate system is utilized. We discuss the Navier-Stokes equation for the case of spatially variable viscosity coefficients.