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

Abstract

For the identification and characterisation of coherent flow structures in turbulent boundary layers and for the examination of the interaction processes between these structures, 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 particular it could be shown that the low-speed streaks in the near wall region of the turbulent boundary layer flow are mainly responsible for the turbulent mixing, especially when they interact with large scale structures coming from the outer part of the boundary layer. This interaction is also of primary importance for the generation of stream-wise vortices. Hairpin-like vortices on the other side are only of secondary importance for the turbulent mixing in the near wall region because these structures do not appear frequently.