These are pockets of turbulence within a laminar flow at Reynolds number around transition. These structures typically move at a speed below that of the fluid flow and have a distinctive trailing edge. The leading edge is not well defined, as the turbulent flow around the trailing edge morphs into larger-scale structures downstream and ultimately laminar flow.

This distinguishes it from a turbulent slug, which has both a well-defined leading and trailing edge (see diagram below):

Puffs are typically generated in experiments by introducing a disturbance to a laminar flow close to transition. Numerically, this technique is not so straightforward and puffs are attained by gradually decreasing the Reynolds number of a turbulent flow. To study puffs, their formation, and evolution we have proposed the use of a gradually expanding pipe. Some preliminary results are shown below.

The above diagram shows streamwise vorticity with higher vorticity present in the darker regions. In the centre of the figure (d), a velocity trace shows the centreline velocity along the length of the pipe. A fully-turbulent field is maintained upstream which propagates through a slight expansion in the pipe (c). The change in pipe diameter results in a slight drop in Reynolds number which causes turbulent puffs to form (a and b). The puffs are well developed by the end of the pipe (e).

• Transient growth of separated flows, C.D.Cantwell, PhD thesis [ pdf ]