Velocity characteristics of the flow in the near wake of a disk

Abstract

Measurements of the velocity characteristics of near-wake flows were obtained with a direction-sensitive laser-Doppler anemometer. The wakes were formed downstream of central disks of diameters 8·9, 12·5 and 14·2 mm which were located on the centreline of a 20·0 mm jet. Detailed measurements were obtained with initial annular-jet velocities of from 9·4 to 39·5 m/s and include values of the axial and radial components of the mean velocity, the three normal stresses and the shear stress. Probability density distributions and energy spectra were also measured.The results show, for example, that the curvature of the annular jet increases with disk diameter and that the ratios of the maximum positive and negative centre-line velocities to the exit velocity increase with decreasing disk diameter and are essentially independent of the initial velocity. The turbulent field is substantially anisotropic with a minimum turbulence intensity of around 30% in the recirculation region; the locations of zero shear stress and zero mean velocity gradient are not coincident. The measured spectra reveal predominant frequencies in a small region of the outer-shear layer and in the vicinity of the jet exit; these discrete frequencies did not propagate downstream nor into the recirculation region.