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Secondary currents in turbulent flows over rough walls

Lead Research Organisation: University of Aberdeen
Department Name: Engineering

Abstract

Abstracts are not currently available in GtR for all funded research. This is normally because the abstract was not required at the time of proposal submission, but may be because it included sensitive information such as personal details.
 
Description The channel length required for the development of the flow, from the channel entrance to full establishment, is often a prerequisite when designing hydraulic structures or planning research experiments in open channels. However, the information on the flow development length (LD) is scarce, and even its definition remains vague. In hydraulic experiments, this lack of knowledge introduces great uncertainty, often making comparisons of findings from different studies questionable. This paper offers a physics-based definition for LD, and reports results of systematic laboratory studies to provide guidance on its quantitative assessment. Our data for uniform flows suggest that up to 100 flow depths (H) are required for mean velocity field (including sidewall secondary currents), turbulent stresses (except streamwise variance), velocity skewness and kurtosis, and depth-scale largescale- motions to become essentially independent of the streamwise coordinate. However, very large-scale-motions, streamwise velocity variance, and roughness-induced secondary currents are found to require longer LD of around 150H. Published in Journal of Hydraulic Research, 2023.61(1), "Flow development in rough-bed open channels: mean velocities, turbulence statistics, velocity spectra, and secondary currents"

Time-averaged velocity fields in uniform open-channel flows over rough beds may exhibit spatial heterogeneities due to the effects of bed roughness and secondary currents (SCs). The latter typically originate from the turbulence anisotropy and spatial heterogeneity introduced by the solid and mixed corners (i.e., between sidewalls and water surface), but may also appear due to roughness spanwise heterogeneities, e.g., associated with patchy vegetation distributions or streamwise sediment ridges on the channel bed. In this project, we propose rigorous conservation equations for momentum, kinetic energy and fluid stresses accounting for the contributions of bed roughness and SCs, separately. Particular attention is given to the terms regulating the energy exchanges between roughness-induced and SC related motions, which are expected to provide information on the physical mechanisms leading to the generation of roughness-induced SCs. The proposed approach is illustrated using a large-eddy simulation of a rough-bed open-channel flow.

We employ spectral-space and real-space quantities to characterize large-scale motions (LSMs) and very-large-scale motions (VLSMs) in open-channel flows (OCFs). The obtained data on velocity auto- and co-spectra, spectral correlation coefficient, and structure functions of the second and third orders are consistent with the hypothesis that VLSMs are a special type of coherent structure rather than an alignment of LSMs, even if active interactions between LSMs and VLSMs most likely take place. Based on the project data, we propose a conceptual velocity spectrum for OCFs and an explanation of the mechanism of VLSMs' contribution to the Reynolds shear stress.
Exploitation Route Experimental design, flow modelling; flow control.
Sectors Aerospace

Defence and Marine

Environment
URL http://tandfonline.com/doi/full/10.1080/00221686.2022.2132311
 
Description The project outcomes have been used in experiments, data analysis and interpretations related to (1) hydrokinetic energy convertors for application in open-channel flows; and (2) in modelling fish swimming performance and drag acting on the fish.
First Year Of Impact 2022
Sector Aerospace, Defence and Marine,Education,Energy,Environment
Impact Types Societal
 
Description Field Stereoscopic Particle Image Velocimetry (FSPIV) system for high-resolution in-situ studies of freshwater and marine ecosystems
Amount £289,054 (GBP)
Funding ID NE/T009004/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 09/2019 
End 03/2020
 
Title Upgrade of the Aberdeen Robotic PIV system 
Description The robotic PIV system for measuring instantaneous velocity fields, developed during the previous EPSRC project (EP/K041088/1) has been upgraded with additional measurement modes including a dual plane measurement mode. 
Type Of Material Improvements to research infrastructure 
Year Produced 2022 
Provided To Others? No  
Impact The upgraded PIV system has been used in the experiments related to this EPSRC project. In addition, it has been actively used in experimental studies of hydrokinetic energy convertors and fish hydrodynamics. 
 
Description Melbourne 
Organisation University of Melbourne
Country Australia 
Sector Academic/University 
PI Contribution Our group shares experimental data on fixed and mobile bed granular flows in water channels to support physical interpretation of wind-tunnel experiments conducted by Professor Ivan Marusic and his group. This is ongoing collaboration within EP/K041088/1 "Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes and quantification".
Collaborator Contribution Professor Ivan Marusic and his group helped with planning, methodology, and interpretations, contributing at all project stages for EP/K041088/1 "Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification". The collaboration is ongoing.
Impact Two successful EPSRC research proposals have resulted from discussions with Professor Ivan Marusic (EP/K041088/1 "Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification"; EP/V002414/1 "Secondary currents in turbulent flows over rough walls").
Start Year 2014
 
Description Open-channel flow turbulence, Tsinghua University 
Organisation Tsinghua University China
Country China 
Sector Academic/University 
PI Contribution Advising on open-channel turbulence; advising on data analysis; contributing to joint publications.
Collaborator Contribution Experiments on open-channel turbulence; joint data analysis; contributing to joint publications.
Impact The preparation of outputs is underway.
Start Year 2024
 
Description River hydraulics and engineering, Hohai University 
Organisation Hohai University
Country China 
Sector Academic/University 
PI Contribution Advising on turbulence measurements in laboratory facilities; advising on data analyses of turbulence measurements; conceptual developments in relation to confluence hydrodynamics and turbulence control; contributing to joint publications.
Collaborator Contribution Running experiments in the Hohai laboratory facilities following a joint experimental design; supporting a visiting PhD student at the University of Aberdeen; contributing to joint publications.
Impact The outputs are still under development.
Start Year 2023
 
Description Shallow turbulent flows and mixing layers 
Organisation French National Institute of Agricultural Research
Department INRA Versailles
Country France 
Sector Academic/University 
PI Contribution Data analysis and interpretation in relation to turbulence structure
Collaborator Contribution Extensive laboratory experiments, data handling, analysis and interpretation
Impact Proust, S., Nikora, V. Compound open-channel flows: effects of transverse currents on the flow structure. Journal of Fluid Mechanics, 2020, 885, A24 3. Proust, S., Berni, C., Nikora, V. Shallow mixing layers over hydraulically smooth bottom in a tilted open channel. Journal of Fluid Mechanics 2022, 951, A17
Start Year 2016
 
Description Hosting students from local schools to demonstrate Fluid Mechanics Laboratory and selected experiments 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Hosting students from local schools to demonstrate Fluid Mechanics Laboratory and selected experiments
Year(s) Of Engagement Activity 2023,2024
 
Description Public lectures 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact The talks have focused on career development of emerging researchers and its enhancement through research publications in specialized peer-reviewed journals

I received feedback from a number of early stage researchers that after my talks their publication style has sharply improved.
Year(s) Of Engagement Activity 2014,2015,2016,2017,2021,2022
 
Description Workshop on careers of emergent researchers 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Workshop on careers of emergent researchers organised during the 40th World IAHR Congress in Vienna in 2024.
Year(s) Of Engagement Activity 2023