Hot-wire investigation of the wake behind cylinders at low Reynolds numbers

The hot-wire technique has been used to measure the regular vortex street pattern behind a cylinder at low Reynolds number. Measurements of mean velocity distribution were made both below and above the critical Reynolds number at which the periodic motion appears. Amplitude and phase measurements gave sufficient information for computation of the instantaneous flow pattern of the vortex system. The important points resulting from the investigation are that (i) the critical Reynolds number at which vortices are shed is 40, (ii) in the range of Reynolds numbers investigated the vortices are not shed directly from the cylinder but appear some distance downstream as an instability of the laminar wake.