This vortex of recirculating liquid has a vital role to play in arriving at the answer to the ultimate question. Flow pathlines in a glass of Guinness. Once the liquid is moving downwards near the glass wall, the liquid motion tries to drag bubbles rising in this region down with it. They move closer to the wall, but eventually escape from the surface of the liquid. However, bubbles smaller than about 0.05 mm diameter cannot resist the downward drag force, and do indeed move downwards close to the wall.
Every worthwhile story needs a human interest element.
So CFD not only answers the ultimate question, but it also has all the right connections. We thank Guinness for providing product photography and moral support.