. TRANSITION

Small surface imperfections such as rough spots, blobs of paint, fly specks, or, on a model, flaws in covering, bumps caused by protruding spars,’ etc., tend to disturb the laminar boundary layer, but at low boundary layer Reynolds numbers (i. e., near the leading edge of the wing), viscosity tends to damp down the disturbances and the laminar flow successfully over-rides them. Low flying speeds and small dimensions encourage the formation of laminar boundary layers on the leading edges of model wings. Even when the surface is not perfect, and no surface ever is, the boundary layer will initially be laminar. As the flow continues to move over the surface, the boundary layer Re rises with distance covered, and the damping effect of the viscosity becomes progressively less. Somewhere a critical point will be reached at which the small air ripples caused by surface irregularities just manage to maintain themselves without being damped out, and a small distance behind this point any minute disturbance will overcome the damping effect altogether. A distinctly wavy or rough surface will cause this sooner, i. e., at a lower Re. The laminae break up rather sharply and the flow makes a transition to turbulence (Figure 3.3). The

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. point or narrow zone on the surface where this occurs is the transition zone, and it is j. associated with a critical boundary layer Reynolds Number. At higher b. l. Re (that is, l behind this zone on the wing) the boundary layer will be turbulent