The significance of coherent flow structures for the turbulent mixing in wall-bounded flows

Abstract

For the identification and charaeterisation of coherent flow struetures In turbulent boundary layers and for the examination of the interaetion processes between these struetures, a flat plate boundary layer flow was investigated at high Reynolds number with the Multiplane Stereo Particle Image Velocimetry. The Multiplane Stereo Particle Image Velocimetry was developed for this particular investigation in order to determine flow quantities for the first time which were not accessible by any other measurement technique before. In addition a new calibration procedure was developed and the generation of tracer particles for optical flow investigations was examined to increase the measurement accuracy. The existence of various coherent structures with different geometrical and dynamical properties could be confirmed and their significance for the turbulent mixing was estimated. In partlcular It could be shown that the lowspeed streaks In the near wall region of the turbulent boundary layer flow are malnly responsible for the turbulent mixing, especially when they Interaet with large scale struetures coming from the outer part of the boundary layer. This interaction 15also of primary Importance for the generation of stream-wise vortices. Halrpln-like vortices on the other side are only of secondary importance for the turbulent mixing In the near wall region because these struetures do not appear frequently.