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B. Chanut - One of the best experts on this subject based on the ideXlab platform.

  • Small-scale tests to investigate the dynamics of finite-sized dry granular avalanches and forces on a wall-like obstacle
    Granular Matter, 2012
    Co-Authors: Paolo Caccamo, Thierry Faug, Herv� Bellot, B. Chanut, Florence Naaim-bouvet
    Abstract:

    Small-scale laboratory tests investigate the force from finite-sized granular avalanches on a wall. First, the reference flows, in absence of the wall, were analysed in a wide range of slopes from a minimum angle for which no flow is possible to a critical angle for which the flow becomes very dilute. The changes in thickness and velocity over time exhibit transitions at the minimum slope angle and at intermediate slopes. Then the normal force exerted on a wall spanning the flow was measured. It is notable that the transitions detected in reference flows had a direct effect on the force. The maximum force was equal to the kinetic force of the incoming flow at high slopes, whereas it scaled like hydrostatic force at lower slopes. This is the effect of the dense-to-dilute transition. Furthermore, the maximum force at low slopes was found to be several times greater than the hydrostatic force of the incoming flow. This finding is explained by the considerable contribution of the Stagnant Zone formed upstream of the wall. Furthermore, the jamming transition was highlighted at the avalanche standstill by the collapse of the residual force on the wall when approaching the minimum angle for which no flow is possible. These results are useful for the design of protection dams against rapid mass movements.

  • A scaling law for impact force of a granular avalanche flowing past a wall
    Geophysical Research Letters, 2012
    Co-Authors: Thierry Faug, P. Caccamo, B. Chanut
    Abstract:

    A scaling law is proposed to describe the force experienced by a wall-like obstacle overflowed by a granular avalanche-flow when a Stagnant Zone is formed upstream from the wall and co-exists with an inertial Zone above without granular jump. It relates the force on the wall relative to the force due to the kinetic energy of the undisturbed incident flow to (i) the Froude number, (ii) the wall height relative to the flow thickness, (iii) the slope angle and (iv) the various parameters associated with the properties of the flowing granular material. The scaling law is compared to small-scale discrete numerical simulations in two dimensions and data from granular laboratory tests. Finally, we discuss on the applicability of the new model to the full-scale granular flows.

  • Force from granular flows on a wall-like obstacle: contribution to snow avalanche dam design
    2011
    Co-Authors: B. Chanut, Thierry Faug, M. Naaim
    Abstract:

    Dense avalanches of granular snow can lead to huge damages on structures: buildings, protection dams, etc. Understanding the influence of obstacles on flows of granular fluids and the force that granular fluids can exert on the obstacle is therefore of crucial interest. Our study deals with both steady and avalanche granular flows down an incline, and overflowing a wall-like obstacle. Small-scale discrete numerical simulations were carried out in various incident flow regimes from a slow dense regime to a rapid dilute regime. In the rapid dilute regime, the mean force is mainly driven by the kinetic energy of the flow. It scales as the velocity square similarly to the inertial regime for newtonian fluids such as water and air. In the slow dense regime, the property of granular materials to behave as a solid leads to the formation of a large Stagnant Zone upstream of the wall-like obstacle. The length of the Stagnant Zone diverges when the slope decreases toward a critical slope determined by the frictional properties of the granular medium. The resulting weight, balanced by basal friction, largely contributes to the total force on the wall. This latter can reach a value several times greater than the typical hydrostatic force of the undisturbed incident flow. This transition is well described by a hydrodynamic continuum model derived from the Euler theorem both in steady and unsteady states. Because of experimental evidence of similar behaviors between snow and granular flows, the continuum model is a good candidate to derive the mean force from major dense snow avalanches likely to overflow protection dams built in avalanche prone areas.

  • Equation for the force experienced by a wall overflowed by a granular avalanche: experimental verification
    Physical Review E : Statistical Nonlinear and Soft Matter Physics, 2011
    Co-Authors: T. Faug, P. Caccamo, B. Chanut
    Abstract:

    The present paper deals with the force experienced by a wall overflowed by a granular avalanche. First, we shortly report new laboratory tests on dry granular avalanches overflowing a wall down a rough channel. In the first step, the thickness and velocity of the control flows without a wall are measured. In the second step, a wall is mounted to obstruct the flow and the normal force experienced by the wall is measured. Then, a set of equations based on depth-averaged momentum conservation and making it possible to derive the time-varying force on the wall is described. The model was proposed and calibrated on 2D discrete numerical simulations in an earlier work [B. Chanut, T. Faug, and M. Naaim, Phys. Rev. E 82, 041302 (2010)]. This model takes into account the fact that a quasi-static Stagnant Zone is established upstream of the wall and coexists with an inertial flowing Zone above. For a large range of slopes, the model's prediction is successfully compared to experimental data with a reasonable estimation of the incoming flow density and in spite of some rough assumptions made to describe the dynamics of the dead Zone. Finally, the results are analysed with regards to previous 2D discrete numerical simulations and we discuss the future work to be undertaken on the dynamics of the dead Zone established upstream of the wall.

