2.5 Particle Velocimetry Measurements
69
Voronoï diagram is used to measure the granular flow. This method can identify the
local pattern formed by neighboring grains, and remain stable for some time [14].
Thus, a combination of RM and the Voronoï diagram should be an excellent tool to
improve particle tracking.
2.5.2 Image Processing
Furthermore, the Relaxation Method (RM) PTV is based on the use of the iterative
relaxation process technique, and it is widely applicable to complex flows with local
rotating and shear motions [15]. In RM, as the reference particle selected by the quasirigidity radius finds many neighbors flowing similarly, the positive estimate of such
movement is increased [16]. The most important advantage of this relaxation method
is that the examination is based on the probability of particle matching between the
first and second frames, defined for every possible pair of particles inclusive of the
probability of those being non-matched (or the probability of the loss of partners).
It is suitable to measure the 2-D flow regime of very slow and dense granular flow
since there exist particles that will disappear when they are flowing toward the outlet.
2.5.3 Flow Correlation and Intermittency
The flow dynamics in a very slow and dense pebble bed is studied and analyzed by
the particle tracking method from macroscopic as well as microscopic perspectives.
The velocity profile and fluctuation characteristics at different heights of the silo bed
are analyzed. By analyzing the velocity evolution of individual particles, intermittent and distinct bulk motion events of pebbles are observed. The time correlations
of particle velocities have periodic and oscillatory tails that are different from the
rapid granular flow with the exponential decaying tails. Two parameters, namely the
correlation time τ c and the intermittency index C 2 , are proposed. How they affect
the whole flow regime on the streamlines are analyzed. The autocorrelation time
τ c seems to follow power-law regressions (τ c ∼ H
α , H is the bed height) with the
reduction of heights on different streamlines. The relationships of autocorrelation,
intermittency, and shear functions are discussed in the whole flow field. Finally, a
further discussion on the features of extremely slow flow as well as the characterization of flow patterns is performed based on the parameter analysis, including the
correlation time, intermittency index, and shear rates. The results are important for
improving the understanding of the physics of pebble flows and the application of
pebble beds in nuclear engineering.
69
Voronoï diagram is used to measure the granular flow. This method can identify the
local pattern formed by neighboring grains, and remain stable for some time [14].
Thus, a combination of RM and the Voronoï diagram should be an excellent tool to
improve particle tracking.
2.5.2 Image Processing
Furthermore, the Relaxation Method (RM) PTV is based on the use of the iterative
relaxation process technique, and it is widely applicable to complex flows with local
rotating and shear motions [15]. In RM, as the reference particle selected by the quasirigidity radius finds many neighbors flowing similarly, the positive estimate of such
movement is increased [16]. The most important advantage of this relaxation method
is that the examination is based on the probability of particle matching between the
first and second frames, defined for every possible pair of particles inclusive of the
probability of those being non-matched (or the probability of the loss of partners).
It is suitable to measure the 2-D flow regime of very slow and dense granular flow
since there exist particles that will disappear when they are flowing toward the outlet.
2.5.3 Flow Correlation and Intermittency
The flow dynamics in a very slow and dense pebble bed is studied and analyzed by
the particle tracking method from macroscopic as well as microscopic perspectives.
The velocity profile and fluctuation characteristics at different heights of the silo bed
are analyzed. By analyzing the velocity evolution of individual particles, intermittent and distinct bulk motion events of pebbles are observed. The time correlations
of particle velocities have periodic and oscillatory tails that are different from the
rapid granular flow with the exponential decaying tails. Two parameters, namely the
correlation time τ c and the intermittency index C 2 , are proposed. How they affect
the whole flow regime on the streamlines are analyzed. The autocorrelation time
τ c seems to follow power-law regressions (τ c ∼ H
α , H is the bed height) with the
reduction of heights on different streamlines. The relationships of autocorrelation,
intermittency, and shear functions are discussed in the whole flow field. Finally, a
further discussion on the features of extremely slow flow as well as the characterization of flow patterns is performed based on the parameter analysis, including the
correlation time, intermittency index, and shear rates. The results are important for
improving the understanding of the physics of pebble flows and the application of
pebble beds in nuclear engineering.