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1

Turbulent kinetic energy budget in a gravel-bed channel flow

Mignot E., Barthélemy E., Hurther D.

Acta Geophysica

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2008

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Vol. 56, no. 3

601-613

EN

The present experimental investigation focuses on the characteristics of near bed turbulence in a fully rough, uniform open-channel flow over a gravel-type bed. Due to bed topography small scale heterogeneity, the flow is not uniform locally in the near bed region and a double averaging methodology is applied over a length scale much larger than the gravel size. The double-averaged Turbulent Kinetic Energy (TKE) budget derived in the context of the present flow over a gravel bed differs from the TKE budget written for flow over a vegetation canopy. The non-constant shape of the roughness function measured in our gravel bed leads to an additional bed-induced production term which is null for vertical roughness elements, such as simplified vegetation elements. The experimental estimation of the terms of the TKE budget reveals that the maximum turbulent activity takes place away from the reference plane, near the roughness crests. However, within the interface sublayer the work of the bed induced velocity fluctuations against the Reynolds stress is of the same magnitude as the main turbulence production term. Consequently, the characteristics of the TKE budget have similarities with uniform flows over canopies and strongly differ from uniform flows over smooth and transitionally rough flows over sedimentlike beds.

2

Spatially-averaged oscillatory flow over a rough bed

Coleman S. E., Nikora V. I., Schlicke T.

Acta Geophysica

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2008

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Vol. 56, no. 3

698-733

EN

A rigorous framework involving flow decomposition and averaging is presented, within which the mechanics of rough-(e.g., rippled-) bed oscillatory flows can be better interpreted and understood. Spatiallyaveraged equations for conservation of fluid mass and momentum are developed for analyses of rapidly-changing bed conditions, e.g., for growing ripples. Where repeated observations of the changing bed conditions are available, the ensemble and spatially-averaged versions of these equations can be used for more detailed analyses of the flow dynamics. The double-averaged (in space and phase or time) equations of mass and momentum conservation are shown to be appropriate for analyses of flows over fixed rough beds and equilibrium ripples. The value of the present framework is highlighted herein by its application to PIV-measured oscillatory-flow velocities, stresses and vorticities over growing and equilibrium wave-induced intermediate-depth orbital-vortex ripples. In particular, discussions are provided regarding the mechanisms by which gravity-induced and pressure-gradient-induced momentum is transferred to the bed, with the analysis framework naturally and explicitly including the combination of the full range of fluid stresses and boundary form and skin friction drag that is important in defining the flow mechanics.

3

Numerical modeling of environmental flows using DAM: Some preliminary results

Walters R. A., Plew D. R.

Acta Geophysica

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2008

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Vol. 56, no. 3

918-934

EN

Development of closures and parameterizations for subgrid scale effects is a significant and longstanding problem in the numerical simulation of environmental flows. The model described herein uses a rigorous approach for developing double-averaged governing equations — first a traditional Reynolds averaging to derive the Reynolds averaged Navier-Stokes equation (RANS), then a volume average to derive a set of double-averaged equations (DANS). An existing finite element flow model is then modified to accommodate these equations. This process gives rise to several new terms that require closures, as well as a new equation for free surface elevation. This paper is directed toward model development and uses several existing closure schemes as test cases.

4

Micro- and macro-dispersive fluxes in canopy flows

Poggi D., Katul G. G.

Acta Geophysica

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2008

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Vol. 56, no. 3

778-799

EN

Resolving every detail of the three-dimensional canopy morphology and its underlying topography remains untenable when modeling high Reynolds number geophysical flows. How to represent the effects of such a complex morphological variability and any concomittant topographic variability into one-dimensional bulk flow representation remains a fundamental challenge to be confronted in canopy turbulence research. Theoretically, planar averaging to the scale of interest should be applied to the time-averaged mean momentum balance; however, such averaging gives rise to covariance or dispersive terms produced by spatial correlations of time-averaged quantities that remain ‘unclosed’ or require parameterization. When the averaging scale is commensurate with few canopy heights, these covariances can be labeled as ‘micro-dispersive’ stresses. When averaging is intended to eliminate low-wavenumber topographic variations, we refer to these covariances as ‘macro-dispersive’ terms. Two flume experiments were used to explore the magnitude and sign of both micro- and macro-dispersive fluxes relative to their conventional Reynolds stresses counterparts: a rod-canopy with variable roughness density and a dense rod canopy situated on gentle hilly terrain. When compared to the conventional momentum flux, the micro-dispersive fluxes in the lowest layers of sparse canopies can be significant (~50%). For dense canopies, the dispersive terms remain negligible when compared to the conventional momentum fluxes throughout. For the macro-dispersive fluxes, model calculations suggest that these terms can be neglected relative to the Reynolds stresses for a deep canopy situated on a narrow hill. For the region in which topographic variations can interact with the pressure, both model calculations and flume experiments suggest that the macro-dispersive fluxes cannot be neglected, and their value can be 20% of the typical Reynolds stresses.

5

Form induced stresses over rough gravel-beds

Aberle J., Koll K., Dittrich A.

Acta Geophysica

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2008

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Vol. 56, no. 3

584-600

EN

This paper presents an analysis of spatial flow heterogeneity over rough gravel beds for shallow flows in terms of form induced stresses. Data from experiments specifically designed with the intention to analyze the flow data with the double-averaging methodology are used to investigate the nature of form induced stresses. It is shown that spatial flow heterogeneity is small at greater distances to the roughness tops (z 100), increases slightly towards z 100, and increases significantly below z 100. Form induced stresses determined over the same bed and with the same slope are found to be independent of discharge. The influence of the number of measuring verticals on the magnitude of form induced stresses is discussed. The distributions of form induced stresses - are used to define the geodetic level of the roughness crest for rough, irregular beds from hydraulic data.

6

Flow structure over square bars at intermediate submergence: Large Eddy Simulation study of bar spacing effect

Stoesser T., Nikora V. I.

Acta Geophysica

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2008

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Vol. 56, no. 3

876-893

EN

Turbulent open-channel flow over 2D roughness elements is investigated numerically by Large Eddy Simulation (LES). The flow over square bars for two roughness regimes (k-type roughness and transitional roughness between d-type and k-type) at a relative submergence of H/k = 6.5 is considered, where H is the maximum water depth and k is the roughness height. The selected roughness configurations are based on laboratory experiments, which are used for validating numerical simulations. Results from the LES, in turn, complement the experiments in order to investigate the time-averaged flow properties at much higher spatial resolution. The concept of the double-averaging (DA) of the governing equations is utilized to quantify roughness effects at a range of flow properties. Double-averaged velocity profiles are analysed and the applicability of the logarithmic law for rough-wall flows of intermediate submergence is evaluated. Momentum flux components are quantified and roughness effect on their vertical distribution is assessed using an integral form of the DA-equations. The relative contributions of pressure drag and viscous friction to the overall bed shear stress are also reported.