TURBULENT SPOTS IN CHANNEL FLOWS

A number of investigations into the formation and development of turbu lent spots in plane Poiseuille, plane Couette and watertable flows are reviewed. Three main observations are drawn from this work. Firstly, the initial development is associated with the transient growth due to the three-dimensional lift-up effect. Secondly, the spreading and pro pagation velocities of the different spots are quite similar. Thirdly, the velocity field inside the spots show essentially all the characte ristics of fully developed turbulent flow, albeit at low Reynolds numb ers. The mechanism behind the rapid spreading of the spots, however, i s not yet fully understood, although observations of wave activity, mo difications of the surrounding flow field and stability calculations p oint in the direction of growth by destabilization.