2204
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
discrete elements located close to the edge of the dark region
had the peak f
x
. The impact pulse is therefore correlated to the
propagation of the dark region where the discrete elements
reduced their momentum notably. As the f
x
decreased steadily
with time after impact, the post-peak f
x
was found to be less
than 30% of the peak f
x
after about 0.14s. This finding suggests
the highly transient nature of the impact.
3.3
Energy dissipation and discharge
Figure 4b shows the relationship of the normalized kinetic
energy (k
e
/ k
e(max)
) of the discrete elements with time in region 1,
2 and 3. The solid line, dotted line and dashed line represent the
computed k
e
/ k
e(max)
in region 1, 2 and 3 respectively. The k
e
is
the sum of the kinetic energy of all the discrete elements in a
region. From Figure 4a and Figure 4b, it can be observed that
k
e
decreased much more rapidly with time than f
x
. In region 1,
k
e
reduced to 30% of the peak k
e
after less than 0.02s while f
x
requires about 0.14s to reduce to 30% of its peak f
x
. The
magnitude of k
e
rose gently following the rapid reduction.
Figure 4c shows the mean discharge rate (q
n
) of the discrete
elements relative to the maximum computed q
n
(q
n(max)
) in
region 1, 2 and 3. Similar to the trend of k
e
, the normalized
discharge rate reduced with time. The reduction of the
discharge rate was less rapid in comparison with the reduction
of k
e
shown in Figure 4b. A rising trend of q
n
is observed
following the reduction. It is interesting to note that both the k
e
and q
n
rose following the sharp reduction of their values. It is
inferred that the deceleration effect caused by the baffles are
more significant during the first impact (i.e. time before 0.04
second for regions 1 to 3) at which the impact pulse propagated
along these regions. The trend of kinetic energy of these
regions beyond 0.14 second is likely to be affected by the
presence of various deposition mechanisms, such as runup,
reflected wave, jet and hydraulic jump, etc (Armanini and
Scotton 1993; Armanini 1997; Sun and Law 2012). Further
research will be carried out to study the influence of these
mechanisms on the computed kinetic energy and discharge.
Based on the above observations of the first impact process, the
baffles reduced the kinetic energy and the discharge of the
discrete elements behind them effectively.
4 CONCLUSIONS
In this study, the three-dimensional discrete element method
was used to analyse a granular surge flow through a row of
baffles. The analysis focused on the short moment at which the
first impact of the discrete elements on the baffles took place.
The propagation of impact pulses in the upstream direction was
observed at the moment of impact. The magnitude of the
impact pulse decreased with the distance upstream from a row
of baffles. The deceleration of the discrete elements was
uniform over the flow depth at the moment of impact. More
than half of the kinetic energy of the discrete element right
behind the row of baffles was dissipated in less than 0.02s. The
kinetic energy of the granular medium behind the row of baffles
decreases more rapidly with time than the unbalanced force.
Based on the findings of the analysis, a single row of baffles is
effective in reducing the kinetic energy and discharge of the
granular surge flow at the moment of impact.
5 ACKNOWLEDGEMENTS
This paper is published with the permission of the Head of the
Geotechnical Engineering Office and the Director of Civil
Engineering and Development, Government of the Hong Kong
Special Administrative Region.
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