Rising in the centre of the glass and free from the friction of the wall, they soar quickly but draw a tiny amount of liquid along with them. This soon establishes an upwards current in the beer. When a bubble reaches the brew's surface the gas escapes into the head, but the fluid it brought with it has nowhere to go. Pushed by the bubbles behind, the current is forced sideways and then back downwards as it hits the glass wall, beginning the cycle over again. As the current returns to the bottom of the glass it meets new bubbles trying to ascend.
Larger examples have sufficient buoyancy to resist the pull and continue to rise.
Therefore the effect is very noticeable. The group chose the Guinness conundrum as a way of demonstrating the sophistication of the latest wave of CFD software and how it could be particularly relevant to the brewing and wine industries, a big export earner for Australia. Modelling the flow of fluids and gases during a manufacturing process is important for producers looking to improve efficiency and quality. That doesn't mean though that answering the Guinness issue was straightforward. Modelling the flow of an air conditioner or the dispersal of poisonous gas from a heating unit would be extremely simple in comparison.