Velocity characteristics in the turbulent near wakes of confined axisymmetric bluff bodies

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Abstract
Measurements of the velocity characteristics and wall pressure are reported for the axisymmetric turbulent flow downstream of three bluff bodies (disks of 25% and 50% area blockage and a cone of 25% blockage) confined by a long pipe. The dimensions of the recirculation regions were found from the mean-velocity components, which were determined by a laser-Doppler velocimeter: the corresponding components of Reynolds stress were also recorded. The lengths and maximum widths of the recirculation bubbles (in bluff-body diameters), recirculating mass-flow rates (normalized by the average velocity in the plane of the baffle, U0, and the baffle diameter) and maximum turbulent kinetic energy (normalized by U02) were as follows: cone 1.55, 0.55, 0.19, 0.11; disk (25% blockage) 1.75, 0.62, 0.31, 0.19; disk (50% blockage) 2.20, 0.55, 0.26, 0.16. The increase in recirculation length with blockage is opposite to the trend in unconfined, annular jets. The distribution of Reynolds stresses is strongly dependent on blockage: for the smaller blockage both the disk and the cone have the maximum value of kinetic energy near the rear stagnation point. It is proposed that this is because the generation of turbulence by normal stresses is more important in the flow consequent on the smaller blockage.The measurements include profiles of the velocity characteristics at, as well as upstream of, the trailing edges of the baffles for use as boundary conditions in numerical solutions of the equations of motion.