Mechanics of Sand Transport - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 49

Fig. 4.18 Parameter space of density and speed in the Venus wind
tunnel as measured by Marshall et al. (1992). The triangles denote
individual tunnel runs. At low speed/low density conditions, the sand

(175-250 l) does not move, and at high speed/high density conditions,
the bed stays plane with no bedforms. Only in a 'sweet spot' do ridge/
wave bedforms with no slip faces, and microdunes (Fig. 4.19 ) form

Fig. 4.19 Periodic microdunes formed in quartz sand (50-200 l) in the working section of the NASA Venus wind tunnel at a speed of 0.79 m/s.
Image courtesy of Ron Greeley

increases the wind stress moving up the stoss (upwind)
slope. Because the stress depends on height and slope, the
stress in fact peaks (s max ) before the highest point. Now, if
the sand flux responded instantly to the wind stress, the
maximum flux (q max ) would be upwind of the crest, and
thus dunes would always grow, regardless of size. However,
the lag (L sat ) in response means the peak sand flux is
downwind of the peak—if it is downwind of the crest, then
the crest will be eroded (see Fig. 4.17 ).
Bagnold (1941) suggested it would control the minimum
size of dunes, and Kok et al. (2012) suggest L sat is 'the only
relevant length scale in the physics of dunes', although that
may be going a little far (not least since the atmospheric
boundary layer thickness seems to be a fundamental limit to

the growth of dunes). The modern physics perspective, able
to simulate computationally a set of arbitrary flow condi-
tions, and the trajectory of particles in such a flow (and their
effect on it), might be profitably applied to experiments
made with planetary wind tunnels. One example parameter
sweep (Marshall and Greeley 1992) is in density/speed space
(Fig. 4.18 ) in the Venus wind tunnel at NASA Ames (see
Chap. 17 ) where bedforms appear (Fig. 4.19 ) in only a
limited part of the space—too low speed and no transport
happens, but too large a speed and bedforms are swept away.
The existence of a finite saturation length also implies an
'inertia' to turbulent changes in wind stress, which has
important implications for transport overall and for field
measurements in particular. Since it takes a finite time for the