The wave structure of turbulent spots in plane Poiseuille flow

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Abstract
The wave packets located at the wingtips of turbulent spots in plane Poiseuille flow have been investigated by hot-film anemometry. The streamwise velocity disturbances associated with the waves were found to be antisymmetric with respect to the channel centreline. The amplitude of the waves had a maximum close to the wall that was about 4% of the centreline velocity. The modified velocity field outside the spot was measured and linear stability analysis of the measured velocity profiles showed that the flow field was less stable than the undisturbed flow. The phase velocity and amplitude distribution of the waves were in reasonable agreement with the theory, which together with the symmetry properties indicate that the wave packet consisted of the locally least stable Tollmien-Schlichting mode.