Laboratory Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 203

material during their merger) and the variety of barchan
forms under different flow direction variations has been
studied (Taniguchi et al. 2012; see also Fig. 17.16 ).
Perhaps the most spectacular examples of this type of
study is that by the Paris group. They used a platform
immersed in a water tank (Fig. 17.17 ), where the platform
could be translated some distance (thereby setting up a
quasi-steady flow for a brief interval). The platform had a
rotating table on which the sediment bed is deposited
(incidentally, a rotating table was used on a beach, with
natural wind, in Rubin and Ikeda's original experiments on
bedform orientation). Thus to simulate reversing winds, the
platform is just moved back and forth. To simulate unidi-
rectional winds, the platform is rotated 180 between each
cycle. And, of course, a range of wind angles can be sim-
ulated by different angles; Reffet et al. (2010) show the
transition from transverse to longitudinal bedforms (e.g.,
Fig. 17.16 ).

Fig. 17.15 Schematic of a steady-flow water flume, with a circular
platform that can be rotated to expose the bedform to varying flow
directions. Graphic courtesy of K. Taniguchi

outwards) and, second, Titan involves much colder temper-
atures which adds to the hazard, as well as the cooling
demands. (For example, if the power to the building failed, a
1.6 bar Titan chamber at 94 K would start warming up,
becoming a 5 bar chamber at room temperature.)
At the time of writing, NASA is constructing a large
Venus simulation chamber with a volume of a little over
1m 3 at the Glenn Research Center in Cleveland. This
chamber, the Extreme Environment Chamber, will be able
to generate not only the pressure and gas composition of
Venus, but also its temperature. While the principal purpose
of the chamber is to test instruments and equipment for
future Venus missions, perhaps some aeolian experiments
can be conducted by introducing some kind of blower or
fan. This facility might also be capable of simulating Titan
conditions in a similar way.

17.5 Abrasion Experiments

The susceptibility of materials to aeolian abrasion can be
expressed as the ratio of the mass of material eroded to
either the mass of the impacting particles or to the number
of impacting particles; the susceptibility to abrasion is not
equivalent to a coefficient of abrasion because the suscep-
tibility can vary with parameters such as impacting particle
velocity and the angle of impact with the target (Greeley
and Iversen 1985, p. 112). How does one go about mea-
suring the many different physical aspects that can con-
tribute to determining the susceptibility of aeolian erosion
for diverse natural materials, since very careful control of
both the impacting particles and the target materials is
required? The solution to this question was a rather unique
experiment where individual particles could be directed at
targets within a chamber, which provided careful control of
both the environmental conditions in the test chamber and
the geometry of the individual sand grain impacts on the
target surface.
The experimental apparatus used to determine the sus-
ceptibility to abrasion for various rocks ended up being a
circular chamber in which individual particles could be
flung at controlled speeds against a prepared rock target
(Figure 4.5 of Greeley and Iversen 1985). Individual sand
grains were dropped into a rotating tube that accelerated the
grain to the desired velocity at the point of impact on the
target plate. The mass of the individual impacting grains
was carefully controlled (through sieving), and the mass of
the target plates before and after a run provided a direct
measurement of the amount of material removed by the
accumulated effect of the impacting sand grains. The angle
of the target surface relative to the direction at which the

17.4 Water Tank Experiments

A range of remarkable experiments has been conducted
with sand in flumes (essentially a water version of a wind
tunnel) and in water tanks where a platform loaded with
sand can be moved back and forth. Clearly, the saltation
threshold is far off what it will be in air. But such studies are
of interest for studying the morphology of bedforms,
because another scaling parameter (see Sect. 5.5 ) falls in a
useful range. This is the saturation length, proportional to
the diameter of the particle divided by the fluid density.
Dunes in air on Earth may have length scales of the order of
a few meters and larger; by substituting water for air (with a
factor of 800 difference in density) and perhaps using
especially fine sand, the saturation length and thus the
bedform wavelength shrink accordingly, and therefore
dunes form with scales of just a centimeter or two. At this
scale, the bedforms (Fig. 7.18 ) and their arrangement can be
conveniently studied.
The interaction between migrating barchans has been
studied (Endo et al. 2004) in a continuous water flume
(Fig. 17.15 , using different colors of sand to track the