Turbulence

Richard Feynmann once described turbulence as “the most important unsolved problem of classical physics”, and indeed it is a flow phenomenon that impacts many aspects of our daily lives. This talk will address the general characteristics that define turbulence, what makes it an important unsolved problem (or, more generally, series of problems), and some of the features that make turbulent flows difficult to observe, simulate, understand and control.

In contrast to laminar flows, turbulent ones are characterized by high mixing due to the existence of a self-sustaining fluctuating field, governed by the continuity, momentum and energy equations in the simplest cases, and consisting of a spatiotemporal range of energetic scales which grows rapidly with Reynolds number. Associated with this field are eddies or ``coherent structures’’, the precise definition of which can be hotly-debated, but which describe regions of high statistical (and often visual) correlation of at least one variable.

We will review approaches to understanding the physics of turbulence: seminal flow visualizations going back to Da Vinci and Reynolds, elegant and key early dimensional scaling analyses, theoretical progress, experimental observations and diagnostic advances, the impact of numerical simulation, and inferences on the mechanisms which incur and sustain turbulence. Where possible the focus will be on the simplest manifestations of turbulence and geometric configurations, with a brief discussion of extensions to more complex flows of engineering interest, such as compressible, inhomogeneous and multi-phase flows. Some deeper discussion will be assigned to recent progress in wall-bounded turbulence. The talk will conclude with a brief outlook on the state-of-the art and some current challenges in the field.