Most flows which occur in practical applications are turbulent. This term denotes a motion in which an irregular fluctuation (mixing, or eddying motion) is superimposed on the main stream. Figures 16.1a-d illustrate this. These are photographs of the turbulent flow in a water channel. The flow has been made visible by powder sprinkled on the surface. The flow velocity is the same in all four photos, but the camera is moved at different velocities along the axis of the channel. It can easily be deduced from the figures whether the longitudinal velocity of the fluid particles is larger or smaller than that of the camera. These figures give us a rather impressive idea of the complexity of turbulent flow.
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