  • Mean force on a wall overflowed by dense granular avalanches: discrete numerical simulations
    2009
    Co-Authors: B. Chanut, Thierry Faug, M. Naaim
    Abstract:

    Using discrete elements simulations, we investigated 2D unsteady granular flows down a rough inclined plane and overflowing a wall, for a large range of slope inclinations. We first characterized flows without any obstacle by measuring time evolution of flow depth, mean velocity and volume fraction. These unsteady flows display similar scalings as ones observed for steady and uniform flows. We then introduced a wall normal to the bottom and we measured the mean force exerted by the incoming flow. Two regimes appear, a dense and a collisional regime, depending on the slope inclination and for which the mean force strongly differs. We attempted to compare the mean force with two typical forces linked to the control flow without obstacle for both regimes. The comparison highlights an important contribution due to a Stagnant Zone formed in front of the obstacle, particularly in the dense regime and the transition towards a static regime.

Donald J. Lyman - One of the best experts on this subject based on the ideXlab platform.

  • effects of a vascular graft natural artery compliance mismatch on pulsatile flow
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J. Lyman
    Abstract:

    Abstract Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or Stagnant Zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

  • Effects of a vascular graft/natural artery compliance mismatch on pulsatile flow.
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J. Lyman
    Abstract:

    Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or Stagnant Zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

Chuen-shii Chou - One of the best experts on this subject based on the ideXlab platform.

  • Moving granular filter bed of quartz sand with louvered-walls and flow-corrective inserts
    Powder Technology, 2007
    Co-Authors: Chuen-shii Chou, Shun-hsing Chen
    Abstract:

    Abstract This study investigated the effect of a simple flow-corrective element, a vertical plate, on the flow patterns and wall stresses in a two-dimensional louvered-wall moving granular filter bed of quartz sand. The static wall stress distributions produced by the granular solids were measured, and compared with the theoretical prediction, using the differential slice and Runge–Kutta (order four) methods. The variations in the dynamic wall stresses over time were obtained using a two-directional pressure gauge. The results show that a vertical plate placed in each stage of the granular filter bed effectively diminished the quasi-Stagnant Zone adjacent to the louvered-wall, and reduced the wall pressure pulsation. For example, the ratio of the Stagnant Zone area in the second stage to the area of quartz sand in the second stage decreased from 50.66% (without an insert) to 27.86% (with a vertical plate). The maximum dynamic normal wall stress on the convergent section of third-stage louvered-wall (louver angle: 40°) declined remarkably from 9.95 kPa (without an insert) to 5.9 kPa (with a vertical plate), and then the reduction of the maximum dynamic normal wall stress was up to 40.7%. Furthermore, a vertical plate installed at an appropriate location satisfies the following requirements: (1) it shrinks the Stagnant Zone and reduces the wall stresses on the louvered wall of a moving granular filter bed; and (2) it solves the potential problem of a roof-type insert, on which the Stagnant Zone may sit.

  • Flow patterns and wall stresses in a moving granular filter bed with an asymmetric louvered-wall and obstacles
    Chemical Engineering and Processing: Process Intensification, 2006
    Co-Authors: Chuen-shii Chou, T.l. Yang, J.c. Chang
    Abstract:

