An experimental study of the inception of large-scale structures in laminar flow through two parallel fins

Abstract

This article presents an experimental investigation aimed to determine the onset conditions for the formation of large-scale structures in flows through two parallel fins. The flow visualization results confirmed the presence of the coherent structures, determined the critical Reynolds number for their formation, and correlated it to the local flow parameters. The value of this critical Reynolds number, when based on local parameters, seems to be constant for all studied gap sizes and equal to 6.48. The study also documented the structures' axial spacing and convective speed, as a function of geometrical (gap width) and dynamical (Reynolds number) parameters. The structures' streamwise spacing, for a fixed gap width, asymptotically approaches a constant value when the Reynolds number is increased. This constant value increases linearly with the gap width. The structures' convective speed varies linearly with the Reynolds number. Distinct curves for each gap size are obtained when bulk quantities are employed. However, when normalized by the local velocity, its variation with the local Reynolds number seems to follow a single linear curve independently of the gap widths. The results showed that the coherent structures are local phenomena, best characterized by local parameters of the interfin flow region.