Transition and separation of the boundary layer
Apart from the study of laminar profiles, the main reason for the interest in knowledge of the transition is in monitoring the effectiveness of transition strips. Diffusive flows of mass, momentum and energy in the boundary layer increase more or less abruptly in the transition from the laminar to the turbulent regime.
In the zones of separation tangential stresses (diffusive flows of momentum) decrease to zero and become negative.
■ The variations in tangential stresses can be highlighted by covering the body with a film of non-volatile liquid.
■ The different diffusion of mass in laminar and turbulent zones may be shown by making use of the sublimation of a solid, such as naphthalene, or the evaporation of a liquid, as in the china clay method.
■ The different diffusion of energy can be highlighted with heat-sensitive coatings.