 |
| Titel |
A novel optical method for estimating the near-wall volume fraction in
granular flows |
| VerfasserIn |
Luca Sarno, Maria Nicolina Papa, Luigi Carleo, Yih-Chin Tai |
| Konferenz |
EGU General Assembly 2016
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250132983
|
| Publikation (Nr.) |
 EGU/EGU2016-13541.pdf |
|
|
|
|
|
| Zusammenfassung |
| Geophysical phenomena, such as debris flows, pyroclastic flows and rock avalanches, involve
the rapid flow of granular mixtures. Today the dynamics of these flows is far from being
deeply understood, due to their huge complexity compared to clear water or monophasic
fluids. To this regard, physical models at laboratory scale represent important tools for
understanding the still unclear properties of granular flows and their constitutive
laws, under simplified experimental conditions. Beside the velocity and the shear
rate, the volume fraction is also strongly interlinked with the rheology of granular
materials. Yet, a reliable estimation of this quantity is not easy through non-invasive
techniques.
In this work a novel cost-effective optical method for estimating the near-wall volume
fraction is presented and, then, applied to a laboratory study on steady-state granular flows. A
preliminary numerical investigation, through Monte-Carlo generations of grain distributions
under controlled illumination conditions, allowed to find the stochastic relationship between
the near-wall volume fraction, c3D, and a measurable quantity (the two-dimensional volume
fraction), c2D, obtainable through an appropriate binarization of gray-scale images captured
by a camera placed in front of the transparent boundary. Such a relation can be well described
by c3D = aexp(bc2D), with parameters only depending on the angle of incidence of light, ζ.
An experimental validation of the proposed approach is carried out on dispersions of
white plastic grains, immersed in various ambient fluids. The mixture, confined in
a box with a transparent window, is illuminated by a flickering-free LED lamp,
placed so as to form a given ζ with the measuring surface, and is photographed by a
camera, placed in front of the same window. The predicted exponential law is found
to be in sound agreement with experiments for a wide range of ζ (10˚ <ζ<45˚
).
The technique is, then, applied to steady-state dry granular flows. A 2m-long
laboratory flume, with an upper reservoir connected to the lower part of the channel
through an adjustable sluice gate, is employed. The granular material, loaded in the
reservoir, is the same of the previous tests. The flume inclination is 30˚ and different
openings of the sluice gate are investigated. The apparatus is also equipped with a
digital scale at the channel outlet for measuring the mass flow, a LED lamp and two
high-speed cameras, placed laterally and above the chute to capture the flow motion
at a given cross section. Side wall and free surface velocity profiles are obtained
through PIV techniques. The volume fraction profiles are also obtained from side
images. The joint information of the velocity and volume fraction profiles allowed to
estimate the mass flow, which is found to be in good agreement with the mass flow,
directly measured by the scale. These results confirm the robustness of the proposed
cost-effective approach for the simultaneous measurement of velocity and volume fraction. |
|
|
|
|
|
|