Qualitative Questions in Fluid Mechanics

In this paper in honour of Professor Leen van Wijngaarden, some propositions about fluid mechanics are discussed. First, basic fluid mechanics research should be judged as much by its progress in clarifying the essential questions about the phenomena of fluid flow and in establishing general concepts, as by its contribution to the solutions of specific problems. In fact, the latter often contribute to the former. Both aspects attract good students to the subject.Second, researchers make more progress and are likely to impress a journal editor when they relate their problems to general physical and/or mathematical considerations, and when they analyse and present their results in a wide but fluid mechanically relevant context, for example through symmetry considerations, invariants (including dimensionless groups, scaling laws and topological constraints), differential properties (or jumps, wiggles and swirls), and through raising new questions and concepts of general significance from studies of specific flows.Lastly, decisions by organisations and individuals about future research directions also benefit from being considered in a wide conceptual framework.