Flow patterns in an axisymmetrical laminar pipe jet with a flat entering velocity profile and a diametrical expansion ratio of 1:2 was investigated using nondisturbing flow visualization. A wavelike instability was discovered in the detached shear layer even at the lowest Reynolds numbers, and for Reynolds numbers greater than 350 tangential motions were observed, resulting in considerable turbulence near the mean position of re-attachment. This turbulence even at low Reynolds explains local high rates of mass and heat transfer. The mean re-attachment position depends on Reynolds number.

Instantaneous mean velocity profiles and wall shear stresses were determined.