The growth of small disturbances in the incompressible axisymmetric wake of a slender body of revolution has been studied experimentally. The experiments were performed in a running water channel at a Reynolds number of 3600 based on the body diameter and free-stream velocity, without any artificial stimulation of the wake. Digital techniques have been used to analyse the output signals of a multichannel hot-film anemometer system.
Analysis of the mean velocity profiles and the velocity fluctuations shows that the wake can be divided into three regimes: the near wake or adjustment region, a region of locally parallel flow, and the far wake. The near wake is characterized by the adjustment of the mean velocity profile from that in the boundary layer of the body to the wake profile. At the same time, the structure of the disturbances in the boundary layer seems to adjust to a different structure in the wake. Large velocity gradients in both the streamwise and the radial direction are present in this region. In the region of locally parallel flow, a helical mode disturbance is superseded by a mode which appears to be mostly a planar oscillation of higher frequency and amplification rate. Higher-order harmonics of dominant frequency components have been identified in these experiments. The far wake is the region in which nonlinear interactions appear to predominate. The experimental measurements and hydrogen bubble photographs indicate the presence of large scale transverse disturbances in this region, associated with roll-up of the fluid.