    This study investigated the flow patterns and wall stresses in a two-dimensional asymmetric louvered-wall moving granular filter bed with an obstacle placed at each stage of the granular filter bed. The obstacles utilized were: (1) a single vertical plate and (2) a wedge-shaped obstacle comprising a left-vertical-plate and a right-inclined-plate. A digital camcorder was used to record the flow pattern histories of the granular solids in the moving granular filter bed. The instantaneous vertical velocity distribution at a certain height from the bottom of the third stage in a two-dimensional asymmetric louvered-wall granular filter bed was determined from two successive flow images. The static stresses and the variations in the dynamic wall stresses with time were obtained using a two-directional pressure gauge, which could simultaneously measure the normal and shear stresses of the granular solids. The preliminary theoretical study of the obstacle's placement in the asymmetric louvered-wall moving granular filter bed was proposed. The magnitude of the average dynamic normal stress on the convergent section of the louvered-wall greatly exceeds that on the vertical section of the louvered-wall. A wedge-shaped obstacle (or a single vertical plate) placed at a proper location, which causes two asymmetric flowing streams in the granular bed, may increase the velocity and refreshing rate of the filter granules in the quasi-Stagnant Zone near the louvered-wall and further the overall dust collection efficiency of the filter granules because the central flowing core above exit is not evident in the granular bed. Additionally, a wedge-shaped obstacle installed at an appropriate location, which consists of a left-vertical-plate and a right-inclined-plate, satisfies the following requirements: (1) it keeps the granules in the left part of the granular bed away from the obstacle, preventing interference therewith; (2) it shrinks Stagnant Zone and declines the normal stress on the convergent section of the louvered-wall in the right part of the granular bed; and (3) it solves the potential problem of a roof-type insert, on which the Stagnant Zone may sit.

  • Flow patterns and wall stresses in a moving granular filter bed with a curved symmetric louvered wall
    Advanced Powder Technology, 2005
    Co-Authors: Chuen-shii Chou, T.l. Yang
    Abstract:

    Abstract This study investigated the flow patterns and wall stresses in a two-dimensional moving granular filter bed with a curved and symmetric louvered wall. A digital camcorder was used to record the flow pattern histories of the granular solids in the moving granular filter bed. The numerical predictions of the flow patterns were implemented using the discrete element method and compared with the experimental results. The variations in the dynamic wall stresses with time were obtained using a pressure gauge, which could simultaneously measure the normal and shear stresses of the granular solids. The effects of the curvature of the louvered wall and the exit width upon the flow patterns and the wall stresses were also elucidated. For a fixed value of radius of curvature, the ratio of the quasi-Stagnant Zone (Q-SZ) area in the third stage to the granular bed area in the third stage increases with decreasing width of the exit, but the average dynamic normal stress on the curved louvered wall increases with increasing width of the exit. For a fixed value of the exit width, both the ratio of the Q-SZ area in the third stage to the granular bed area in the third stage and the average dynamic normal stress on the curved louvered wall increase with increasing radius of curvature. Additionally, the curved louvered wall can not only shrink the area of the Q-SZ in the granular bed, but also reduce the dynamic normal stress on the louvered wall.

  • The flow patterns and stresses on the wall in a symmetric louvered-wall moving granular filter bed
    Powder Technology, 2003
    Co-Authors: Chuen-shii Chou, Jiri Smid, J.t. Kuo, Shu-san Hsiau
    Abstract:

    Abstract The flow patterns and stresses on the wall in a symmetric two-dimensional louvered-wall moving granular filter bed were investigated. The static wall stress distributions produced by the granular solids were measured and compared with the theoretical prediction using the differential slice and Runge–Kutta (order four) methods. The variations in the dynamic wall stresses with time in a moving granular filter bed were obtained. In addition, the effect of the louver angle upon the flow patterns and wall stresses was investigated. Four different flow regions were observed in a moving granular filter bed. As the angle of the louver decreases, the quasi-Stagnant Zone area adjacent to the side wall becomes smaller and the static normal stress acting on the convergent section of the side wall becomes larger. The magnitude of the static normal stress acting on the convergent section is approximately 10 times as large as that acting on the vertical section. When the normal stress measured by pressure gauge installed on the upper stage decreases to zero, the normal wall stress measured by pressure gauge installed on the adjacent lower stage then begins to descend and fluctuate under the static normal wall stress during granular material withdrawal. Employing the results obtained using stress measurements and image processing, the pressure pulsation phenomenon in a symmetric two-dimensional louvered-wall moving granular filter bed may be further understood.

  • Numerical simulation of flow patterns of disks in the asymmetric louvered-wall moving granular filter bed
    Powder Technology, 2000
    Co-Authors: Chuen-shii Chou, Jiri Smid, J.t. Kuo, C.y. Tseng, Shu-san Hsiau
    Abstract:

    Abstract Using the Discrete Element Method (DEM) model [P.A. Cundall, O. Strack, Geotechnique, 29 (1979) 47.], the granular solid flow pattern histories, velocity field and quasi-Stagnant Zone development close to the louvered walls of six kinds of 2-D asymmetrical moving granular filter beds were studied numerically. The numerical results reported here and the experimental results [J.T. Kuo, J. Smid, S.S. Hsiau, C.Y. Wang, C.S. Chou, Filtr. Sep., 35(6) (1998) 529.] provide fundamental and important information for designing moving granular bed high-temperature flue gas cleanup filters.

