Spanwise periodicity and the existence of very large scale coherence in turbulent boundary layers

Abstract

Multiple plane stereo PIV results and data from a rake of ten hot-wire probes are used to investigate the largest scale structures in the log and wake regions of a zero-pressure-gradient turbulent boundary layer. Vector fields from streamwise-spanwise plane stereo PIV reveal long low- and high-speed regions, with a length that often exceeds the viewing window (> 2δ). Instantaneously, a remarkable degree of spanwise organisation is also evident in these fields. This manifests as a persistent spanwise stripiness in the u component of the PIV vector field. Almost all trace of this spanwise organisation is lost in the mean statistics, presumably due to the multitude of scales that are naturally ..