Transient growth and transition induced by random distributed roughness

Recent experiments on transient disturbances generated by three-dimensional roughness have used spanwiseperiodic arrays of geometrically simple cylindrical roughness elements. Connecting these laboratory experiments to more realistic situations requires the study of surfaces with distributed roughness. This is accomplished in this work by numerically generating quasi-random rough surfaces and manufacturing these surfaces using rapid-prototyping technology. Measurements of the disturbances that the rough surface creates in a Blasius boundary layer are obtained for three test configurations corresponding to roughness-based Reynolds numbers of [Re.sub.k] = 164, 227 and 301. The two lower values give laminar flow; the highest value results in localized transition approximately 140 mm downstream of the leading edge of the roughness. All three configurations exhibit transient growth of steady disturbances. Unsteady fluctuations indicate that transition in the [Re.sub.k] = 301 configuration is likely an example of a bypass transition mechanism in which the unsteady-disturbance growth outpaces the stabilizing relaxation of the steady flow. Measurements above the roughness surface in the [Re.sub.k] = 227 configuration provide a phenomenological model for distributed receptivity.