Sandy F C Stewart - One of the best experts on this subject based on the ideXlab platform.

  • effects of a vascular graft natural artery compliance mismatch on pulsatile flow
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J. Lyman
    Abstract:

    Abstract Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or Stagnant Zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

  • Effects of a vascular graft/natural artery compliance mismatch on pulsatile flow.
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J. Lyman
    Abstract:

    Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or Stagnant Zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

Shu-san Hsiau - One of the best experts on this subject based on the ideXlab platform.

  • Study of Flow Patterns in Two-Stage Mode of Moving Granular Bed Filter
    Aerosol and Air Quality Research, 2017
    Co-Authors: Chang Chia-wei, Shu-san Hsiau, Yi-shun Chen, Yan-pin Chyou, Jiri Smid
    Abstract:

    ABSTRACTThe use of moving granular bed filters (MGBFs), which hold an important role in the gasification or combustion of coal and biomass, has been growing. Furthermore, these filters have great potential to be developed for the high-temperature gas cleanup of advanced power generation systems. However, the creation of a Stagnant Zone due to defective designs in MGBFs can cause serious issues such as plugging. Therefore, in order to minimize these problems, flow-corrective inserts for filter vessels have been researched, most of which use mono-sized filter granules. This study proposed a new method that introduced two granule sizes in one filter vessel, called the two-stage filtration mode. Nevertheless, the design theory of a mass flow vessel by Johanson could not be satisfied by using two sizes of filter granules to diminish the Stagnant Zone. The flow patterns in a two-dimensional and cross-flow moving granular bed were evaluated in six test configurations based on different geometric designs in a filter. The two kinds of filter granules consisted of coarse and fine silica sands. The findings revealed that the flow patterns of filter granules were influenced by the vessel geometry. The optimal design for two-stage filtration diminished the Stagnant Zone in 165 minutes.

  • The flow patterns and stresses on the wall in a symmetric louvered-wall moving granular filter bed
    Powder Technology, 2003
    Co-Authors: Chuen-shii Chou, Jiri Smid, J.t. Kuo, Shu-san Hsiau
    Abstract:

    Abstract The flow patterns and stresses on the wall in a symmetric two-dimensional louvered-wall moving granular filter bed were investigated. The static wall stress distributions produced by the granular solids were measured and compared with the theoretical prediction using the differential slice and Runge–Kutta (order four) methods. The variations in the dynamic wall stresses with time in a moving granular filter bed were obtained. In addition, the effect of the louver angle upon the flow patterns and wall stresses was investigated. Four different flow regions were observed in a moving granular filter bed. As the angle of the louver decreases, the quasi-Stagnant Zone area adjacent to the side wall becomes smaller and the static normal stress acting on the convergent section of the side wall becomes larger. The magnitude of the static normal stress acting on the convergent section is approximately 10 times as large as that acting on the vertical section. When the normal stress measured by pressure gauge installed on the upper stage decreases to zero, the normal wall stress measured by pressure gauge installed on the adjacent lower stage then begins to descend and fluctuate under the static normal wall stress during granular material withdrawal. Employing the results obtained using stress measurements and image processing, the pressure pulsation phenomenon in a symmetric two-dimensional louvered-wall moving granular filter bed may be further understood.

  • Numerical simulation of flow patterns of disks in the asymmetric louvered-wall moving granular filter bed
    Powder Technology, 2000
    Co-Authors: Chuen-shii Chou, Jiri Smid, J.t. Kuo, C.y. Tseng, Shu-san Hsiau
    Abstract:

    Abstract Using the Discrete Element Method (DEM) model [P.A. Cundall, O. Strack, Geotechnique, 29 (1979) 47.], the granular solid flow pattern histories, velocity field and quasi-Stagnant Zone development close to the louvered walls of six kinds of 2-D asymmetrical moving granular filter beds were studied numerically. The numerical results reported here and the experimental results [J.T. Kuo, J. Smid, S.S. Hsiau, C.Y. Wang, C.S. Chou, Filtr. Sep., 35(6) (1998) 529.] provide fundamental and important information for designing moving granular bed high-temperature flue gas cleanup filters.