Name: Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions (Video) | JoVE Journal
Uploaded: 2018-02-22T15:00:00.000+00:00
Duration: 11 min 51 s
Description: Learn two methods to characterize incipient particle motion in regular substrates from laminar to turbulent flow. This approach quantifies sediment bed geometry effects on particle dynamics across industrial and natural applications.

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The authors are thankful to unknown referees for valuable advice and to Sukyung Choi, Byeongwoo Ko and Baekkyoung Shin for the collaboration in setting up the experiments. This work was supported by the Brain Busan 21 Project in 2017.

The authors have nothing to disclose

We present two different experimental methods for characterizing the incipient particle motion as a function of the sediment bed geometry. For that purpose, we use a monolayer of spheres regularly arranged according to a triangular or quadratic symmetry in such a way that the geometrical parameter simplifies to a single geometry. In the creeping flow limit, we describe the experimental method using a rotational rotameter to induce the laminar shear flow as in previous studies39,<sup cla...

Incipient particle motion is encountered in a wide range of industrial and natural processes. Environmental examples include the initial process of sediment transport in river and oceans, bed erosion or dune formation among others 1,2,3. Pneumatic conveying4, removal of pollutants or cleaning of surfaces5,6 are typical industrial applications involving the onset of particle motion.
Due to the broad range of applications, the onset of particle motion has been extensively studied over a century, mostly under turbulent conditions7,8,9,10,11,12,13,14,15. Many experimental approaches have been applied to determine the threshold for the onset of motion. The studies include parameters such as the particle Reynolds number13,16,17,18,19,20, the relative flow submergence21,22,23,24 or geometrical factors as the angle of repose16,18,25, exposure to the flow26,27,28,29, relative grain protrusion29 or streamwise bed slope30.
The current data for the threshold including turbulent conditions are broadly scattered12,31 and the results often seem inconsistent24. This is mostly due to the inherent complexity of controlling or determining flow parameters under turbulent conditions13,14. Besides, the threshold for sediment motion strongly depends on the mode of motion, i.e. sliding, rolling or lifting17 and the criterion to characterize the incipient motion31. The latter may be ambiguous in an erodible sediment bed.
During the last decade, experimental researchers have studied incipient particle motion in laminar flows32,33,34,35,36,37,38,39,40,41,42,43,44, where the wide spectrum of length scales interacting with the bed is avoided45. In many practical scenarios implying sedimentation, the particles are quite small and the particle Reynolds number remains lower than about 546. On the other hand, laminar flows are able to generate geometric patterns as ripples and dunes as turbulent flows do42,47. Similitudes in both regimens have been shown to reflect analogies in the underlying physics47 so important insight for the particle transport can be obtained from a better controlled experimental system48.
In laminar flow, Charru et al. noticed that the local rearrangement of a granular bed of uniformly sized beads, so-called bed armouring, resulted in a progressive increase of the threshold for the onset of motion until saturated conditions were achieved32. Literature, however, reveals different thresholds for saturated conditions in irregularly arranged sediment beds depending on the experimental set-up36,44. This scattering may be due to the difficulty of controlling particle parameters such as orientation, protrusion level and compactness of the sediments.
The main goal of this manuscript is to describe in detail how to characterize the incipient motion of single spheres as a function of geometrical properties of the horizontal sediment bed. For that purpose, we use regular geometries, consisting of monolayers of fixed beads regularly arranged according to triangular or quadratic configurations. Regular substrates similar to that we use are found in applications such as for the template-assembly of particles in microfluidic assays49, self-assembly of microdevices in confined structured geometries50 or intrinsic particle-induced transport in microchannels51. More importantly, using regular substrates allows us to highlight the impact of local geometry and orientation and to avoid any dubiety about the role of the neighborhood.
In laminar flow, we observed that the critical Shields number increased by 50% only depending on the spacing between the substrate spheres and thus on the exposure of the bead to the flow38. Similarly, we found that the critical Shields number changed by up to a factor of two depending on the orientation of the substrate to the flow direction38. We noticed that immobile neighbors only affect the onset of the mobile bead if they were closer than about three particle diameters41. Triggered by the experiment findings, we have recently presented a rigorous analytical model that predicts the critical Shields number in the creeping flow limit40. The model covers the onset of motion from highly exposed to hidden beads.
The first part of this manuscript deals with the description of the experimental procedure used in previous studies at shear Reynolds number, Re*, lower than 1. The laminar flow is induced with a rotational rheometer with a parallel configuration. In this low Reynolds number limit, the particle is not supposed to experience any velocity fluctuation20 and the system matches the so-called hydraulically smooth flow where the particle is submerged within the viscous sublayer.
Once incipient motion at laminar flow is established, the role of turbulence can become clearer. Motivated by this idea, we introduce a novel experimental procedure in the second part of the protocol. Using a Göttingen low-speed wind tunnel with open jet test section, the critical Shields number can be determined in a wide range of Re* including the hydraulically transitional flow and the turbulent regime. The experimental results can provide important insight about how forces and torques act on a particle due to the turbulent flow depending on the substrate geometry. Besides, these results can be used as a benchmark for more sophisticated models at high Re* in a similar way that past work in laminar flow has been used to feed semi probabilistic models52 or to validate recent numerical models53. We present some representative examples of applications at Re* ranging from 40 to 150.
The incipient criterion is established as the motion of the single particle from its initial equilibrium position to the next one. Image processing is used to determine the mode of onset of motion, i.e. rolling, sliding, lifting39,41. For that purpose, the angle of rotation of mobile spheres that were manually marked is detected. The algorithm tracks the position of the marks and compares it with the center of the sphere. A preliminary set of experiments was conducted in both experimental set-ups to clarify that the critical Shields number remains independent of finite size effects of the set-up and relative flow submergence. The experimental methods are thus designed to exclude any other parameter dependent on the critical Shields number beyond Re* and geometrical properties of the sediment bed. The Re* is varied using different fluid-particle combinations. The critical Shields number is characterized as a function of the burial degree, <img alt="Greek letter zeta symbol, used in mathematics and physics equations." src="/files/ftp_upload/57238/57238eq001.jpg" data-altupdatedat="1788250015000" data-alt="Equation 01">, defined by Martino et al.37 as <img alt="Static equilibrium equation: tan φ / E; used in force analysis and vector resolution diagrams." src="/files/ftp_upload/57238/57238eq002.jpg" data-altupdatedat="1788250015000" data-alt="Equation 02"> where <img alt="Φ symbol, often used in equations, representing variables like magnetic flux or phase angle." src="/files/ftp_upload/57238/57238eq003.jpg" data-altupdatedat="1788250015000" data-alt="Equation 03"> is the angle of repose, i.e. the critical angle at which motion occurs54, and <img alt="Equation of static equilibrium, ΣFx=0, diagram for educational physics concepts." src="/files/ftp_upload/57238/57238eq004.jpg" data-altupdatedat="1788250015000" data-alt="Equation 04"> is the exposure degree, defined as the ratio between the cross-sectional area effectively exposed to the flow to the total cross-sectional area of the mobile bead.

