# Xiang-Zao Kong Publications Content

## Publications

### THESES

 2 Kong, X.-Z. Experimental investigation of air injection in saturated unconsolidated porous media, Dissertation ETH Zurich, 128 pp., 2010. AbstractThe work described in this thesis is primarily concerned with the construction and study of laboratory scale models for the process of air injection into liquid-saturated grain packing. Experiments, both in two-dimensional (2D) and three-dimensional (3D) setups, were carried out using water-saturated packings of glass beads and/or packings of crashed fused silica glass grains saturated with a glycerin-water solution. High resolution digital images of the invasion patterns were recorded and analyzed. During air injection into a vertically-placed 2D glass bead packing saturated with water, three stages were identified, a tree-like pattern, a fluidized pattern, and a migrating single-channel pattern. The expansion of the tree-like pattern behaves in a diffusion-like manner as the air branches advance upward randomly, and finally reach a more or less constant width. The starting position of the fluidized pattern was quantitatively estimated via balancing the pressure forces between the effective stress due to the weight of the grains and the pressure resistance on the displaced fluid combined with the capillary pressure. Four dynamic regimes were distinguished: regime (i) where the fluidization stops somewhere between the top of the packing and the injection orifice, a transition regime (ii), regime (iii) where the fluidization reaches the injection orifice, and regime (iv) where the deformation of the packing appears as soon as the air is injected. A critical injection rate $$Q_{f}$$ is defined to identify the transition regime. The value of $$Q_{f}$$ can be determined via $$Q_{a}$$, where $$Q_{a}$$ is calculated as averaged flux per channel. The regime (iv) is characterized by a characteristic injection rate $$Q_{c}$$, which is estimated by balancing the pressure gradient of the air flow and the overburden pressure gradient of the medium. The phenomenon of the migrating channel is measured quantitatively in two parts, before and after breakthrough. Before breakthrough, the characteristic measurements concern maximum vertical advance, maximum horizontal advance, air volumetric fraction, ratio of total surface area to volume, specific surface area of the air phase, and box-counting dimension. After breakthrough, the characteristic measurements focus on mean horizontal position of air channel, horizontal shifting distance, lateral movement distance, and lateral movement width. Before breakthrough, the maximum vertical height of the air structure approximately advances linearly with time. The maximum horizontal advance reaches a maximum value and then levels off for the rest of the time. Air volumetric fraction decreases monotonically with time, and finally levels off asymptotically to an approximate constant. In all cases, the air volumetric fraction for packings of small grains is larger than that for packings of large grains. The ratio of total surface area to volume varies in time similarly to the air volumetric fraction. However, the ratio of total surface area to volume can clearly be grouped according to the grain size, which is also true for the specific surface area of the air phase. Both can be scaled with the Bond number with a power of -0.5. After breakthrough, the migration process is studied by analyzing the mean horizontal position, horizontal shifting distance, lateral movement distance, and lateral movement width of the air channel. The results indicate that over 99% of the horizontal shifting distance is less than 10 mm. Furthermore, the its probability density function indicates that the air channel oscillates more frequently in the packing of small grains than in the packing of large grains. The interaction of the air flow with the grains and the liquid leads to a mobilization of the grains, in which air channels migrate and grain clusters undergo shearing. The channel migration comes to a stop after some time, leaving one thin and stable preferential channel for air flow. Assuming Hagen-Poiseuille’s formula to be applicable, the size of the preferential channel should exceed a lower threshold $$D_{ch}$$ so that a mechanical equilibrium at the channel interface is maintained, but it should stay below an upper threshold $$D_{max}$$ so that a stable air channel is sustained. A rearrangement of the grains is observed which is caused by a pulsation effect. It induces a compaction process, in which the individual grains are disassembled from the region of non-zero shear rate and then reassembled into the compacted clusters of the region of zero shear rate. It also induces a size segregation process, in which smaller grains move into the spaces beneath larger grains. By using high-speed image acquisition through laser scanning, the 3D dynamic air plume is recorded by sequential tomographic imaging. Due to the overlap between adjacent laser sheets and the light reflection, air bubbles are multiply exposed in the imaging along the scanning direction. A “curvature” method, based on a threshold on the curvature of grey-value in scanning direction, is proposed to remove the redundant pixels. The respective results are discussed by comparing the reconstructed air plume volume with the injected one and by evaluating the morphological consistency of the obtained air plume. The reconstructed air plume is further investigated with respect to its growth characteristics, such as breakthrough, air volume fraction, and air channel migration. 