The speed Of a flow affects its properties in a number of ways. At low enough speeds, the inertia of the fluid may be ignored and we have creeping flow. This regime is of importance in flows containing‘ small particles (suspensions), in flows through porous media or in narrow passages (coating techniques, micro-devices). As the speed is increased, inertia becomes important but each fluid particle follows a smooth trajectory; the flow is then said to be laminar. Further increases in speed may lead to instability that