Experimental investigation on axisymmetric turbulent wakes with zero momentum defect

Abstract

An experimental investigation of a turbulent axisymmetric wake with zero momentum defect was carried out. The experiment was conducted in a low speed wind tunnel with a circular tube mounted parallel to the stream. A controlled amount of air was injected into the stream at the end of the model to cancel the drag produced by the turbulent boundary layer on the model. The measurements on the mean flow and the fluctuation quantities were carried up to 90 diameters downstream. By adjusting the strength of the injections the behavior of the pure wake, the co-flowing jet and the matched injection were examined, and the self similar properties both in the mean velocity and the turbulent intensity were found to exist in these cases. Rapid decays off the centerline velocity and the maximum turbulent intensity were observed in the matched injection case. The effect of the initial condition was studied by artificially thickening the boundary layer on the model, and it was observed that the wake relaxes into the final decay law sooner. The unmatched cases, both strong and weak injections, were also investigated; the strong injection case was found to relax into the self-similar weak jet profile and the weaker injection case was observed to approach to the similar wake profile. Flow visualization was conducted on the plane of symmetry and the entrainment process by the engulfing large eddies was visualized in the pure wake and in the co-flowing jet. A lack of large rotations was observed in the matched injection case and the lack of production of turbulent energy was attributed to the faster decay of the flow properties.