68
2 Experiments in Pebble Flows
of the mixing zone. Pebbles move collectively with their neighbors so that the tworegion arrangement can be established and maintained, and the mixing zone can be
constrained to a reasonable size. Analysis based on observations shows that the flow
of the pebble bed is intermittent and paroxysmal.
In the experiment, the pebble flow in the drainage pebble mode is considerably
different from that in the recirculation mode. The pebble flow in a pebble packing is
related to not only the geometric design and material parameters but also the mode
of discharging pebbles. In other words, the pebble flow is the quasi-static flow of
being discharged one by one, or the steady flow of being quickly discharged. The
competition mechanism that generally exists in the quasi-static pebble flow is one
of the essential reasons that causes resistance and diffusion between pebbles. The
existence of the stagnant zone is the critical factor to cause the non-uniformity of the
overall flow field in the pebble packing. At the same time, the internal collapse will
happen under certain conditions, which also cause the non-uniformity of the flow
field. The self-organizational mechanism causes the mixing of the different pebbles
during the quasi-static pebble flow. Internal equilibrium arches exist all over the pebble packing, which will temporarily cut off the propagation of the disturbance of the
voids. The friction coefficient influence on the overall flow field is very complicated
and presents the great nonlinearity and strong coupling effects, which deserves to be
deeply investigated.
2.5 Particle Velocimetry Measurements
2.5.1 Measurement Techniques
Concerning the measurement method, Particle Tracking Velocimetry (PTV) [11,
12] shows some advantages. However, the conventional PTV methods depend on
the minimum displacement matching algorithm [13], which fails in measuring the
very slow pebble flow with dense dispersion of pebbles, fluctuation motion with
many-body interactions, and local flow gradients within several particle diameters
[14]. The inter-frame displacement becomes a significant issue for the precision of
measurement.
The Relaxation Method (RM) of PTV utilizes an iterative relaxation process
technique applicable to complex flows with local rotating and shear motions [15].
In the RM, the reference particle selected by the quasi-rigidity radius may have
many neighboring pebbles moving similarly [16]. The most important advantage
of this relaxation method is that examination is based on matching probability of
particles between the first and second frames, considering every possible pair of
particles including the nonmatching case (loss of partners). This method is suitable
to measure the 2-D very slow dense granular flow where particles may disappear in
the next frame when they are drained out from the outlet, but the disappeared particles
account for a smaller percentage of the total particle number. On the other hand, the
2 Experiments in Pebble Flows
of the mixing zone. Pebbles move collectively with their neighbors so that the tworegion arrangement can be established and maintained, and the mixing zone can be
constrained to a reasonable size. Analysis based on observations shows that the flow
of the pebble bed is intermittent and paroxysmal.
In the experiment, the pebble flow in the drainage pebble mode is considerably
different from that in the recirculation mode. The pebble flow in a pebble packing is
related to not only the geometric design and material parameters but also the mode
of discharging pebbles. In other words, the pebble flow is the quasi-static flow of
being discharged one by one, or the steady flow of being quickly discharged. The
competition mechanism that generally exists in the quasi-static pebble flow is one
of the essential reasons that causes resistance and diffusion between pebbles. The
existence of the stagnant zone is the critical factor to cause the non-uniformity of the
overall flow field in the pebble packing. At the same time, the internal collapse will
happen under certain conditions, which also cause the non-uniformity of the flow
field. The self-organizational mechanism causes the mixing of the different pebbles
during the quasi-static pebble flow. Internal equilibrium arches exist all over the pebble packing, which will temporarily cut off the propagation of the disturbance of the
voids. The friction coefficient influence on the overall flow field is very complicated
and presents the great nonlinearity and strong coupling effects, which deserves to be
deeply investigated.
2.5 Particle Velocimetry Measurements
2.5.1 Measurement Techniques
Concerning the measurement method, Particle Tracking Velocimetry (PTV) [11,
12] shows some advantages. However, the conventional PTV methods depend on
the minimum displacement matching algorithm [13], which fails in measuring the
very slow pebble flow with dense dispersion of pebbles, fluctuation motion with
many-body interactions, and local flow gradients within several particle diameters
[14]. The inter-frame displacement becomes a significant issue for the precision of
measurement.
The Relaxation Method (RM) of PTV utilizes an iterative relaxation process
technique applicable to complex flows with local rotating and shear motions [15].
In the RM, the reference particle selected by the quasi-rigidity radius may have
many neighboring pebbles moving similarly [16]. The most important advantage
of this relaxation method is that examination is based on matching probability of
particles between the first and second frames, considering every possible pair of
particles including the nonmatching case (loss of partners). This method is suitable
to measure the 2-D very slow dense granular flow where particles may disappear in
the next frame when they are drained out from the outlet, but the disappeared particles
account for a smaller percentage of the total particle number. On the other hand, the