The flow and passive scalar fields in a time-space developing turbulent spot within a Poiseuille flow are analyzed using direct numerical simulations realized in a large computational domain. The turbulent core of the spot is populated by the quasistreamwise vortices as in the buffer layer of the fully developed wall turbulence. There is a massive clustering of the outer-layer vortices in the immediate vicinity of the spot periphery near the centerline. These long coherent chains are induced by the wave area. They provoke large-scale-motions (LSM) as a detailed spectral density analysis of the fluctuating flow and scalar fields reveals. The outer spectral core is dense and there is a clear scale separation with respect to the inner core. A bridge between the outer and inner spectral cores set up near the wall. The size and location of the large-scale spectral core are independent of the wall normal distance. An adequately modified existing model, based on the long and short term dispersion of the structures, describes relatively well the bounds of the outer spectral core. By their very nature, there is a restrictive similarity between the effect of the LSM in a turbulent spot and that of the passive attached Townsend eddies in a fully developed turbulent flow.

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• Flow instability
• Structure & turbulence of boundary layers
• Turbulence