Effect of adverse pressure gradients on the structure of wall turbulence

Results of an experimental study of the structure of wall turbulence in a symmetric diffuser with inlet cross section of 40 x 100 mm, total expansion angle of 8 deg and a 400 mm long duct leading to the diffusor are presented. The turbulence characteristics were measured in four sections along the diffusor. Stroboscopic flow imaging, with subsequent computer processing of the photographs, was used. Profiles of the longitudinal and transverse (normal to the wall) components of the velocity vector, velocity fluctuations, and Reynolds stress are presented. It is shown that an adverse pressure gradient significantly modifies the magnitudes and nature of the distribution of velocity fluctuations in the wall zone of the boundary layer, and significantly decreases the correlation between longitudinal and transverse fluctuations. Data on the intermittency of the transition from separationless to separated flow over the surface are described.