Suspensions are considered from the perspective of development of their bulk fluid mechanics. Suspension mechanics has had a fluid mechanical basis since its inception, as description of the motion of such a mixture requires consideration of the coupling of fluid motions to the surface stresses driving motion on the particles. While it thus sits on a very firm foundation of prior study of the microhydrodynamics, i.e., the microscale forces and motions of the particles and their relation to the properties, the bulk fluid mechanics of suspensions is still in its early stages. This work first outlines the foundations and then proceeds to describe some basic developments from studies exploring the behavior of suspensions as bulk fluids. The focus is on the near-hard-sphere suspension of solid particles in Newtonian liquid, for which a coherent and extensive, though far from complete, understanding of properties and phenomena exists; an overview of the state of understanding of rheology and migration phenomena is provided. This is followed by a consideration of part of the rather limited body of work which explores a central fluid mechanical paradigm of the viscous-inertial balance, i.e., the influence of the bulk Reynolds number $Re$, emphasizing its impact on two flows studied by the author, namely stability of bulk suspensions in pipe and Taylor-Couette (TC) flows. In the TC flow, states not seen in pure fluids under similar conditions are observed. Similarly, elevated Reynolds number flow in bifurcating conduit flows is shown to challenge our ability to predict behavior based on continuum descriptions of the particle phase, indicating directions for productive study.

• Received 12 August 2020
• Accepted 20 October 2020

DOI:https://doi.org/10.1103/PhysRevFluids.5.110519

This article appears in the following collection:

2020 Invited Papers

Physical Review Fluids publishes a collection of papers associated with the invited talks presented at the 72nd Annual Meeting of the APS Division of Fluid Dynamics.