Interaction of the wake from bluff bodies with an initially laminar boundary layer.

An experimental investigation was conducted to determine the mechanism and conditions governing transition induced by small bluff bodies suspended in a laminar boundary layer. The experiment was performed in the inlet section of a tube. The mechanism by which transition occurred appeared to be a local effect dependent upon the stability characteristics of the element wake rather than the stability characteristics of the boundary layer over the range of boundary-layer Reynolds numbers examined. That is, transition of the boundary layer appeared to involve the direct seeding of turbulence in the boundary layer from the wake of the element, rather than amplification of wavelike disturbances as considered in classical boundary-layer stability theory. In the central region of the boundary layer, the critical value of the element Reynolds number which resulted in transition of the boundary layer coincided, approximately, with the Reynolds number for incipient transition in the wake of the element. Laminar vortices shed by disturbance elements at subcritical values of element Reynolds numbers were found to decay in propagating downstream. For elements located outside the boundary layer, transverse turbulence contamination of the boundary layer was found to depend on diffusion of the turbulent wake below a critical (trigger) value of y/d. Turbulent patches were generated by elements traversing the boundary layer when the instantaneous slip velocity Reynolds number was slightly higher than the value of critical Reynolds number for elements fixed within the boundary layer.

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