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Spatially-averaged flow statistics within a canopy of large bluff bodies: Results from direct numerical simulations

Coceal O., Thomas T. G., Belcher S.

Acta Geophysica

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2008

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Vol. 56, no. 3

862-875

EN

The flow within a canopy of large bluff bodies is highly turbulent and spatially heterogeneous. Results from direct numerical simulations over groups of cubical obstacles are analysed using the double-averaging methodology. The obstacles occupy a significant fraction of the canopy space; this gives rise to substantial dispersive stresses within the canopy. The underlying bluff-body turbulent dynamics is different from typical canopy turbulence, and this is reflected in the double-averaged statistics. The spatially-averaged velocities, stresses and drag force depend significantly upon the layout of the obstacles. An ongoing challenge is to parameterise these spatially-averaged quantities in terms of the obstacle geometry and layout.

2

Interface between turbulent flows above and within rough porous walls

Pokrajac D., Manes C.

Acta Geophysica

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2008

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Vol. 56, no. 3

824-844

EN

This paper explores the concept of a macroscopic boundary between turbulent flows above and within rough permeable walls. The macroscopic boundary and the associated conditions for macroscopic flow variables have been thoroughly investigated for laminar, but not for turbulent flows. The literature on laminar flows follows two main conceptual models of the boundary: sharp boundary with step changes in macroscopic variables and gradual boundary with smooth changes of variables. The former approach is usually associated with the two-domain simulation models and the latter one with the single-domain models. This paper presents the derivation of the step conditions for velocity and shear stress at the macroscopic boundary between turbulent boundary layer and turbulent porous media flows. The physical meaning of the main terms in the shear stress condition is discussed in order to clarify the relationship between two-domain and single-domain simulation models.

3

Double-averaging methodology and its application to turbulent flow in and above vegetation canopies

Finnigan J., Shaw R.

Acta Geophysica

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2008

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Vol. 56, no. 3

534-561

EN

Double averaged equations for atmospheric boundary layer flows are introduced as natural extensions of single averaged Reynolds equations. We show that in circumstances where double averaged equations are needed, the two fundamental properties of Reynolds averaging are violated. First, we consider double-averaging in free air turbulence, where the aim is to separate coherent motions from background turbulence. We illustrate the different properties of the main operators that have been used and the physical meaning of the terms that result. Second, in canopy flows, the multiply connected nature of the canopy airspace leads to a different set of departures from the standard Reynolds equations. We establish the physical meaning of the extra terms that arise. Finally we briefly discuss the problems, both practical and theoretical, that arise when we use double averaged equations to interpret real data.

4

Double-averaged velocity and stress distributions for hydraulically-smooth and transitionally-rough turbulent flows

Cameron S. M., Nikora V. I., Coleman S. E.

Acta Geophysica

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2008

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Vol. 56, no. 3

642-653

EN

We analyse experimental measurements of turbulent open-channel flow over hydraulically-smooth and transitionally-rough beds using the double-averaging methodology. Oil with a viscosity of 15×10-6 m2/s is used instead of water so that transitional-range roughness Reynolds numbers can be achieved with large (11.1 mm) roughness elements, allowing spatial variations in the mean velocity field to more easily be measured. Distributions of double-averaged velocities, turbulence intensities, form-induced intensities, and viscous, Reynolds, form-induced and total shear stresses are studied with comparisons made between distributions for hydraulically-smooth, transitionally-rough, and fully-rough boundaries. Measured streamwise turbulence intensities for all experiments peaked at a constant distance from the bed (z ++d + = 15) when elevation scale is adjusted using the zero-plane displacement d for the logarithmic velocity distribution. This collapse suggests that turbulence intensity distributions may be useful in assessing appropriate values of d for transitionally-rough and fully-rough boundaries. Form-induced normal and shear stresses above the roughness tops were found to collapse towards a common curve independent of roughness Reynolds number.