The flow in an axisymmetric contraction fitted to a fully developed pipe flow is experimentally and numerically studied. The reduction in turbulence intensity in the core region of the flow is discussed on the basis of the budgets for the various turbulent stresses as they develop downstream. The contraction generates a corresponding increase in energy in the near-wall region, where the sources for energy production are quite different and of opposite sign compared to the core region, where these effects are caused primarily by vortex stretching. The vortices in the pipe become aligned with the flow as the stretching develops through the contraction. Vortices which originally have a spanwise component in the pipe are stretched into pairs of counter-rotating vortices which become disconnected and aligned with the mean flow. The structures originating in the pipe which are inclined at an angle with respect to the wall are rotated towards the local mean streamlines. In the very near-wall region and the central part of the contraction the flow tends towards twocomponent turbulence, but these structures are different. The streamwise and azimuthal stresses are dominant in the near-wall region, while the lateral components dominate in the central part of the flow. The two regions are separated by a rather thin region where the flow is almost isotropic.

Direct numerical simulations (DNS) were carried out to investigate the turbulence statistics and coherent structures in a fluid flow through a conical diffuser, called an Azad diffuser, for three opening angles of 2 • , 4 • , and 8 • . The adverse pressure gradient affected the axisymmetric flow such that the streamwise component of the turbulence intensity was significantly increased in the outer region of the diffuser. The maximum Reynolds shear stress increased as the opening angle increased. The premultiplied energy spectra showed that the most energetic wall-normal height and its wavelength increased when the opening angle increased. Time-resolved instantaneous flow fields and twopoint correlation functions provided evidence for the streamwise merging of low-speed streaking structures. The outer vortical structures shifted toward the diffuser axis as the flow passed the diffuser throat. The swirling motions of the individual hairpins in the outer region were stronger in the diffuser flow than in the straight pipe flow.

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