Behind the Cover: November 2023

Each month, Physics Today editors explore the research and design choices that inspired the latest cover of the magazine.

Gently nudge a fluid and it moves in an orderly way, tracing out parallel streamlines in what is known as laminar flow. But force the fluid so vigorously that the ratio of inertial to viscous forces grows large, and chaotic, turbulent eddies emerge. Those convection extremes occur in Earth’s oceans and atmosphere, in the stars, and in uncountable other natural and industrial systems.

Scientists’ understanding of convection in systems driven to turbulence remains incomplete. In their November Physics Today cover story , Detlef Lohse and Olga Shishkina examine the problem with an idealized model: the Rayleigh–Bénard convection cell, a container that is heated from below and cooled from above. The temperature gradient drives convection. This month’s cover image is a snapshot of fluid temperature in a numerically simulated cell. Tiny plumes of rising warm (red) and sinking cool (blue) fluid are pervasive. The colors in the cell correspond to both temperature and buoyant motion.

Lohse and Shishkina are especially interested in the regime at which the Rayleigh number Ra—the nondimensionalized value of the temperature difference between the hot bottom plate and the cold top plate—becomes extremely large. The data informing the cover image are taken at a height near the lower plate at Ra ~ 10¹³. That value is small enough that even though the interior of the cell has become turbulent, the regions along the wall remain laminar. For even larger Ra, nearly every part of the container becomes turbulent. In that ultimate regime, as the authors call it, the amount of heat transported through the cell dramatically increases.

Lorien Williams, the Physics Today artist who designed the cover, found the plume structure so stunning that she “wanted it to take up as much space as possible.” To make that structure pop and appear “larger than life,” she shifted the circular image upward until it overlapped the Physics Today header. Because the image is so detailed, she chose to keep the background text white and unobtrusive. The simple but thick font, Cubano, makes the wording easy to read against the busy image.