沙漠研究 22-1, 267 -270 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－

Investigation into Sand Movement around a Column Using a Wind Tunnel
Hiroaki MOCHIZUKI*1), Yukari OKADA2), Sanjay KUMAR3) and Sangeeta SINHA3)
Abstract: In the present study, sand movement around a column was investigated in the laboratory using a wind tunnel and ahigh-speed digital video camera. The thickness of the sand layer in the test section was kept at 0.28 m. It was observed that thesand from upstream and also from both sides of the obstacle (column) moved faster due to accelerated sand-driving wind compared tothose particles away from the column or downstream. This caused visible erosion of the sand layers in the upstream and also fromboth sides of the column. These eroded sand particles were scoured deeper than those at initial horizontal surface. However,movement of the sand became steadily slower when the scoured depth increased. The scoured area around the column fanned out
from the column with time. The column began to oscillate as the sand below the bottom of the column was scoured, and finally, it
overturned windward. In the present investigation, the relation between the scoured depth around the column and blowing time ofwind, also the relation between the depth of the buried column and the time required until the column overturned, and the mechanismof the sand movement around and below the column were clarified in detail.
Keywords: Desert, Sand movement, Scour around column, Wind tunnel

1. Introduction
The effect of sand movement due to obstacles is very
important for stability of civil structures and desert plantations.
In desert plantations, it is known to have significant effect onsurvival rate and uprooting of trees. The movement of sandparticles due to the wind in dry lands is affected by several
factors, the major ones being:, wind velocity, inclination of thedeposited sand, size of sand grain and most importantly, theposition and geometrical nature of the obstacle. For example,
when a wood, a stake, a seedling, etc. are standing on a flatsand surface, sand grain in surface is scoured or depositedaround them in a very unique way. Sand movement patterns
have been widely investigated and documented by (Bagnold1938, 1954) who experimentally studied the relation between
wind velocity and sand movement. Later, the creep motion inaeolian sand transport was studied using two granular flow
models, Saint-Venant model and Navier-Stokes type equations(Wang and Zheng, 2004). Also, a method for measuring sandcreep movement was experimentally developed using ahigh-speed digital camera (Wang et al., 2009). Patterns anddynamics of desert dunes were simulated by a computer(Hatano and Hatano, 2001). The hillslope evolution by
nonlinear creep and landsliding was investigated (Joshua et al.,2001). Modeling and simulation method were proposed tosynthesize sandy terrain with vegetation covers (Wang and Hu,2009). But, phenomenon of sand pattern construction andmovement at small scale around the obstacles on the sandsurface has not yet been clarified in detail. The reason why
the column in wind overturns windward is due to the fact thatthe sand on the upstream side of the column is scoured fasterthan the downstream side as investigated experimentally(Hayashi et al., 1994) and numerically (Kawamura et al.,1999). The sand movement around a cube on the flat sandsurface was experimentally and numerically investigated under
the approach of sand erosion and deposition (Tominaga, 2007).
But, sand movement around obstacles has not yet been totally
clarified in previous investigations. So, in the present
investigation, the detailed mechanism of sand movement
around a column was experimentally observed using a video
camera and a laser distance meter. The present results arevery important to optimize the strength required for civil
construction and also for desert vegetation survival, etc.

2. Materials and Methods
A wind tunnel was schematically shown in Figure 1. Air
from a blower entered into a test section through a convergent
section with 1.82 m length, 1.90 m width and 1.00 m height.
The test section was 2.78 m length, 1.18 m width, 0.76 m
height and had a flat sand bed of thickness 0.28 m to ensurethat sand movement in this experiment was similar to thenatural one. So, air was discharged into atmosphere throughthe air channel with height 0.48 m. The test section had a sideand upper walls made of transparent acrylic resin to observe
sand movement from the side and top views. The outer andinner diameters of the column were 76 mm and 71mm,*Corresponding author: moonbeam@agri.kagoshima-u.ac.jp
1-21-24, Korimoto, Kagoshima, Japan Tel:+91-099-285-8612
1) Department of Environmental Science and Natural Resources, Faculty of Agriculture, Kagoshima University
2) Faculty of Agriculture, Kobe University

3) Centre for Renewable Energy and Environment , Bihar, India

Fig.1. Configuration of experimental apparatus.

respectively. The column was positioned at 0.55 m upstreamof the exit and 0.42 m from the side wall of the test section.
The height of the column over the sand surface was kept atconstant of 23.5 cm to give the same wind power. Theheights below the sand surface were 0, 20, 40, 60 and 80 mm
to estimate the effect of the depth buried in the sand.
Furthermore, sand movement around the column was observed
by a camera (Casio Digital Camera, EX-F1). The upper andlower surfaces of the column were made of transparent acrylic
resin with thickness of 5 mm to observe sand movement below
the bottom of the column. Sand particles used in theexperiment were sifted through several screens and weighted
using an electronic balance (Type BL-320H). Sand grains of
99% in the sand bed were smaller than 0.3 mm in diameter.
Wind velocity in the test section was measured by anAnemometer (Type SK-93F). The depth of scoured sand wasmeasured by a laser distance meter (Type LS-411). Blowing
time was measured by a stopwatch.