<table><tbody><tr><td>MCR 302 Rotational Rheometer</td><td>Anton Paar</td><td>Induction of shear laminar flow</td><td></td></tr><tr><td>Measuring Plate PP25</td><td>Anton Paar</td><td>Induction of shear laminar flow</td><td></td></tr><tr><td>Peltier System P-PTD 200</td><td>Anton Paar</td><td>Keep temperature of silicon oils constant in the system at laminar flow</td><td></td></tr><tr><td>Silicone oils with viscosities of approx. 10 and 100 mPas</td><td>Basildon Chemicals</td><td>Fluid used to induced the shear in the particles</td><td></td></tr><tr><td>Soda-lime glass beads of (405.9 &amp;plusmn; 8.7) &amp;mu;m</td><td>The Technical Glass Company</td><td>Construction of the regular substrates for laminar flow conditions</td><td></td></tr><tr><td>Opto Zoom 70 Module 0.3x-2.2x</td><td>WEISS IMAGING AND SOLUTIONS GmbH</td><td>Imaging system for recording the bead motion in the rheometer</td><td></td></tr><tr><td>2 x TV-Tube 1.0x, D=35 mm, L=146.5 mm</td><td>WEISS IMAGING AND SOLUTIONS GmbH</td><td>Imaging system for recording the bead motion in the rheometer</td><td></td></tr><tr><td>UI-1220SE CMOS Camera</td><td>IDS Imaging Development Systems GmbH</td><td>Imaging system for recording the bead motion in the rheometer</td><td></td></tr><tr><td>UI-3590CP CMOS Camera</td><td>IDS Imaging Development Systems GmbH</td><td>Imaging system for recording the bead motion in the rheometer</td><td></td></tr><tr><td>Volpi IntraLED 3 - LED light source&amp;nbsp;</td><td>Volpi USA</td><td>Imaging system for recording the bead motion in the rheometer</td><td></td></tr><tr><td>Active light guide diameter 5mm</td><td>Volpi USA</td><td>Imaging system for recording the bead motion in the rheometer</td><td></td></tr><tr><td>300 Watt Xenon Arc Lamp</td><td>Newport Corporation</td><td>Imaging system for recording the bead motion in the rheometer</td><td></td></tr><tr><td>Wind-tunnel with open jet test section, G&amp;ouml;ttingen type&amp;nbsp;</td><td>Tintschl BioEnergie und Str&amp;ouml;mungstechnik AG</td><td>Induction of turbulent flow</td><td></td></tr><tr><td>Glass spheres of (2.00 &amp;plusmn; 0.10) mm</td><td>Gloches South Korea</td><td>Construction of the regular substrates for turbulent flow conditions</td><td></td></tr><tr><td>Alumina spheres of (5.00 &amp;plusmn; 0.25) mm</td><td>Gloches South Korea</td><td>Targeted bead for experiments</td><td></td></tr><tr><td>CTA Anemometer DISA 55M01</td><td>Disa Elektronik A/S&amp;nbsp;</td><td>Measurement of&amp;nbsp; flow velocity in the wind tunnel</td><td></td></tr><tr><td>Miniaure Wire Probe Type 55P15</td><td>Dantec Dynamics</td><td>Measurement of&amp;nbsp; flow velocity in the wind tunnel</td><td></td></tr><tr><td>HMO2022 Digital Oscilloscope, 2 Analogue. Ch., 200MHz</td><td>Rohde &amp;amp; Schwarz</td><td>Measurement of&amp;nbsp; flow velocity in the wind tunnel</td><td></td></tr><tr><td>Phantom Miro eX1 High-speed Camera</td><td>Vision Research IncVis</td><td>Imaging system for recording the bead motion in the wind-tunnel</td><td></td></tr><tr><td>Canon ef 180mm f/3.5 l usm macro lens</td><td>Canon</td><td>Imaging system for recording the bead motion in the wind-tunnel</td><td></td></tr><tr><td>Table LED Lamp</td><td>Gloches South Korea</td><td>Imaging system for recording the bead motion in the wind-tunnel</td><td></td></tr></tbody></table>

