A whoosh bottle demonstration typically involves igniting ethanol vapor in a large bottle with a narrow opening. The rapidly expanding gas creates the whooshing noise as it vacates the container. This crowd-pleasing experiment is often used to portray the volatility of vapors or demonstrate some interesting thermodynamics.

This particular video caught my eye. Give it a watch. A user in the comments wonders if he observes hexagonal patterns along the flame's surface. Now, I doubt these patterns are hexagonal, but there is definitely an interesting structure emerging. What is happening along the surface to cause that structure? How can we explain its dynamics? I'm tempted to think the dynamics lead us to the cylindrical Bessel equation (see vibrating membrane for depictions of said modes), at least in a simplified model, but there's a lot going on here. And it doesn't help that I know little about fluids, let alone fluids that are on fire.

Consider the following picture a starting point since we can model the container as a cylinder from here on out. The initial condition involves a strong peak atop a fairly flat circular flame.

flame delta peak initial condition

The combustion along the boundary then occurs more rapidly for some reason. Perhaps the flame propagates more quickly along any residual liquid ethanol along the walls. Structure then begins to form in the center. I wonder if the imperfections at the glass boundary are causing those radial ridges, but why would the boundary of the flame cause a surface fluctuation to propagate towards the center in this case? The center of the flame ought to respond to pressure variations of the gas underneath it, right?

flame settles into structure

The structure is most eminent below. It almost looks like each cell oscillates, while the network as a whole seems to keep the same node and edge formation in this brief time, though this observation probably isn't reliable.

flame structure


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