In b , the air starts to move back towards the centre almost as soon as it has finished moving away from it. The flow lines in diagram b show that the decompression is far gentler in b and this means that less energy is lost. This fact also shows up as a reduced coefficient of drag. Moving water this quickly looses energy and contributes to a higher back pressure than with d , which has a smoother velocity profile. Here, the water is moved towards the centre gradually and therefore the pressure drop is much less.
Diagrams have fins removed for clarity Two of the following diagrams are from ready made bottles.
The Normal bottle is a standard 2 litre bottle without modofications. The Velocity profile shows that virtually all of the acceleration occurs within the last 6 cm of the bottle and apart from the concentration of flow at the neck, most of the rest of the acceleration occurs within about 1 cm. The Soda bottle is an almost conically shaped bottle. The Velocity Profile for this shaped bottle shows that the acceleration between 14 and 4 cm is a fairly smooth curve. Getting a good shape is an artform if you do not have moulds or other specialised equipment.