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.
Original abstract
as written by the authors arXiv:2602.18236
We report an experimental investigation of turbulent Rayleigh-Benard convection in a rectangular cell of large aspect ratio ($$\Gamma = 10$$) over the Rayleigh number range $$5.4\times10^7 \le Ra \le 7.2\times10^9$$ and Prandtl number range $$4.3 \le Pr \le 67.3$$. Planar particle image velocimetry measurements show that the flow self organises into several horizontally stacked convection rolls, and repeated experiments under identical parameters (both $$Ra$$ and $$Pr$$) reveal that the number of rolls varies within the range of 3 to 7 with 6 being the most probable, which demonstrates the presence of multiple flow states. When $$Pr$$ is increased to 67.3, the number of roll like structures increases significantly, indicating a structural transition from a roll dominated to a plume dominated flow. This transition is reflected in the global momentum transport, for $$Pr \leq 18.3$$ the Reynolds number scales as $$Re \sim Ra^{0.58}Pr^{-0.97}$$, whereas the scaling is changed to $$Re \sim Ra^{0.72}$$ when $$Pr$$ reaches 67.3. Within individual rolls, we further examine the Reynolds numbers based on horizontal and vertical velocity components, $$Re_{u,\text{roll}}$$ and $$Re_{w,\text{roll}}$$, and find that the former increases while the latter decreases with roll size (quantified as the aspect ratio of the roll $$\Gamma_\text{roll}$$) due to continuity constraints, with their ratio following $$Re_{w,\text{roll}}/Re_{u,\text{roll}} \sim \Gamma_\text{roll}^{-0.61}$$. We impose different initial flow conditions (roll structures) with controlled perturbations, and demonstrate that the initial condition can influence the final turbulent state. We show that the number of horizontally stacked rolls regulates the global transport, larger number of rolls induces greater vertical momentum and heat transfer.
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.