Physical units helpful for multiscale modelling

Abstract

The work addresses the problem of the interoperability of modelling codes, especially in the context of the relatively limited bandwidth of current High-Performance Computers (HPC). Many codes employ non-dimensionalisations that effectively set units by the scalings they choose for length, time and other key quantities. These units can be difficult to unpick in a multiscale environment, but the natural choice of SI units usually leads to a need to treat a much large range of number with negative consequences for HPC deployment. A compromise, applicable to both particle and mesh-based codes, is sought whereby the user may set length- and time-scales in SI units appropriate to the plasma or other modelling problem under consideration. Application is made both analytically and to existing plasma software that demonstrates reduced need for number range relative to SI. An algorithm is presented that enables treatment of the often tricky problem of changing units with minimal user intervention. The code work indicates use of single-precision (32-bit real number representations) may be adequate for particle-mesh modelling in an unexpectedly large range of circumstances.