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Statistical structure of high-Reynolds-number turbulence close to the free surface of an open-channel flow

Published online by Cambridge University Press:  14 January 2003

ISABELLE CALMET
Affiliation:
Ecole Centrale de Nantes, UMR 6598, 1 rue de la Noe, 44321 Nantes cedex 3, France
JACQUES MAGNAUDET
Affiliation:
Institut de Mécanique des Fluides de Toulouse, UMR 5502, 2 avenue Camille Soula, 31400 Toulouse, France

Abstract

Statistical characteristics of turbulence in the near-surface region of a steady open- channel flow are examined using new data obtained in a high-Reynolds-number large-eddy simulation using a dynamic subgrid-scale model. These data, which correspond to a Reynolds number Re* = 1280 based on the total depth and shear velocity at the bottom wall, are systematically compared with those found in available direct numerical simulations in which Re* is typically one order of magnitude smaller. Emphasis is put on terms involved in the turbulent kinetic energy budget (dominated by dissipation and turbulent transport), and on the intercomponent transfer process by which energy is exchanged between the normal velocity component and the tangential ones. It is shown that the relative magnitude of the pressure–strain correlations depends directly on the anisotropy of the turbulence near the bottom of the surface-influenced layer, and that this anisotropy is a strongly decreasing function of Re*. This comparison also reveals the Re*-scaling laws of some of the statistical moments in the near-surface region, especially those involving vorticity fluctuations. Velocity variances, length scales and one-dimensional spectra are then compared with predictions of the rapid distortion theory elaborated by Hunt & Graham (1978) to predict the effect of the sudden insertion of a flat surface on a shearless turbulence. A very good agreement is found, both qualitatively and quantitatively, outside the thin viscous sublayer attached to the surface. As the present high-Reynolds-number statistics have been obtained after a significant number of turnover periods, this agreement strongly suggests that the validity of the Hunt & Graham theory is not restricted to short times after surface insertion.

Type
Research Article
Copyright
© 2003 Cambridge University Press

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