Absolute stability mechanism of a swept cylinder laminar boundary layer with imposed spanwise periodic conditions.

The linear impulse response of a laminar three-dimensional boundary layer with imposed spanwise periodic conditions is theoretically analyzed. Because of the imposed spanwise periodicity, a unidirectional absolute instability is physically significant. The base flow is obtained through a direct numerical simulation of the flow along a swept cylinder, with periodic boundary conditions enforced at the spanwise boundaries. It is shown that this flow is absolutely stable. However, it exhibits crossflow modes of zero group velocity whose damping rate in time is quite slow. Moreover, these crossflow modes are spatially amplified. A direct numerical simulation of the same configuration but in the presence of surface roughness has enabled the observation of these traveling crossflow waves. The introduction of the roughness element in the computation creates initial transients that play the role of an impulselike forcing, which generates the traveling crossflow waves as predicted by the theoretical analysis. Depending on the roughness chordwise location, the frequency and damping rate in time of the crossflow waves change in agreement with the theoretical results. [ABSTRACT FROM AUTHOR]