The organized motions in 'puffs' in transitional pipe flow.

The turbulence production process in a puff in transitional pipe flow was studied by means of flow visualization. The technique of water soluble dye injection and laser sheet lighting was employed for visualization. It is found that the upstream laminar fluid enters the relatively slower moving turbulent puff around the pipe centre as a jet. This 'jetting action' generates large scale ring vortices like that in an axisymmetric mixing layer. These structures then undergo a chaotic breakdown to smaller scales followed by relatively long regions of decay and relaminarization. This formation of ring vortices and their breakdown is a continuous process and seems to be responsible for the self sustenance of an equilibrium puff as it moves downstream. (A)