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Large eddy simulation of the velocity-intermittency structure for flow over a field of symmetric dunes

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journal contribution
posted on 04.06.2018, 09:30 by Chris KeylockChris Keylock, Kyoungsik S. Chang, George S. Constantinescu
Owing to their frequent occurrence in the natural environment, there has been significant interest in refining our understanding of flow over dunes and other bedforms. Recent work in this area has focused, in particular, on their shear-layer characteristics and the manner by which flow structures are generated. However, field-based studies, are reliant on single-, or multi-point measurements, rather than delimiting flow structures from the velocity gradient tensor, as is possible in numerical work. Here, we extract pointwise time series from a well-resolved large eddy simulation as a means to connect these two approaches. The at-a-point analysis technique is termed the velocity-intermittency quadrant method and relates the fluctuating, longitudinal velocity, u 1 ′ (t) , to its fluctuating pointwise Hölder regularity, . Despite the difference in boundary conditions, our results agree very well with previous experiments that show the importance, in the region above the dunes, of a quadrant 3 (u 1 ′ < 0, α 1 ′ > 0) dominant near the wall and quadrant 2 dominant close to the lower part of the separated shear layer. These results are consistent with a near-wall advection of vorticity into a region downstream of a temporarily foreshortened reattachment region, and the entrainment of slow moving and quiescent fluid into a faster, more turbulent shear layer. A comparison of instantaneous vorticity fields to the velocity-intermittency analysis shows how the pointwise results reflect larger-scale organisation of the flow. We illustrate this using results from two instantaneous datasets. In the former, extreme velocity-intermittency events corresponding to a foreshortened recirculation region (and high pressures on the stoss slope of the dune immediately downstream) arise, and the development of intense flow structures occurs as a consequence. In the other case, development of a 'skimming flow' with relatively little exchange between the inner and outer regions results in exceedances because of the coherence associated with this high velocity, high turbulence outer region. Thus, our results shed further light on the characteristics of dune flow in the near-wall region and, importantly for field-based research, show that useful information on flow structure can be obtained from single-point single velocity component measurements.

History

School

  • Architecture, Building and Civil Engineering

Published in

Journal of Fluid Mechanics

Volume

805

Pages

656 - 685

Citation

KEYLOCK, C.J., CHANG, K.S. and CONSTANTINESCU, G.S., 2016. Large eddy simulation of the velocity-intermittency structure for flow over a field of symmetric dunes. Journal of Fluid Mechanics, 805, pp.656-685.

Publisher

© Cambridge University Press

Version

AM (Accepted Manuscript)

Publication date

2016-09-23

Copyright date

2016

Notes

This article has been published in a revised form in Journal of Fluid Mechanics http://doi.org/10.1017/jfm.2016.519. This version is free to view and download for private research and study only. Not for re-distribution, re-sale or use in derivative works. © Cambridge University Press.

ISSN

0022-1120

eISSN

1469-7645

Language

en