Introduction

Although sometimes ignored, there is no doubt that hydrodynamical processes play a central role in virtually all areas of astrophysics. If they are neglected in the analyses of observations and the modelling, the results for any object must become questionable; the same is therefore true of the understanding of basic astrophysical phenomena and processes that result from such investigations.

Investigations of astrophysical fluid dynamics are hampered by both theoretical and observational problems. On the theoretical side it is evident that the systems being studied are so complex that realistic analytical investigations are not possible. Furthermore, the range of scale, extending in the case of stars from the stellar radius to scales of order 100m or less, entirely prevents a complete numerical solution. Observationally, the difficulty is to find data that are sensitive to the relevant processes, without being overwhelmed by other, similarly uncertain, effects. Progress in this field therefore requires a combination of physical intuition combined with analysis of simple model systems, possibly also experiments analogous to astrophysical systems, detailed numerical simulations to the extent that they are feasible, together with a judicious choice of observations and development and application of analysis techniques that can isolate the relevant features. Douglas Gough has excelled in all these areas.

In this brief introduction we make no pretense of reviewing the whole vast field of hydrodynamical processes in astrophysics, or even in stars.