1 Kong, X.-Z. Investigation on some dynamical characteristics of granular material, MSc Thesis University of Science and Technology of China, 60 pp., 2006. AbstractThis paper presents experimental and theoretical studies on segregation of granular media subjected to external vertical sinusoidal vibration, vibration energy absorption properties of granular materials, dynamic behaviors of sandpile formation, and the stop-and-go motion in vertical pipe flow. Results of our studies not only extend the knowledge of non-linear properties of granular media under some dynamical conditions, such as vibrating, shearing, and flowing etc, but also is a theoretical instruction in the processing, storing and transporting of granular media in applied engineering. In the experiments with vertical sinusoidal vibration of granular material in containers with bottleneck(s), new segregation patterns and convection properties were found: (1)Under the condition of two-dimensional containers with one bottleneck, with the variation of the width of bottleneck, big and small particles show “Two Side segregation Pattern”, “Left Side segregation Pattern” and a pattern that big particles segregate to the upper-left part of the container. Furthermore, the angle of segregation interface and the angle of free-surface of granular bed follow a determinate rule, and the convection rolls of granular bed turn relatively and regularly. (2)Under the condition of three-dimensional container with one bottleneck, big particles aggregate and form a “Ring-like Segregation” pattern, and the position and length of big particle cluster can be adjusted by changing the vibration frequency. And mechanism of segregation is proposed. (3)Under the condition of three-dimensional container with a series of bottlenecks along its vertical axis, according to the differences of “Mobility” of each particle, particles will segregate alternantly. The simulation results of discrete element method (DEM) are in agreement with the experimental observations. The discovering of these new segregation patterns are of theoretical significance to studying the complex dynamical properties of granular materials, which also would be the engineering instruction for avoiding or enhancing segregation in the processing and transporting of granular materials. A DEM simulation of particle segregation patterns controlled by vibration frequency is carried out. It was found that: at low vibration frequency, large particles clustered on the top of the granular bed; as the frequency raised, large particles sinked into the granular assembly; at very high vibration frequency, large particles went up again. The mechanism of the transition of large particles clustering on the top—sinking—rising is studied in detail. The relation within inputted energy, void fraction and the effective restitution coefficient are found out. A Cellular Automata (CA) simulation of dynamic of sandpile formation is performed. A kinetic energy sandpile model, taking into account of grain inertia and the moving directions of the toppling grain, is developed and used to study the behaviour of sandpiles. In this model, the inertial effects are based on the toppling kinetic energy. The CA model reproduces the phenomenon of sandpile formation by revolving rivers which was found in previous experimental study in literature, and the relation between anglar speed and the height of sandpile is obtained. The simulation resluts reveal the non-symmetry of the sandpile formation and the selectivity of motion path of sand. The concepts and measuring method of internal friction are successfully applied to the measurement of low frequency vibration energy absorption properties of granular materials. An Invert Torsion Pendulum is designed to measure the energy absorption properties of granular materials under low frequency vibration using the free-attenuation mode. The vibration energy absorption of granular material decreases non-linearly with the increment of vibration amplitude under low frequency vibration. These results here can provide evidence for the understanding of some granular collective behaviors. When particles discharge from an open-top capillary pipe, it was found that, with particles of a particular size range, the outflux fluctuates greatly with time and the bulk dense granular flow in the pipe shows stop-and-go motion when the filling height is much larger than a threshold. When the filling height reduces towards the threshold, undergoing a transitional stage, the outflux and the bulk movement become much more stable. A heuristic theory taking into account of the granular compaction and interstitial air pressure effect is proposed to explain the appearing and disappearing of the stop-and-go motion. Finally, the conclusion of this paper and the prospect of further work are presented.

