A previous paper (Brennen (1968b)) reported the observation and analysis of wave patterns on the surface of fully developed cavities behind a series of axisymmetric headforms in No.2 water tunnel at Ship Division, NPL. Comparison of theory and experiment appeared to confirm that these waves, which appeared a short distance after separation, grew in amplitude as they were convected downstream and then under certain conditions broke up into turbulence, were the amplified result of a select frequency instability in the separated or cavity surface boundary layer.
The small vertical tunnel (figure 1) was employed to study and extend observation of the same phenomenon to smaller headforms and Reynolds numbers. An additional intention was to investigate the effect of small quantities of polymer additive on the behaviour of this instability. But, a more dramatic phenomenon was manifest with the addition of these drag-reducing chemicals, leaving the original objective unattainable
