Structures of a turbulent spot - an investigation by means of multi-plane stereo PIV

The multi-plane stereo PIV (MSPIV) technique has been applied to an investigation of the spatial and temporal development of turbulent spots in a laminar flat-plate boundary layer flow with a slight adverse pressure gradient. On the basis of a large number of evaluated instantaneous 3- and 2-component velocity vector fields, measured synchronously and separated in space, or with different time separations in one or two planes, the technique enabled the possibility of determining several statistical quantities of fluid mechanical significance: ensemble averages and velocity fluctuations (u´, v´, w´), rms-fields, Galilean accelerations, probability density functions (PDF) and space-time-correlations of the velocity fluctuations, the instantaneous Reynolds stresses (u´v´) of all four quadrants and the y-component of vorticity. The shape and role of coherent substructures for the growth and turbulent mixing of the spot were the focus of this investigation. The average substructures have been identified as hairpin-like vortices arising at the rear of the spot and as wall near stream-wise streaks with strong shear layers, similar to those in fully turbulent boundary layer flows, but in this case more orderly. The interpretation of the results of the statistical analysis and of the instantaneous velocity vector fields leads to the development of a model which can describe, as the main source for turbulent mixing, the interaction of the dominant substructures and the fluid exchange normal to the wall