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Nick Hutchins
Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota 55455, USA

Bharath Ganapathisubramani
Department of Aerospace Engineering and Mechanics, University of Minnesota 107 Akerman Hall, 110 Union Street SE, Minneapolis, MN 55455, USA; Department of Aeronautics, Imperial College London, Prince Consort Road, London, SW7 2AZ

Ivan Marusic
Department of Mechanical Engineering, The University of Melbourne, Melbourne, VIC, 3010, Australia


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 present in wall-bounded turbulence. A 'dejittering' technique has been developed, whereby the instantaneous vector fields are sorted according to dominant spanwise Fourier modes. By applying statistical tools to the sorted subsets, we are able to extract a clear view of the spanwise organisation. We show that the instantaneous spanwise behaviour of the streamwise velocity component (u) is well described by single spanwise sinusoidal modes extending a considerable distance in the wall-normal and streamwise directions. Results are confirmed in the various PIV data-sets. Since the PIV fails to adequately capture the full streamwise extent of the low-speed regions, a rake of hot-wire probes is also employed to capture a continuous view of the spanwise coherence. It is found that the low-speed regions are in fact extremely persistent in the streamwise direction, often exceeding 205 in length. The fact that these long features meander appreciably in the spanwise direction will limit the overall streamwise length-scale as witnessed by a single probe or single point statistic. For instance, in the log region of turbulent boundary layers, premultiplied one-dimensional spectra of the streamwise velocity component (kxΦuu) seem to reveal a peak characteristic lengthscale of 5 − 7δ.