3. Results and Discussion
Before the observation of sand movement, wind velocity
was measured at 0.4 m upstream of the column and between
0.2 m and 0.62 m from the side wall. The mean velocity wasabout 7.0 ms-1 and the margin of error is plus- minus 0.2 ms-1.
The direction of the wind was checked by measuring thegeometrical shape and propagation of sand ripples. The
pattern formation of the sand ripples was taken by the camera
before all experiments. As a result, their directions werealmost perpendicular to the side wall of the test section in theregion between 10 cm and 60 cm upstream of the column
During the experiment, since sand moves downstream in thetest section, the height of the flat sand surface decreased little
by little in the upstream region of the test section. So theexperiment was stopped every fifteen minutes, after new sandwas supplied, the experiment was started again.
Upper and side views of sand movement around thecolumn and below the bottom of the column are shown inphotos and their respective diagrams in Figure 2. Figures 2

Fig.2. Sand movement around and below the column.

(a)~(d) are for the photos from the upper direction of the test
section, and a diagram in below-left in Figure 2 indicatescommon sand movement patterns of Figures 2 (b)~(d).
Figure 2 (a) shows the picture for no wind. Figure 2 (b)shows the picture at 240 second later from the start of windblowing. In these figures, the sand layer is scoured in arc
pattern in the region located in the side surfaces of the column.
In the region from the side to back side surfaces, the sand isshallowly scoured in downstream direction. Behind thecolumn, an elliptical pattern is observed in the region

surrounded by “abcd” as shown by the oblique lines in thediagram. This pattern becomes larger in Figures 2 (c) and (d).
The reason why this is formed is due to the fact that the sandmoving from both sides of the column collides at this regionand separates. Also, the sand reverses toward the column inthe region shown in white between the elliptical region and thecolumn. So the sand layer contacting the back side of thecolumn becomes higher than those at the front- and side walls.
In this way, sand is largely affected and scoured in the region of“oabco” around the column. Sand gradually fans out asshown by arrows in the diagram. In Figure 2 (d), sand thatwas in contact with the front half side wall of the column istotally scoured. But the column does not overturn in thepresent experiment. In previous investigations, it wasdescribed that the column overturned windward in the case ofFigure 2 (d). The difference is explained in the following
sentences.
After Figure 2 (d), the Figures 2 (e1), (e3) and theirrespective Figures 2 (e2), (e4) show at 990 sec the momentright before the sand start to scour at the bottom. After thismoment, the sand movement below the bottom of the column
starts and the sand is scoured as shown in Figures 2 (f1)~(i1)and their respective Figurse 2 (f2)~(h2), Figures 2 (f3)~(h3)and their Figures 2 (f4)~(i4) with time. In Figures 2 (e1)~(i1),the camera is focused in the bottom of the column. So theouter circle of the figures corresponds to the diameter of theupper surface of the column and the inner circle to the bottom
surface. Figures 2 (e3)~(h3) are from oblique upward
direction and their corresponding Figures 2 (e4)~(i4) and theFigure 2 (i4) are from the side direction. In Figures 2 (f3) ~(h3), dotted lines are the circumference of the bottom. The
sand is scoured and an air channel is generated below the lines.
In the Figure 2 (f1) and the Figure 2 (f2), small arc regions
shown in white at an angle of 45 degrees are observed, and in
Figure 2 (g1) and Figure 2 (g2), these arcs are observed in theform of two clear crescents. These indicate that the sandscour below the bottom starts at the angle of 45 degrees fromthe upstream (right) end of the column as also shown inFigures 2 (f3)~(h3). This is due to the fact that the sandmoves with the fastest velocity at 45 degrees. In (h1) and(h2), two crescents contact each other and combine. Twocrescents correspond to caves in the Figures 2 (f4)~(h4) and do
not support the column. In this way, the scouring of the sandbelow the bottom gradually expands and the column oscillates,
and finally, as shown in Figure 2 (i1), the Figure 2 (i4), and theFigure 2 (i1), the column overturns windward, because thecolumn loses the balance since the scoured sand area becomeslarger than the no- scoured one (shown in black). At thismoment the column overturns windward within about 45
degrees from the horizontal line of the column in Figure 2 (i1).

