Environment and receptivity

In an environment where initial disturbance levels are small, the transition Reynolds number of a boundary layer is dependent upon the nature and spectrum of the disturbance environment, the signatures in the boundary layer of these disturbances and their excitation of the normal modes, receptivity, and finally the linear and nonlinear amplification of the growing modes. The progress in understanding receptivity to free stream vorticity and acoustic disturbances is reviewed. The technique of the initial value problem, which promises to be a significant additional tool for providing guidance toward the resolution of receptivity issues is outlined.