2201
A Numerical Study of Granular Surge Flow through a Row of Baffles
Une étude numérique des écoulements granulaires à travers une rangée de chicanes
Law R.P.H., Lam A.Y.T., Choi K.Y.
Geotechnical Engineering Office, Civil Engineering and Development Department, The Government of the Hong Kong
Special Administrative Region, Hong Kong, China
ABSTRACT: A numerical study that utilizes three-dimensional discrete element method was undertaken to investigate the impact
process and the dynamic interaction between granular surge flow and baffles. In the numerical analyses, the granular flow medium and
the baffles were modelled as frictional spherical discrete elements and rigid square objects respectively. The location, velocity and
forces acting on the individual discrete elements during the impact and interaction process were captured and recorded in the analyses.
A detailed assessment of the numerical output data indicates that a single row of baffles is effective in reducing the kinetic energy and
discharge of the granular surge flow.
RÉSUMÉ : Une étude numérique utilisant une approche par éléments discrets en 3D a été mise en oeuvre pour étudier le processus
d'impact et l'interaction dynamique entre un écoulement granulaire et des chicanes. Dans cette étude, le milieu granulaire a été modélisé
par des objets sphériques et les chicanes par les objets carrés. Les grains comme les chicanes sont considérées comme parfaitement
rigides. L'emplacement, la vitesse et les forces agissant sur les éléments discrets au cours du processus d'impact et l'interaction ont été
ainsi calculés. Une évaluation détaillée des données de sortie indique qu’une seule rangée de chicanes est efficace pour la réduction de
l'énergie cinétique et l'évacuation de l'écoulement granulaire.
KEYWORDS: baffles, granular flow, discrete element method, energy dissipation, impact pulse, discharge.
1 INTRODUCTION
Granular surge flows such as debris flows and snow avalanches
are dangerous natural hazards. Baffles have been employed to
dissipate the kinetic energy of such flows and reduce the
entrainment of channel bed deposits. Rows of baffles could be
constructed across the flow path in order to reduce the flow
velocity, the entrainment potential and dynamic impact force on
downstream structures. Baffles constructed across the debris
flow path are potentially more advantageous than the
conventional rigid debris-resisting barriers because baffles are
generally easier to construct without the need for extensive site
formation works. However, the dynamic interaction of granular
surge flow with baffles is still an emerging area of research to
both the academics and the practitioners.
Discrete element method was introduced in geomechanics by
Cundall and Strack in 1979 and was later adopted as a research
tool by other researchers (e.g. Muir et al., 2008; Sibille et al.,
2008; Thompson et al., 2009). The discrete element method is
an appropriate and useful tool for modelling debris flow and
snow avalanche because of the granular flow nature (Zwinger
2000; Nicot 2004; Hutter et al, 2005; Pudasaini & Hutter 2007)
of these phenomena. In a previous study by Law 2008, a series
of flume model tests and three dimensional discrete element
analyses were conducted to investigate the impact behaviour of
granular surge flow on a rigid barrier. The study has been
extended to investigate granular surge flow through a row of
baffles. This paper focuses on the numerical study of the
impact process and the dynamic interaction between baffles and
granular surge flow using the discrete element method.
2 NUMERICAL STUDY
2.1
Introduction
The granular medium was modelled as incompressible frictional
discrete elements and the planar rigid surfaces were used to
model the baffles and the ground surface. In the analyses, the
location and velocity of the individual discrete elements
together with the forces acting on each element were captured
and recorded in order to study the impact process and the
dynamic interaction between baffles and granular surge flow.
The displacement of individual discrete elements is independent
of one another, and they only interacted when coming into
contact with each other or with the baffles and other rigid
boundaries. The motion of each discrete element was calculated
on the basis of the forces acting on it by the Newton’s laws of
motion. The displacements and rotations of the discrete
elements were computed in the numerical analyses.
2.2
Numerical model setup and test procedure
Figure 1 shows the geometry of the flow path and the baffles.
The granular medium comprises a total of 30,000 spherical
discrete elements and all the discrete elements have a uniform
diameter of 0.05m. The material density of each discrete
element is 2,650 kg/m
3
. The discrete elements were not
intended to simulate motion of individual particles in the
granular medium. Instead, the macroscopic behaviour of the
granular medium was represented by the contact behaviour
between the discrete elements. Planar rigid surfaces were used
to model the baffles and the ground surface.
