Scanning PIV of turbulent flows over and through rough porous beds using refractive index matching

Rousseau, Gauthier; Ancey, Christophe

doi:10.1007/s00348-020-02990-y

Cited by 29 publications

(20 citation statements)

References 59 publications

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“…2009; Detert, Nikora & Jirka 2010; Dey & Das 2012; Voermans et al. 2017; Rousseau & Ancey 2020) or direct Navier–Stokes simulations (Fang et al. 2018; Kuwata & Kawaguchi 2019; Shen et al.…”

Section: Theoretical Developmentsmentioning

confidence: 99%

An experimental investigation of turbulent free-surface flows over a steep permeable bed

Rousseau

Ancey

2022

J. Fluid Mech.

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Steep streams involve shallow, supercritical turbulent flows over a permeable bed made up of coarse particles. They usually exhibit higher flow resistance and stronger mass and momentum exchanges between the stream and subsurface flow than low-gradient streams. Describing their flow dynamics using generalised Manning–Strickler equations has led to empirical relationships with weak predictive power (errors between predictions and data of over one order of magnitude). We studied shallow turbulent flows by employing a mesoscopic approach based on the double-averaged Navier–Stokes equations. More specifically, we were concerned with the possibility of modelling the turbulent and dispersive shear stress equations using simple algebraic equations. To that end, we studied shallow, supercritical turbulent flows over a sloping bed made up of randomly packed spherical particles. Using visualisation techniques based on particle velocimetry imaging and refractive index matched scanning, we were able to reconstruct the velocity field throughout the bed and stream, far from the sidewalls, and estimate the contributions of the dispersive and turbulent shear stresses to the total shear stress. The dispersive shear stress represented less than 20 % of the turbulent shear stress, but because it was concentrated within a thin layer (called the roughness layer) where it outweighed the turbulent shear stress, it had a significant influence on the mean velocity profile. We proposed an algebraic closure equation for dispersive shear stress, based on the mixing-length model used for turbulent shear stress, and we found that it captured closely the mean-velocity and turbulence-intensity profiles of shallow flows over horizontal or sloping permeable beds. Our data suggest that flow dynamics was affected largely by turbulence damping, drag forces and dispersion within the roughness layer, which may explain why steep streams differ from low-gradient streams.

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Section: Theoretical Developmentsmentioning

confidence: 99%

An experimental investigation of turbulent free-surface flows over a steep permeable bed

Rousseau

Ancey

2022

J. Fluid Mech.

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“…This technique is possible in a laboratory environment if the fluid temperature is stable (as its refractive index retains a known value). This is called refractive index matching (RIM) and has been used not only to study granular flows but also other fluids (Budwig 1994;Li et al 2005;Wiederseiner et al 2011;Dijksman et al 2012;Bai and Katz 2014;Clément et al 2018;Rousseau and Ancey 2020).…”

Section: Refractive Index Matchingmentioning

confidence: 99%

“…The second was a combination of an aqueous sodium iodide solution (for studies using this combination, see Narrow et al 2000;Bai and Katz 2014;Clément et al 2018) and borosilicate glass beads, but the interstitial fluid was too dense and the beads showed positive buoyancy. Finally, we retained borosilicate glass for the particles and a mixture of ethanol and benzyl alcohol for the interstitial fluid-a combination recently used by van der Vaart et al (2015) and Rousseau and Ancey (2020). The liquid mixture's viscosity was close to that of water: ≈ 3 cP.…”

Section: Refractive Index Matchingmentioning

confidence: 99%

A conveyor belt experimental setup to study the internal dynamics of granular avalanches

Trewhela

Ancey

2021

Exp Fluids

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This paper shows how a conveyor belt setup can be used to study the dynamics of stationary granular flows. To visualise the flow within the granular bulk and, in particular, determine its composition and the velocity field, we used the refractive index matching (RIM) technique combined with particle tracking velocimetry and coarse-graining algorithms. Implementing RIM posed varied technical, design and construction difficulties. To test the experimental setup and go beyond a mere proof of concept, we carried out granular flow experiments involving monodisperse and bidisperse borosilicate glass beads. These flows resulted in stationary avalanches with distinct regions whose structures were classified as: (i) a convective-bulged front, (ii) a compact-layered tail and, between them, (iii) a breaking size-segregation wave structure. We found that the bulk strain rate, represented by its tensor invariants, varied significantly between the identified flow structures, and their values supported the observed avalanche characteristics. The flow velocity fields’ interpolated profiles adjusted well to a Bagnold-like profile, although a considerable basal velocity slip was measured. We calculated a segregation flux using recent developments in particle-size segregation theory. Along with vertical velocity changes and high expansion rates, segregation fluxes were markedly higher at the avalanche’s leading edge, suggesting a connection between flow rheology and grain segregation. The experimental conveyor belt’s results showed the potential for further theoretical developments in rheology and segregation-coupled models. Graphic Abstract

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“…Importantly, measurements were performed between 1 cm to 4 cm away from the sidewall to neglect the side boundary layer effects as observed in Figure 3. Uniformity and repeatability were examined in Rousseau (2019) and in Rousseau & Ancey (2020), which provide further information on the experimental procedure and findings.…”

Section: Velocimetry and Scanningmentioning

confidence: 99%

“…In this proceeding paper, we summarize the experimental work and the model based on the double-averaged momentum equation. Further details on the experimental procedure can be found in Rousseau & Ancey (2020). In the results part, the model is compared with experiments and well-known flow resistance equations used in coarse-grained river studies.…”

Section: Introductionmentioning

confidence: 99%

Modeling turbulent stream flows over rough permeable beds

Rousseau¹,

Pascal²,

Ancey³

2020

River Flow 2020

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In steep streams, turbulent flows run over coarse sediments that are similar in size to flow depth. Under these conditions, a significant part of the flow may seep through the permeable bed. Based on experimental data, this paper presents a model able to reproduce the vertical structure of flows over rough permeable beds in low relative submergence conditions. Experiments were performed on open-channel flows passing over tilted coarse-grained beds with slopes within the 0.5% -4% range. Fluid velocities were measured by Particle Image Velocimetry (PIV) and a technique called Refractive Index-Matched Scanning (RIMS), allowing the interior of the bed to be examined. By applying the double averaging methodology, porosity and mean velocity, as well as turbulent and dispersive stresses profiles were collected from the subsurface to the free surface. A turbulent boundary layer over the rough bed was observed while experiments were run at intermediate Reynolds numbers, i.e. Re = O(1000). Under these flow conditions, viscosity played a non-negligible role through the van Driest damping effect. Based on the Prandtl mixing length theory, we propose a model for the turbulent stress that takes into account the continuous porosity profile, damping and dispersive effects. Finally, we show a good agreement between the model and classic flow resistance laws employed for river studies. Our model contrasts with existing boundary-layer models which generally assume a discontinuous porosity profile at bed interface, whether the bed is permeable or impermeable.

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Scanning PIV of turbulent flows over and through rough porous beds using refractive index matching

Cited by 29 publications

References 59 publications

An experimental investigation of turbulent free-surface flows over a steep permeable bed

An experimental investigation of turbulent free-surface flows over a steep permeable bed

A conveyor belt experimental setup to study the internal dynamics of granular avalanches

Modeling turbulent stream flows over rough permeable beds

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