Pseudo-Sound Behind an Obstacle on a Cylinder in Axial Flow

Pseudo-sonic fluctuations of the wall pressure behind a ring-shaped obstacle on a flexible extended cylinder in longitudinal flow of fluid have been experimentally investigated. Integral and spectral statistical characteristics of pressure fluctuation field behind the obstacle have been obtained and its influence on the structure of the turbulent boundary layer has been investigated. Putting the obstacle in the interior of the boundary layer changes the structure of the whole boundary layer. As the obstacle diameter increases, the intensity of the near-wall pressure fluctuations grows. The maximum intensity is observed in the near wake of the obstacle. At the distances exceeding 100 diameters of the obstacle, the turbulent boundary layer recovers. An increase of the obstacle diameter and flow velocity results in a growth of the low-frequency spectral components of pressure fluctuations and in a decay of the high-frequency components, compared to a boundary layer on hydraulically smooth cylinder. The turbulent boundary layer behind the obstacle is saturated with large-scale vortical structures. The biggest contribution to the energy of the pseudo-sonic pressure fluctuation field is made by the vortices shedded from an obstacle transverse to the flow, the frequencies of those conforming to the Strouhal number Sh approximate to 0.1. In the subcritical regime of the separation flow on the ring obstacle, the Strouhal number is inversely proportional to the Reynolds number.