When the flow rate of a fluid exceeds a certain critical value, it is no longer laminar. The flow becomes extremely irregular and complex, changing constantly with time; there is no steady state or constant pattern. Such unpredictable, chaotic flow is termed turbulence. The transition from laminar to turbulent flow is often abbrupt - a smooth laminar flow will suddenly become unstable once the critical speed is reached. Instabilities can be introduced by an object in the flow path, such as a bump on a pipe wall.
In a real fluid there will be a strong adheseive force between the object and the molecules of the fluid passing close to it. This layer of fluid will adhere to the object and remain at rest relative to it. The speed of the fluid increases from zero to free-stream value as the distance from the object increases. This variation in speed takes place in a thin layer, called the boundary layer.
When the flow speed is relatively low, the velocity changes can be made easily by the fluid and the flow remains smooth, the streamlines deviating around the object and returning to parralel laminar flow after passing it.
At higher speeds, the fluid will not be able to make a smooth transition from zero speed at the surface to a high speed nearby. The result is turbulence and instability, characterised by breaking up and curling of streamlines, a decreased flow rate, noise, and substantial mixing - indeed, turbulence is required for mixing of a fluid to occur.
For example, normal blood flow in human arteries is laminar, but is at high enough speeds that a relatively small obstruction such as a lump on an interior wall will destroy the laminar flow pattern. Turbulent flow is noisy, and it is this noise that can be used in stethescope diagnosis of circulatory problems.