JFM Perspectives is a very recent undertaking of Journal of Fluid Mechanics where a detailed survey of the current state of research in a particular topic in fluid mechanics is given. Starting in 2016, so far there are only 11 articles which are usually 50-80 pages long. When I joined the group James suggested me to go through the very first one which is written on the topic ‘route to turbulence in pipe’ by our own Prof. Dwight Berkeley from Maths Department. JFM Perspectives coves a wide range of topics like premixed flame dynamics, surfactants, binary mixtures and granular suspension as well as theoretical and modelling aspects of fluid turbulence. It is good place to start if someone not only wants to have elemental understanding but also wants to catch-up with current advances in one of these topics. I will give a very brief summary of one of the articles which I am currently interested in.
Theories of binary fluid mixtures: from phase-separation kinetics to active emulsions- This article deals with the binary mixture where micro-structure and flow are strongly coupled. Some of the examples of relevant processes:- fluid–fluid demixing from an initially uniform state followed by diffusive coarsening on imposing macroscopic shear flow; and Brownian motion and coalescence of emulsion droplets (Ostwald ripening).
A modelling framework is discussed where fluid is represented using velocity (a vector) and a mesoscopic scalar order parameter is used to represent the composition of mixture. The evolution equation of order parameters contains conservative (currents) and a non-conservative (relaxation) dynamics. The coupling between fluid momentum and the order parameter appears as additional stress term in Navier Stokes equation. These stresses can be purely interfacial (for simple fluids) or also elastic (for liquid-crystals) which can be derived from an underlying free energy expressed as a functional of the relevant order parameters. This article introduces the conventional continuum models of binary fluids, first covering some well-studied cases such as the thermodynamics and kinetics of phase separation, and emulsion stability and then introduces more complicated fluid such as nematic liquid crystal which require tensor order parameter. The use of mesoscopic order parameters, in situations where sharp structural features are involved, is emphasized. The article is concluded with the introduction of a currently growing filed of active emulsions in which one of the binary components consists of living/self propelled particles.