Very-large-scale motions in rough-bed open-channel flow

Acknowledgements The study has been supported by two EPSRC/UK grants, ‘High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport’ (EP/G056404/1) and ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (EP/K041169/1). Discussions with I. Marusic and comments of three anonymous reviewers are greatly appreciated.Peer reviewedPublisher PD

The structure of gravel-bed flow with intermediate submergence: a laboratory study

The paper reports an experimental study of the flow structure over an immobile gravel bed in open channel at intermediate submergence, with particular focus on the near-bed region. The experiments consisted of velocity measurements using three-component (stereoscopic) Particle Image Velocimetry (PIV) in near-bed horizontal plane and two-component PIV in three vertical planes that covered three distinctly different hydraulic scenarios where the ratio of flow depth to roughness height (i.e., relative submergence) changes from 7.5 to 10.8. Detailed velocity measurements were supplemented with fine-scale bed elevation data obtained with a laser scanner. The data revealed longitudinal low-momentum and high-momentum "strips'' in the time-averaged velocity field, likely induced by secondary currents. This depth-scale pattern was superimposed with particle-scale patches of flow heterogeneity induced by gravel particle protrusions. A similar picture emerged when considering second-order velocity moments. The interaction between the flow field and gravel-bed protrusions is assessed using cross correlations of velocity components and bed elevations in a horizontal plane just above gravel particle crests. The cross correlations suggest that upward and downward fluid motions are mainly associated with upstream-facing and lee sides of particles, respectively. Results also show that the relative submergence affects the turbulence intensity profiles for vertical velocity over the whole flow depth, while only a weak effect, limited to the near-bed region, is noticed for streamwise velocity component. The approximation of mean velocity profiles with a logarithmic formula reveals that log-profile parameters depend on relative submergence, highlighting inapplicability of a conventional "universal'' logarithmic law for gravel-bed flows with intermediate submergence

Secondary currents and very-large-scale motions in open-channel flow over streamwise ridges

Financial support was provided by the EPSRC/UK grant ‘Bed friction in roughbed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (grant EP/K041088/1). Discussions with I. Marusic (University of Melbourne) and B. Ganapathisubramani (University of Southampton) are greatly appreciated.Peer reviewedPublisher PD

Drag forces on a bed particle in open-channel flow : Effects of pressure spatial fluctuations and very-large-scale motions

The study has been supported by two EPSRC/UK grants, ‘High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport’ (EP/G056404/1) and ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (EP/K041088/1). I.M. also acknowledges the support of the Australian Research Council.Peer reviewedPublisher PD

Heterogeneity in catchment properties: a case study of Grey and Buller catchments, New Zealand

International audienceThe scaling behaviour of landscape properties, including both morphological and landscape patchiness, is examined using monofractal and multifractal analysis. The study is confined to two neighbouring meso-scale catchments on the west coast of the South Island of New Zealand. The catchments offer a diverse but largely undisturbed landscape with population and development impacts being extremely low. Bulk landscape properties of the catchments (and their sub-basins) are examined and show that scaling of stream networks follow Hack's empirical rule, with exponents ?0.6. It is also found that the longitudinal and transverse scaling exponents of stream networks equate to ?l ?0.6 and ?w? 0.4, indicative of self-affine scaling. Catchment shapes also show self-affine behaviour. Further, scaling of landscape patches show multifractal behaviour and the analysis of these variables yields the characteristic parabolic curves known as multifractal spectra. A novel analytical approach is adopted by using catchments as hydrological cells at various sizes, ranging from first to sixth order, as the unit of measure. This approach is presented as an alternative to the box-counting method as it may be much more representative of hydro-ecological processes at catchment scales. Multifractal spectra are generated for each landscape property and spectral parameters such as the range in ? (Holder exponent) values and maximum dimension at ?0, (also known as the capacity dimension Dcap), are obtained. Other fractal dimensions (information Dinf and correlation Dcor) are also calculated and compared. The dimensions are connected by the inequality Dcap?Dinf?Dcor. Such a relationship strongly suggests that the landscape patches are heterogeneous in nature and that their scaling behaviour can be described as multifractal. The quantitative parameters obtained from the spectra may provide the basis for improved parameterisation of ecological and hydrological models. Keywords: fractal, multifractal, scaling, landscape, patchines

Turbulent friction in flows over permeable walls

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Meandering of instantaneous large-scale structures in open-channel flow over longitudinal ridges

The results of large-eddy simulations of open-channel flows over spanwise heterogeneous surface ridges at two representative spanwise spacings are presented. Flows at moderate Froude and Reynolds numbers over smooth channel beds with streamwise-orientated rectangular ridges are considered. The ridge spacing has a profound effect on the flow: at small spacing relatively small secondary cells occur, whilst at large ridge spacing secondary cells occupy the entire flow depth. The instantaneous flow features secondary flow instabilities and the meandering of alternating low- and high-momentum regions. The quasi-periodical nature of the meandering of the instantaneous large-scale motion is visualised and quantified for both ridge spacings. Although time-averaged clockwise and counter-clockwise secondary current cells are symmetrical about the ridge-axis, they exhibit quasi-periodical increase and decrease in size as well as lateral and vertical movement in space over the meandering period

Bed-load effects on hydrodynamics of rough-bed open-channel flows

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Friction factor decomposition for rough-wall flows : theoretical background and application to open-channel flows

Financial support was provided by the EPSRC/UK project ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (grants EP/K041088/1 and EP/K04116/1). I.M. acknowledges the support of the Australian Research Council (grant FL120100017). The large-eddy simulations were carried out at Cardiff University’s high performance computer, which is part of the Supercomputing Wales project. Useful and stimulating discussions with M. Fletcher (Arup), P. Samuels (HR Wallingford), T. Schlicke (Scottish Environment Protection Agency) and J. Wicks (Jacobs) have been instrumental for this project and are gratefully acknowledged. The editor and three reviewers provided insightful comments and helpful suggestions that have been gratefully incorporated in the final version.Peer reviewedPublisher PD