Aspects of turbulent/non-turbulent interfaces

A distinct boundary between turbulent and non-turbulent regions in a fluid of otherwise constant properties is found in many laboratory and engineering turbulent flows including jets, mixing layers, boundary layers, and wakes. Generally, the flow has mean shear in at least one direction within the turbulent zone, but the non-turbulent zones have no shear (adjacent laminar shear is a dierent case, e.g. transition in a boundary layer). There may be purely passive dierences between the turbulent and non-turbulent zones, e.g. small variations in temperature or scalar concentration, for which turbulent mixing is an important issue. The boundary has several major characteristics of interest for the present study. Firstly, the boundary advances into the non-turbulent fluid, or in other words, non-turbulent fluid is entrained. Secondly, the change in turbulence properties across the boundary is remarkably abrupt; strong turbulent motions come close to the non-turbulent fluid, promoting entrainment. Thirdly, the boundary is irregular with a continually changing convoluted shape, which produces statistical intermittency. Its shape is contorted at all scales of the turbulent motion