Laminar sublayer, persistence

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

When the flow in the boundary layer is turbulent, streamline flow persists in a thin region close to the surface called the laminar sub-layer. This region is of particular importance because, in heat or mass transfer, it is where the greater part of the resistance to transfer lies. High heat and mass transfer rates therefore depend on the laminar sublayer being thin. Separating the laminar sub-layer from the turbulent part of the boundary... 

The transition from laminar to turbulent flow on a smooth plate occurs in the Reynolds number range 2 x 10 to 3 x 10 , as shown in Fig. 3.10-1. When the boundary layer is turbulent, a thin viscous sublayer persists next to the plate. The drag caused by the viscous shear in the boundary layers is called skin friction and it is the only drag present for flow past a flat plate. 

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). 

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