Mechanical Engineering - Research Publications

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    Some predictions of the attached eddy model for a high Reynolds number boundary layer
    Nickels, T. B. ; Marusic, I. ; Hafez, S. ; Hutchins, N. ; Chong, M. S. (Royal Society Publishing, 2007-01)
    Many flows of practical interest occur at high Reynolds number, at which the flow inmost of the boundary layer is turbulent, showing apparently random fluctuations invelocity across a wide range of scales. The range of scales over which these fluctuationsoccur increases with the Reynolds number and hence high Reynolds number flows aredifficult to compute or predict. In this paper, we discuss the structure of these flows anddescribe a physical model, based on the attached eddy hypothesis, which makespredictions for the statistical properties of these flows and their variation with Reynoldsnumber. The predictions are shown to compare well with the results from recentexperiments in a new purpose-built high Reynolds number facility. The model is alsoshown to provide a clear physical explanation for the trends in the data. The limits ofapplicability of the model are also discussed.
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    Evidence of very long meandering features in the logarithmic region of turbulent boundary layers
    Hutchins, N. ; MARUSIC, IVAN (Cambridge University Press, 2007)
    A regime of very long meandering positive and negative streamwise velocity fluctuations, that we term ‘superstructures’, are found to exist in the log and lower wake regions of turbulent boundary layers. Measurements are made with a spanwise rake of 10 hot-wires in two separate facilities (spanning more than a decade of Ret) and are compared with existing PIV and DNS results. In all cases, we note evidence of a large-scale stripiness in the streamwise velocity fluctuations. The length of these regions can commonly exceed 20d. Similar length scales have been previously reported for pipes and DNS channel flows. It is suggested that the true length of these features is masked from single-point statistics (such as autocorrelations and spectra) by a spanwise meandering tendency. Support for this conjecture is offered through the study of a synthetic flow composed only of sinusoidally meandering elongated low- and high-speed regions. From detailed maps of one-dimensional spectra, it is found that the contribution to the streamwise turbulence intensities associated with the superstructures appears to be increasingly significant with Reynolds number, and scales with outer length variables (d). Importantly, the superstructure maintains a presence or footprint in the near-wall region, seeming to modulate or influence the near-wall cycle.