Stresses and Drag in Turbulent Bed Load From Refractive Index‐Matched Experiments

• Published in: Geophysical research letters Vol. 45; no. 14; pp. 7000 - 7009
• Main Authors: Ni, Wei‐Jay, Capart, Hervé
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.07.2018
• Subjects: Bed load; Channel flow; Computational fluid dynamics; Constitutive relationships; Drag; Dynamics; Experiments; Fluid flow; Fluidization; Fluidized beds; Fluvial sediments; Forces (mechanics); Imaging techniques; Kinetic theory; Lasers; Load matching; Local flow; Momentum; Momentum balance; Open channel flow; Profiles; Refractive index; refractive index matching; Refractivity; Reynolds stress; Rivers; Sediment load; Sediment transport; Statistical methods; Statistics; Transport; Turbulence; Velocity; Vertical profiles
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL077571

We report steady open‐channel flow experiments that resolve the internal dynamics of turbulent bed load layers at subgrain diameter resolution. Optical access is gained by using materials of matched refractive index, and a laser light sheet is scanned across the medium to capture both the solid and liquid motions over a 3‐D volume. The imaging measurements yield vertical profiles of granular concentration, solid and liquid mean velocity, and velocity fluctuation statistics including the granular temperature and the Reynolds stress. This makes it possible to determine all principal contributions to the momentum balance of each phase. In particular, experimental profiles are obtained for the stresses and interphase drag force. They are used to test constitutive relationships derived from kinetic theory, turbulence theory, and fluidization cell experiments. Plain Language Summary Turbulent bed load is responsible for the transport of coarse sediment by rivers and waves. Because grains are typically opaque and highly concentrated near the bed, one cannot look inside, and intrusive probes would otherwise disturb the local flow. In this work we acquire measurements inside turbulent bed load layers by making the medium transparent, using refractive index‐matched solid and liquid materials. We then scan a laser sheet across a 3‐D volume to capture liquid and granular motions and acquire measurements of key properties including solid and liquid velocities, granular concentration, and statistics of the velocity fluctuations. We deduce the principal contributions to the momentum balance of each phase and test relations describing stresses and drag forces. The measurements support the applicability of continuum two‐phase models to turbulent bed load transport. Such models provide key tools for the study of bed load but could not previously be tested in such detail. Key Points Granular and liquid motions in turbulent bed load are acquired from laser scans We deduce the principal contributions to the momentum balance of each phase The data allow testing of constitutive relations for stresses and drag