Structures of a turbulent spot an investigation by means of multiplane stereo PIV

Kurzfassung

Turbulent spots have been investigated in a laminar flat-plate boundary-layer flow by means of multiplane stereo Particle Image Velocimetry (PIV) in order to underline the significance of the coherent substructures for the growth and turbulent mixing of the spot. For different downstream positions, at Reynolds numbers between 448 < Re < 550, instantaneous velocity vector fields of the spot have been measured for several planes parallel to the wall. Additionally, a statistical analysis of the turbulent flow field has been carried out, including calculations of ensemble averages, rms-fields, Galilean accelerations, probability density functions and space-time-correlations of the velocity fluctuations, the Reynolds stresses of all four quadrants and the y-component of the vorticity. The results of this analysis led to the development of a model which allows to describe the interaction of the dominant substructures and the fluid exchange normal to the wall. The average substructures have been identified as hairpin-like vortices and streaks with strong shear layers, similar to those in fully turbulent boundary layer flows, but they are more orderly