Suspension and Sedimentation of
Particles in a Horizontal Channel
Supported by the National Science Foundation under Grants #CTS-9702723 & CTS-9871156
Particle/wall turbulence interaction is an important topic to many natural and industrial processes such as particle deposition in materials processing, pneumatic transport of granular materials, and sediment transport within rivers and marine flows. Although much work has been done on this topic, there are still many aspects of the particle turbulence interaction within the wall-bounded region that is only known qualitatively, or under limited conditions due to restrictions by existing technology or theoretical simplifications. One such important area is the particle-fluid interaction mechanism that is responsible for the suspension and sedimentation of relatively large, heavy particulates within horizontal, wall-bounded shear flow. Since the development of the contemporary understanding of turbulent burst and sweep structures within boundary layers, it has been speculated that this mechanism is primarily responsible for the for the suspension and interaction of the particles within the flow (Sumer & Oguz, 1978; Sumer & Diegaard, 1981; Kaftori, et al, 1995a, 1995b, 1998; Nino & Garcia, 1996). While this work has revolutionized our phenomenological understanding of the flow, continued model development has not matched this progress due to a lack of quantitative measures of these processes. In light the of the above discussion, the current work has focused on trying to resolve some of these issues by utilizing our two-phase PIV technique to make simultaneous measurements of both the particulate and carrier phase. These measurements allow for the quantification of the important particle/fluid interaction statistics, as well as providing representative instantaneous vector fields of the carrier fluid structure responsible for the interaction.
Preliminary Result
The
flow consists of a fully developed turbulent channel
flow at a Reynolds number of Ret
= uth/n
= 570. The
dispersed phase consists of nominal 200 mm silica
glass spheres with a specific gravity of 2.5,
introduced at a bulk loading faction of 5x10-4.
The wall shear velocity of the flow is
approximately 2.8 m/s, which is slightly higher than
the average settling velocity of the particles (2.5
cm/s) and is sufficient to suspend them within the
flow.
Figure 2: Average velocity: (a) streamwise velocity of the single-phase flow, the fluid and particulates of the two-phase flow, (b) conditionally averaged streamwise, and (c) wall-normal velocity depending on whether the particle is moving away or towards the wall.
From this figure it can be observed that the
presence of the mobile particles has noticeably
altered the mean flow from the original single-phase
conditions. The mean fluid velocity is reduced
slightly in the outer region of the flow, which is
similar to those effects created by stationary
rough-wall elements. It is also found from these
profiles that the effective friction velocity is
increased by the presence of the particles by
approximately 7% for the current conditions. From the
same figure, it can be observed that the particles
primarily lag the mean fluid motion as one moves away
from the wall. This was also observed in previous
measurements of lower density particles (Kafotori,
Hetstroni, & Banerjee, 1995b), which was
attributed to particles selectively residing in
regions of low-speed fluid, rather than having a
sizable slip velocity. By examining the conditionally
averaged fluid velocity (Figure 2b and 2c), it can be
seen that particles moving away from the wall tend to
reside in regions of low-speed streamwise fluid
velocity, while particles settling towards the wall
show no mean bias, which is consistent with previous
results.
Figure 3 - click image for larger view
The above measurements indicate a preferential structure for the fluid motion that is responsible for the suspension of the sediment particles, and through the spatial information provided by the two-phase PIV, it is possible to examine several possible structures that contribute to this process. Figure 3 shows two instantaneous snapshots of the simultaneous fluid and particulate motion. The color contour lines indicate negative contributions to the Reynolds stress (i.e. those regions where slower fluid is moving away from the wall, or high-speed fluid is moving towards the wall). The filled grayscale contours indicate the relative swirling strength of the fluid, which is a kinematic measure of the circular motion of the fluid. The top figure depicts a flow structure that is similar to the hairpin vortex packets identified by Adrian, Meinhart, and Tomkins (2000) in a zero pressure gradient boundary layer. The signature of these structures is observed as compact regions of high swirl (indicating the head of a hairpin structure), followed by a negative Reynolds stress contribution that results from the induced flow of low-speed fluid up through the legs of the hairpin. From the figure, the suspension of the particles as a result of this structure can clearly be observed. The lower figure shows different type of structure that is also responsible for lifting of particles into the outer flow, and has the appearance of a counter-rotating pair of vortices. The magnified region of the lower figure shows the flow in detail, from which two high-swirl regions can be identified where the rotation of the fluid is in opposite directions. In between the two vorticies, low speed fluid is ejected upstream and away from the wall, carrying with it numerous particles.
Work is currently continuing on this project to increase the statistical sample, provide time-resolved (cinemagraphic) PIV measurements of the entrainment events, and to perform multi-dimensional measurements (either through scanning techniques or stereo PIV) to improve our understanding of these complex, three-dimensional structures.
Current publications and conference presentations on the above work:
1. Kiger, K. T. & Pan, C., Suspension mechanisms of solid particulates in a horizontal turbulent channel flow, Second International Synposium on Turbulence and Shear Flow Phenomena, Stockholm, Sweden, June 27-29, 2001.
2. Kiger, K., Pan, C., Particle suspension and sedimentation mechanisms in a horizontal turbulent channel flow, American Physical Society Division of Fluid Dynamics 52nd Annual Meeting, Washington, D.C., November 19-21, 2000.
Bibliography:
Adrian, R.J., Meinhart, C.D., & Tomkins, C.D., (2000) “Vortex organization in the outer region of the turbulent boundary layer”, J. Fluid Mech., 422, pp. 275-290.
Kaftori, D., Hetsroni, G., & Banerjee, S., (1995a) “Particle behavior in the turbulent boundary layer. I. Motion, deposition, and entrainment”, Phys. Fluids, 7(5), pp. 1095-1106.
Kaftori, D., Hetsroni, G., & Banerjee, S., (1995b) “Particle behavior in the turbulent boundary layer. II. Velocity and distribution profiles”, Phys. Fluids, 7(5), pp. 1107-1121.
Kaftori, D., Hetsroni, G., & Banerjee, S., (1998) “The effect of particles on wall turbulence”, Int. J. Multiphase Flow, 24(3), pp. 359-386.
Niño, Y., & Garcia, M.H., (1996) “Experiments on the particle-turbulence interactions in the near-wall region of an open channel flow: implications for sediment transport”, J. Fluid Mech., 326, pp. 285-319.
Sumer, M., & Oğuz, B., (1978) “Particle motions near the bottom in turbulent flow in an open channel”, J. Fluid Mech., 86, pp. 109-127.
Sumer, M., & Deigaard, R., (1981) “Particle motions near the bottom in turbulent flow in an open channel. Part 2”, J. Fluid Mech., 109, pp. 311-337.
| research | publications | biography | teaching | home |