visually based characterization of the incipient particle motion in regular substrates: from laminar to turbulent conditions

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to turbulent flow conditions are presented. For that purpose, the incipient motion of a single bead is studied on regular substrates that consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic symmetries. The threshold is characterized by the critical Shields number. The criterion for the onset of motion is defined as the displacement from the original equilibrium position to the neighboring one. The displacement and the mode of motion are identified with an imaging system. The laminar flow is induced using a rotational rheometer with a parallel disk configuration. The shear Reynolds number remains below 1. The turbulent flow is induced in a low-speed wind tunnel with open jet test section. The air velocity is regulated with a frequency converter on the blower fan. The velocity profile is measured with a hot wire probe connected to a hot film anemometer. The shear Reynolds number ranges between 40 and 150. The logarithmic velocity law and the modified wall law presented by Rotta are used to infer the shear velocity from the experimental data. The latter is of special interest when the mobile bead is partially exposed to the turbulent flow in the so-called hydraulically transitional flow regime. The shear stress is estimated at onset of motion. Some illustrative results showing the strong impact of the angle of repose, and the exposure of the bead to shear flow are represented in both regimes.

Figure 1(a) represents a sketch of the experimental set-up used to characterize the critical Shields number in the creeping flow limit, Section 1 of the protocol. The measurements are conducted in a rotational rheometer that was modified for this specific application. A transparent Plexiglas plate of 70 mm in diameter was carefully fixed to a parallel plate of 25 mm in diameter. The inertia of the measuring system was therefore readjusted bef...

Two different methods for characterizing the incipient particle motion of a single bead as a function of the sediment bed geometry from laminar to turbulent flow are presented.

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to ...

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Learn two methods to characterize incipient particle motion in regular substrates from laminar to turbulent flow. This approach quantifies sediment bed geometry effects on particle dynamics across industrial and natural applications.

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions

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