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### REFEREED PUBLICATIONS IN JOURNALS

 2 Kong, X.-Z. Experimental investigation of air injection in saturated unconsolidated porous media, Dissertation ETH Zurich, 128 pp., 2010. AbstractThe work described in this thesis is primarily concerned with the construction and study of laboratory scale models for the process of air injection into liquid-saturated grain packing. Experiments, both in two-dimensional (2D) and three-dimensional (3D) setups, were carried out using water-saturated packings of glass beads and/or packings of crashed fused silica glass grains saturated with a glycerin-water solution. High resolution digital images of the invasion patterns were recorded and analyzed. During air injection into a vertically-placed 2D glass bead packing saturated with water, three stages were identified, a tree-like pattern, a fluidized pattern, and a migrating single-channel pattern. The expansion of the tree-like pattern behaves in a diffusion-like manner as the air branches advance upward randomly, and finally reach a more or less constant width. The starting position of the fluidized pattern was quantitatively estimated via balancing the pressure forces between the effective stress due to the weight of the grains and the pressure resistance on the displaced fluid combined with the capillary pressure. Four dynamic regimes were distinguished: regime (i) where the fluidization stops somewhere between the top of the packing and the injection orifice, a transition regime (ii), regime (iii) where the fluidization reaches the injection orifice, and regime (iv) where the deformation of the packing appears as soon as the air is injected. A critical injection rate $$Q_{f}$$ is defined to identify the transition regime. The value of $$Q_{f}$$ can be determined via $$Q_{a}$$, where $$Q_{a}$$ is calculated as averaged flux per channel. The regime (iv) is characterized by a characteristic injection rate $$Q_{c}$$, which is estimated by balancing the pressure gradient of the air flow and the overburden pressure gradient of the medium. The phenomenon of the migrating channel is measured quantitatively in two parts, before and after breakthrough. Before breakthrough, the characteristic measurements concern maximum vertical advance, maximum horizontal advance, air volumetric fraction, ratio of total surface area to volume, specific surface area of the air phase, and box-counting dimension. After breakthrough, the characteristic measurements focus on mean horizontal position of air channel, horizontal shifting distance, lateral movement distance, and lateral movement width. Before breakthrough, the maximum vertical height of the air structure approximately advances linearly with time. The maximum horizontal advance reaches a maximum value and then levels off for the rest of the time. Air volumetric fraction decreases monotonically with time, and finally levels off asymptotically to an approximate constant. In all cases, the air volumetric fraction for packings of small grains is larger than that for packings of large grains. The ratio of total surface area to volume varies in time similarly to the air volumetric fraction. However, the ratio of total surface area to volume can clearly be grouped according to the grain size, which is also true for the specific surface area of the air phase. Both can be scaled with the Bond number with a power of -0.5. After breakthrough, the migration process is studied by analyzing the mean horizontal position, horizontal shifting distance, lateral movement distance, and lateral movement width of the air channel. The results indicate that over 99% of the horizontal shifting distance is less than 10 mm. Furthermore, the its probability density function indicates that the air channel oscillates more frequently in the packing of small grains than in the packing of large grains. The interaction of the air flow with the grains and the liquid leads to a mobilization of the grains, in which air channels migrate and grain clusters undergo shearing. The channel migration comes to a stop after some time, leaving one thin and stable preferential channel for air flow. Assuming Hagen-Poiseuille’s formula to be applicable, the size of the preferential channel should exceed a lower threshold $$D_{ch}$$ so that a mechanical equilibrium at the channel interface is maintained, but it should stay below an upper threshold $$D_{max}$$ so that a stable air channel is sustained. A rearrangement of the grains is observed which is caused by a pulsation effect. It induces a compaction process, in which the individual grains are disassembled from the region of non-zero shear rate and then reassembled into the compacted clusters of the region of zero shear rate. It also induces a size segregation process, in which smaller grains move into the spaces beneath larger grains. By using high-speed image acquisition through laser scanning, the 3D dynamic air plume is recorded by sequential tomographic imaging. Due to the overlap between adjacent laser sheets and the light reflection, air bubbles are multiply exposed in the imaging along the scanning direction. A “curvature” method, based on a threshold on the curvature of grey-value in scanning direction, is proposed to remove the redundant pixels. The respective results are discussed by comparing the reconstructed air plume volume with the injected one and by evaluating the morphological consistency of the obtained air plume. The reconstructed air plume is further investigated with respect to its growth characteristics, such as breakthrough, air volume fraction, and air channel migration. 