Mechanical Engineering - Research Publications

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    Dominant spanwise Fourier modes, and the existence of very large scale coherence in turbulent boundary layers
    Hutchins, N. ; Ganapathisubramani, B. ; Marusic, I. ( 2004)
    Multiple plane stereo PIV results and data from a rake often hot-wire probes are used to investigate the largest scalestructures in a zero-pressure-gradient turbulent boundary layer.Instantaneous vector fields from stereo PIV in spanwise-streamwiseplanes reveal long low- and high-speed regions,with a length that often exceeds the viewing window (> 2d).Also evident is a remarkable degree of spanwise organisation,that manifests as a persistent spanwise stripiness in the u componentof the PIV vector field. Almost all trace of such spanwiseorganisation is lost in the mean statistics, presumably dueto the multitude of scales naturally present in wall-bounded turbulence.This can be overcome by ‘de-jittering’ the instantaneousvector fields. By sorting the data according to dominantspanwise fourier modes, and then applying simple statisticaltools to the sorted subsets, we are able to extract a clear viewof spanwise organisation. Results are confirmed in the variousPIV data-sets. Since the PIV fails to adequately capture the fullstreamwise extent of the low-speed regions, a rake of hot-wireprobes is also employed to capture a continuous view of thespanwise coherence. It is found that the low-speed regions arein fact extremely persistent in the streamwise direction, oftenexceeding 20 d in length. The fact that these long features meanderappreciably in the spanwise direction will limit the overallstreamwise length-scale as witnessed by a single probe or singlepoint statistic. For instance, premultiplied one-dimensionalspectra of the streamwise velocity (kxFuu) at this z/d show apeak contribution for characteristic lengthscales of 5-7d.
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    Inclined cross-stream stereo PIV measurements in turbulent boundary layers.
    Hutchins, N. ; Hambleton, W. ; Marusic, I. (CIMNE, 2004)
    By arranging the laser light-sheet and image plane of a stereo PIV system ininclined spanwise/wall-normal planes (inclined at both 45± and 135± to the x-axis) we have obtained a unique quantitative view of the turbulent boundarylayer in planes aligned both with and against the principle vorticity axis of aproposed hairpin model. These experiments have been repeated across a range ofReynolds numbers (Re¿ ¼ 800 ¡ 3050). In-plane swirl results indicate the presenceof inclined eddies, arranged about low-speed regions (with circumstantialevidence suggesting that these occasionally group into packet-like formations).Two-point correlations show that outer scaling is the correct way to quantifythe characteristic spanwise lengthscale across the range of Re¿ .