Why bubbles in Guinness go down
Computer simulation used to depict movement in glass
Dec 21, 1999 - A computer simulation has been used to show even though bubbles float up, they also go down in a glass of Guinness.
Professor Clive Fletcher and students at the University of New South Wales, Sydney, Australia, used FLUENT computational fluid dynamics (CFD) software from Fluent Inc. to illustrate that bubbles go both up and down in a glass of Guinness.
CFD simulations allow engineers to graphically depict fluid flows, pressure, temperature, and chemical concentration at any location. Fletcher's team used this software to simulate the motion of the bubbles and discovered that, as expected, most bubbles do move upwards. The bubbles in the center of the glass, free from the effects of the glass wall, move upwards most quickly and drag liquid with them.
But the liquid moving up in the center of the glass, having nowhere else to go, must eventually turn towards the walls and start to move downward. The liquid moving downward near the walls tries to drag down bubbles with it. Larger bubbles have sufficient buoyancy to resist but smaller bubbles (less than 0.05 mm) are continuously dragged to the bottom of the glass.
These results can be seen in animation at http://www.fluent.com/news/pressrel/guinness/tsld001.htm.
Fluent's corporate headquarters are located in Lebanon, N.H.
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