LAMINAR BOUNDARY LAYERS

Laminar flow causes considerably less skin friction than turbulent In a laminar boundary layer the air moves in very smooth fashion, as if each tiny layer of the fluid was a separate sheet or lamina, sliding past the others with only slight stickiness or viscous stress between. There is no movement of particles of air up or down from layer to layer. The lowest lamina is stuck to the surface. The layer above it slides smoothly over this immobile layer, and the next above smoothly over that and so on until at the outermost limit of the boundary layer, the last lamina of all is moving almost at the speed of the main stream. The total thickness of the whole boundary layer may be a few hundredths of a centimetre. If measurements are made of the speed of flow at each level within this layer, diagrams such as Figure 3.1 may be drawn. Each arrow represents the flow speed at a point above the surface. It is found that the velocity increases fairly steadily from bottom to top. The laminae near the surface are creeping along, those next above move only slightly faster. It is this slow, smooth movement of the layers near the surface that reduces the skin friction. But because these layers are so slow, and receive little traction fri>m the main stream, they are all too easily brought to a standstill.