1 Kong, X.-Z. Investigation on some dynamical characteristics of granular material, MSc Thesis University of Science and Technology of China, 60 pp., 2006. AbstractThis paper presents experimental and theoretical studies on segregation of granular media subjected to external vertical sinusoidal vibration, vibration energy absorption properties of granular materials, dynamic behaviors of sandpile formation, and the stop-and-go motion in vertical pipe flow. Results of our studies not only extend the knowledge of non-linear properties of granular media under some dynamical conditions, such as vibrating, shearing, and flowing etc, but also is a theoretical instruction in the processing, storing and transporting of granular media in applied engineering. In the experiments with vertical sinusoidal vibration of granular material in containers with bottleneck(s), new segregation patterns and convection properties were found: (1)Under the condition of two-dimensional containers with one bottleneck, with the variation of the width of bottleneck, big and small particles show “Two Side segregation Pattern”, “Left Side segregation Pattern” and a pattern that big particles segregate to the upper-left part of the container. Furthermore, the angle of segregation interface and the angle of free-surface of granular bed follow a determinate rule, and the convection rolls of granular bed turn relatively and regularly. (2)Under the condition of three-dimensional container with one bottleneck, big particles aggregate and form a “Ring-like Segregation” pattern, and the position and length of big particle cluster can be adjusted by changing the vibration frequency. And mechanism of segregation is proposed. (3)Under the condition of three-dimensional container with a series of bottlenecks along its vertical axis, according to the differences of “Mobility” of each particle, particles will segregate alternantly. The simulation results of discrete element method (DEM) are in agreement with the experimental observations. The discovering of these new segregation patterns are of theoretical significance to studying the complex dynamical properties of granular materials, which also would be the engineering instruction for avoiding or enhancing segregation in the processing and transporting of granular materials. A DEM simulation of particle segregation patterns controlled by vibration frequency is carried out. It was found that: at low vibration frequency, large particles clustered on the top of the granular bed; as the frequency raised, large particles sinked into the granular assembly; at very high vibration frequency, large particles went up again. The mechanism of the transition of large particles clustering on the top—sinking—rising is studied in detail. The relation within inputted energy, void fraction and the effective restitution coefficient are found out. A Cellular Automata (CA) simulation of dynamic of sandpile formation is performed. A kinetic energy sandpile model, taking into account of grain inertia and the moving directions of the toppling grain, is developed and used to study the behaviour of sandpiles. In this model, the inertial effects are based on the toppling kinetic energy. The CA model reproduces the phenomenon of sandpile formation by revolving rivers which was found in previous experimental study in literature, and the relation between anglar speed and the height of sandpile is obtained. The simulation resluts reveal the non-symmetry of the sandpile formation and the selectivity of motion path of sand. The concepts and measuring method of internal friction are successfully applied to the measurement of low frequency vibration energy absorption properties of granular materials. An Invert Torsion Pendulum is designed to measure the energy absorption properties of granular materials under low frequency vibration using the free-attenuation mode. The vibration energy absorption of granular material decreases non-linearly with the increment of vibration amplitude under low frequency vibration. These results here can provide evidence for the understanding of some granular collective behaviors. When particles discharge from an open-top capillary pipe, it was found that, with particles of a particular size range, the outflux fluctuates greatly with time and the bulk dense granular flow in the pipe shows stop-and-go motion when the filling height is much larger than a threshold. When the filling height reduces towards the threshold, undergoing a transitional stage, the outflux and the bulk movement become much more stable. A heuristic theory taking into account of the granular compaction and interstitial air pressure effect is proposed to explain the appearing and disappearing of the stop-and-go motion. Finally, the conclusion of this paper and the prospect of further work are presented.