Momentum and energy transfer in open-channel flow over streamwise ridges

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

Large and very large scale motions in roughbed open-channel flow

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).Peer reviewedPublisher PD

On application of empirical mode decomposition for turbulence analysis in open-channel flows

The study has been supported by the EPSRC/UK grants: “Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification” (EP/K041088/1) and “Secondary currents in turbulent flows over rough walls” (EP/V002414/1).Peer reviewedPublisher PD

Effects of Streamwise Ridges on Hydraulic Resistance in Open-Channel Flows

Funding Information: Financial support was provided by the EPSRC/UK grant “Bed Friction in Rough-Bed Free-Surface Flows: A Theoretical Frame-work, Roughness Regimes, and Quantification” (Grant No. EP/ K041088/1). Discussions with I. Marusic (University of Melbourne) and B. Ganapathisubramani (University of Southampton) are greatlyPeer reviewedPostprin

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

Funding Information: The work presented in this paper is supported by the EPSRC under Project Numbers EP/R022135/1, EP/V002384/1 and EP/V002414/1. The simulations were carried out on UCL’s supercomputer Kathleen. The first author is funded by UCL’s Department of Civil, Environmental and Geomatic Engineering. The authors are thankful to the reviewers for their useful comments. Publisher Copyright: © 2023, The Author(s).Peer reviewedPublisher PD

Flow development in rough-bed open channels : mean velocities, turbulence statistics, velocity spectra, and secondary currents

Acknowledgements: The authors wish to express their gratitude to Roy Gillanders for the help provided in the laboratory and to the School of Engineering of the University of Aberdeen for the support. The comments and suggestions of the Associate Editor and two anonymous reviewers helped to improve the final version of the paper and are much appreciated. Funding The study has been supported by three Engineering and Physical Sciences Research Council/UK grants: “High-resolution numerical and experimental studies of turbulence-induced sediment erosion and near-bed transport” (EP/G056404/1), “Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification” (EP/K041088/1) and “Secondary currents in turbulent flows over rough walls” (EP/V002414/1). Publisher Copyright: © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.Peer reviewedPublisher PD

Hydraulic resistance in open-channel flows over self-affine rough beds

Acknowledgements The authors wish to express their gratitude to Stephan Spiller for advice regarding the silicone moulds, to Cameron Scott for assisting with manufacturing of the roughness elements and Davide Collautti for help with conducting experiments.Peer reviewedPublisher PD

Turbulent drag reduction by spanwise wall forcing. Part 1: Large-eddy simulation

Turbulent drag reduction through streamwise travelling waves of spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the drag reduction at friction Reynolds numbers $Re_\tau = 951$ and $4000$. The actuation parameter space is restricted to the inner-scaled actuation (ISA) pathway, where drag reduction is achieved through direct attenuation of the near-wall scales. The level of turbulence attenuation, hence drag reduction, is found to change with the near-wall Stokes layer protrusion height $\ell_{0.01}$. A range of frequencies is identified where the Stokes layer attenuates turbulence, lifting up the cycle of turbulence generation and thickening the viscous sublayer; in this range, the drag reduction increases as $\ell_{0.01}$ increases up to $30$ viscous units. Outside this range, the strong Stokes shear strain enhances near-wall turbulence generation leading to a drop in drag reduction with increasing $\ell_{0.01}$. We further find that, within our parameter and Reynolds number space, the ISA pathway has a power cost that always exceeds any drag reduction savings. This motivates the study of the outer-scaled actuation (OSA) pathway in Part 2, where drag reduction is achieved through actuating the outer-scaled motions

Turbulent drag reduction by spanwise wall forcing. Part 2: High-Reynolds-number experiments

Here, we present measurements of turbulent drag reduction at high friction Reynolds numbers in the range of $4500 \le Re_\tau \le 15000$. The efficacy of the approach, using streamwise travelling waves of spanwise wall oscillations, is studied for two actuation regimes: (i) inner-scaled actuation (ISA), as investigated in Part 1 of this study, which targets the relatively high-frequency structures of the near-wall cycle, and (ii) outer-scaled actuation (OSA), which was recently presented by Marusic et al. (Nat. Commun., vol. 12, 2021) for high-$Re_\tau$ flows, targeting the lower-frequency, outer-scale motions. Multiple experimental techniques were used, including a floating-element balance to directly measure the skin-friction drag force, hot-wire anemometry to acquire long-time fluctuating velocity and wall-shear stress, and stereoscopic-PIV (particle image velocimetry) to measure the turbulence statistics of all three velocity components across the boundary layer. Under the ISA pathway, drag reduction of up to 25% was achieved, but mostly with net power saving losses due to the high-input power cost associated with the high-frequency actuation. The low-frequency OSA pathway, however, with its lower input power requirements, was found to consistently result in positive net power savings of 5 - 10%, for moderate drag reductions of 5 - 15%. The results suggest that OSA is an attractive pathway for energy-efficient drag reduction in high Reynolds number applications. Both ISA and OSA strategies are found to produce complex inter-scale interactions, leading to attenuation of the turbulent fluctuations across the boundary layer for a broad range of length and time scales

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