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When two solids slide past one another, we have an empirical model for friction that works very well in many circumstances: a force tangential to the metal interface that is proportional to the normal force across the interface. So then what can we expect when fluid matter moves in such a way that adjacent layers of fluid move at different speeds? How do we model the mutual interaction and, more generally, how do we model the distribution of forces throughout a body of flowing matter? Given such models, how can we then use well-posed external conditions (such as forces like gravity and forces due to contact with solid boundaries) as well as initial conditions to predict the motion of the fluid? When does internal friction (viscosity) dominate and when does inertia dominate? Indeed, what constitutes a full statement of the state of a fluid, considering that different regions can have different velocities, different internal stresses, and possibly different temperatures and densities. It appears that we need a description in terms of fields, both vector fields (for velocity), tensor fields (for stresses), and scalar fields (for temperature, density, pressure, etc.) How do even go about measuring such fields? Once motions are predicted, under what conditions are these motions stable? If instability occurs, what new patterns emerge? Empirical observation (including famous sketches by Leonardo daVinci) shows that fluid motion can go through sequences of transitions to ultimately become very complex, with a mix of organized swirling motion and randomness. We call such a highly complex state of flow turbulence. It is both problematic (it dissipates energy) and useful (it produces mixing).What indeed are the uses of turbulence and what are the methods and devices that can be employed to either enhance it or minimize its effects?

Image source: Michael Belisle, Public domain, via Wikimedia Commons

General resources

American Physical Society organizational units

• Division of Fluid Dynamics (DFD)

Open problems

• https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics#Fluid_Dynamics

PIRA bibliography

• Fluid Mechanics

Physicslabrefs bibliography

• fluid dynamics and surface waves

Books

Emrich, R. J., Ed. (1981), Fluid Dynamics, Part A, Methods of Experimental Physics, v. 18A (Academic).

Emrich, R. J., Ed. (1981), Fluid Dynamics, Part B, Methods of Experimental Physics, v. 18B (Academic).

Granger, R. A. (1994), Experiments in Heat Transfer and Thermodynamics (Cambridge)

Granger, R. A. (1988), Experiments in Fluid Mechanics (Holt Rinehart).

National Committee for Fluid Mechanics Films (1972), Illustrated Experiments in Fluid Mechanics (MIT Press).

Rathakrishnan, E. (2017), Instrumentation, Measurements, and Experiments in Fluids (CRC Press).

Schetz, J. A. and A. E. Fuhs, Eds. (1996), Handbook of Fluid Dynamics and Fluid Machinery, Vol. 2: Experimental and Computational Fluid Dynamics (Wiley).

Tropea, C., A. L. Yarin, and J. F. Foss (2007), Springer Handbook of Experimental Fluid Mechanics (Springer).

Webster, J. G., Ed. (2000), Mechanical Variables Measurement: Solid, Fluid, and Thermal (CRC Press).

ALPhA immersions

• Taylor-Couette Flow (second Taylor-Couette Flows)

• Imaging Shock Waves

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Labs at other universities and colleges

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