For a family of oscillatory Stokes layers, the spatio-temporal evolution of impulsively excited disturbances is investigated, using direct numerical simulations of the linearised Navier-Stokes equations. The semi-infinite planar Stokes layer is modified to incorporate a low-amplitude, highfrequency harmonic, which provides a simplified model of the external noise found in physical experiments. For the unmodified Stokes layer, impulsively excited disturbances are known to form family-tree-like structures, composed of multiple wavepackets. The long-term behaviour that is encompassed within these structures is studied, together with the effects upon them of the alterations to the base flow. In the absence of any base-flow modification, the disturbances are discovered to exhibit a subharmonic pattern of temporal growth, with a periodicity that is twice that of the basic state. It is also shown that when linear instability first arises, it takes an absolute rather than a convective form. Inclusion of a high-frequency harmonic into the basic state is found to have a strong and destabilising impact upon the impulse response. The development of the family-treelike disturbance structure can be disrupted, changing the character of the absolute instability and promoting its appearance at much reduced Reynolds numbers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.