We know we aren’t the first to ponder the phenomenon of watching bubbles sink when a pint of Guinness is poured. Is it magic within the stout that causes this defiance again the laws of physics? Or is there something else going on with this tasty elixir?
And really, what good is keeping all our scientific knowledge bottled up if we can’t pop the top off it and apply it to more social pursuits? In the spirit of St. Patrick’s Day, we did just that.
Bubbles are the circulation engine
More studies than you realize have been conducted about the sinking bubble phenomenon that occurs within a freshly poured pint of Guinness. In one recent study, three Irish mathematicians investigated the shape of the glass. In their flow simulations, they found the rise or fall of the bubbles is directly related to the shape of the glass into which the beer is poured. A glass with a smaller base (like a stout glass) is going to result in a higher bubble density near the middle, causing entrained bubbles to rise up in the center and fall along the side and bottom of the glass.
In 1959, Guinness began using nitrogen to pressurize their beer instead of carbon dioxide, giving a creamier taste, but it also resulted in formation of much smaller bubbles, which are the key to Guinness’ characteristic presentation. The small bubbles rise through the liquid much slower than the large carbon dioxide bubbles in other beers. Slow enough that they can get trapped in the downward flow at the edge of the glass. Voila! sinking bubbles.
To better illustrate this phenomenon, we captured a video of this beautiful dance between gas and liquid. The close-up view shows off the complex wavy motion of the liquid draining down the edge of the glass after a pour, while the bubbles collectively drive, and are driven by the liquid circulation.
Figure 1: The flow patterns after pouring a pint of Guinness are mesmerizing.
Now it’s our turn
Our curiosity was piqued. However, because Guinness is a mostly opaque liquid, we can only see what’s happening near the outer edge of the glass. We can’t see all those bubbles rising in the middle of the glass—unless we use CFD, of course. Challenge accepted.
Instead of showing the liquid flow patterns as in some of the previous studies, we chose to use computational fluid dynamics (CFD) to show the motion of the bubbles – that is what we are looking at in the glass after all.
Figure 2: Each bubble is color coded. That color stays with the bubble throughout the entire simulation; this color coding helps to visualize the circulation patterns within the glass.
Besides being more visually interesting for this experiment, simulating bubbly flows is useful for a number of our client’s applications. For instance, fermentation tanks, bioreactors, and waste water treatment plants often use bubbles to stir up the liquid, keeping all the cells and solids in suspension. Simulating bubbles helps us see what’s happening inside the tank to make sure things stay well mixed. The bubbles also provide an incredible amount of surface area for mass transfer—they allow a mixture to breathe.
Bubbles can also cause problems — like with spillways for high head dams. Water flowing over the spillway plunging into a pool of water from great heights will entrains air bubbles deep underwater. The pressure deep down in the water turns the air bubbles into high levels of dissolved nitrogen and oxygen in the water. When fish breathe in water down deep and then come to the surface, they end up with bubbles in their blood just like a SCUBA diver getting the bends. Not good. By using CFD, we can determine if dissolved gas concentrations are too high, and then design modifications to the spillway to better protect the fish.
Why did we model the bubbles in a pint of a Guinness? We did it because we can. Previous work had set a high bar, but we tapped into our experience and hopped on this project. We poured over the literature and immersed ourselves in the problem, knowing we could handle the pressure and rise to the challenge. And maybe because we like beer also. We’re always looking for ways to improve our understanding of flow related phenomena so we can stay ahead of the curve, and continue to help our clients solve whatever flow related problems they can brew up.