Stationary fluid, persistence

As flow rate was increased, the thin stream of colored water in the center of the pipe would begin to waver and oscillate back and forth across the pipe. The flow rate at which this occurred would vary according to the pipe diameter. Eventually, as flow continued to be increased, the colored filament would break up altogether (see Fig. 48.3b). The final stage of this flow development where the colored filament breaks up altogether and disappears we nowadays understand to be an illustration of fully developed turbulent flow. The existence of eddy currents randomly distributed across the whole cross-section of the pipe causes effective mixing throughout the fluid as it flows in all but a very narrow layer of stationary fluid, termed "the laminar sublayer," which persists next to the wall of the pipe itself (see Fig. 48.4). 

Figure 48.4 A narrow layer of stationary fluid, the laminar sublayer, persists next to the wall during turbulent flow.

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