Fig. 3. Blowing time and scoured depth.

Fig. 4. Buried depth and overturned time.

Because sand around and below the column does not movesymmetrically at the same time, the column does not necessary
overturn windward. In this way, the detailed mechanism ofoverturn of the column was clarified for the first time in thepresent experiment.
Figure 3 shows the relation between blowing time andscoured depth at four points at the distance of 2 mm away fromthe column. The depth at the point ① is lower than the ones
at points ②～④. This is due to the fact that the sandreverses behind the column as explained in Figure 2. The
scoured depth fast decreases at first and slowly with time.
This is due to the movement of the sand that became slowly
with time in the scoured depth.
Figure 4 shows the relation between buried depth andoverturned time of the column. Values at five measuring
depths are shown by length connecting maximum andminimum values. Also though the value at 0 mm can not beclearly judged since it is smaller than other values, the meanvalue is about 45 sec. The overturned time gradually
increases for the buried depth in a period shorter than 40 mm,
but when time value is bigger, the buried depth increases.
From the present results, it could be expected that when woods
and seedlings are planted and stakes are stood in the dried sandland, the buried depth and the direction of wind are the mostimportant factors.
In the present experiment, the fundamental mechanism ofoverturn of the column was described. But the wind velocity
was fixed at about 7 ms-1, therefore it is necessary toinvestigate cases with different wind velocities. Also, theouter diameter of the column was 76 mm and the height of thecolumn over the sand surface was 235 mm. So it is necessary
to conduct experiments to clarify the cases with different
diameters and heights of the column. Furthermore, in thepresent experiment, the column was tested as the obstacle.
But the shapes of the obstacles are more complicated in nature.
Therefore it is necessary to take the shapes of the obstacles into
consideration. Also, woods and seedlings have manybranches, so the effects of branches on their stability should beinvestigated in future. Furthermore, since in some cases,
woods, seedlings and stakes are planted or stood in near
distance, it will be necessary to investigate sand movement
including their interactions.

4. Conclusion
In the present investigation, the mechanism of sandmovement around a column was described in detail. The
sand layer around the column was scoured deeper in the frontand both sides than those away from the column ordownstream. The sand layer at the front and side wall
surfaces of the column are scoured faster than one at the backside. Sand scouring below the bottom starts at 45 degrees
from wind direction. When the sand below the column isscoured and the column loses the balance, the column
overturns. When the buried depth of the column is bigger, thetime when the column overturns is bigger.

References
Bagnold R.A. (1938): The measurement of sand storms.
Proceedings of the Royal Society of London. Series A,Mathematical and Physical Sciences, 167:282-291.
Bagnold R.A. (1954): The Physics of Blown Sand and Desert
Dunes. Dover Publications, Inc. Mineola, New YorkWang Z.T., Zheng X.J. (2004): Theoretical prediction of creep
flux in aeolian sand transport. Powder Technology, 139:123-128.
Wang Y., Wang D., Wang L., Zhang Y. (2009): Measurementof sand creep on a flat sand bed using a high - speed digital
camera. Sedimentology, 6: 705-1712
Hatano Y., Hatano N. (2001): Dune Morphology and SandTransport. Forma, 16: 65-75.
Joshua J., Roering J.W., Kirchner L.S., Sklar W.E.D. (2001):
Hillslope evolution by nonlinear creep and landsliding: An
experimental study, Geology, 29(2): 143-146.
Wang N., Hu B-G. (2009): Aeolian sand movement andinteracting with vegetation: A GPU Based Simulation andVisualization Method, Third International Symposium on
Plant Growth Modeling, Simulation, Visualization andApplications: 401-408.
Hayashi T., Waka R., Kamichika M. (1994): Studies onenvironmental problem in arid land. Proc. MIE International
Forum Symposium, Global Environment, Friendly Energy
Technol.: 385-389.
Kawamura T., Kan M., Hayashi T. (1999): Numerical Study ofthe flow and the sand movement around a circular cylinder
standing on the sand. Transactions of the Japan Society of
Mechanical Engineers. B, 65(629): 124-129.
Tominaga Y. (2007): Wind tunnel experiment and CFD
analysis of sand erosion/deposition around an obstacle.
Nagare, 26: 301-306.