A Boiling Pot Reveals the Whims of Turbulence express
A pot of water heated from below is a simple example of turbulence. Hot fluid rises, cold fluid sinks, and the flow spontaneously breaks into rotating cells that look like horizontal rolls. In a new study, physicists took a wide rectangular container (ten times longer than its height) and tracked how many such rolls appear.
Repeating the experiment under the same conditions, the scientists got anywhere from three to seven rolls, most often six. This means that a turbulent flow has several possible states, and which one sets in is unknown in advance. When they made the fluid more viscous (the Prandtl number increased from 4.3 to 67.3), the rolls began to break apart, and the flow turned from a system of large vortices into chaotic "plumes" — jets of hot fluid rising like bubbles in a lava lamp.
The number of rolls directly affects heat transfer: the more there are, the faster heat moves from bottom to top. Inside each roll, the horizontal velocity grows with its size, while the vertical velocity drops — fluid in a wide roll takes longer to circle around before rising. This is like a carousel: on a big carousel, you ride in circles longer and rise more slowly.
The takeaway is simple: turbulence is not chaos without rules, but a system with several stable states. Which state the flow chooses determines how quickly a room warms up from a radiator or an engine cools down. By controlling the initial conditions, you can nudge the flow toward a desired regime — for example, more efficient cooling.
The Prandtl number for water is about 7, while for motor oil it is over 1000. That is why rolls barely form in oil: heat is transferred by jets, not rotating cells.