RESUMO

In this experimental study the response of the turbulent boundary layer to a sudden change in surface condition, from rough to smooth wall, is examined. The flow field is interrogated by hot-wire anemometry and particle imaging velocimetry while the local skin friction is determined by Preston tube measurements. An internal boundary layer represents the demarkation between the characteristic flow of the upstream rough wall condition and the region developing over the smooth surface. Two distinct energetic regions within the boundary layer are observed downstream of the roughness. Within the internal layer the flow over the smooth surface and the corresponding near-wall turbulence establishes under the influence of the outer region that persists from the incoming rough-wall. Energy contained in the outer region diminishes downstream and the behaviour of the near-wall begins to resemble that of a smooth wall boundary layer in equilibrium. Above the internal layer the structure of the outer region appears to be independent of the near-wall condition, however, the structure of the nearwall region depends on the local near wall condition.