Abstract
The spatio-temporal evolution of a downsized model for a desert dune is observed experimentally in a narrow water flow channel. A particle tracking method reveals that the migration speed of the model dune is one order of magnitude smaller than that of individual grains. In particular, the erosion rate consists of comparable contributions from creeping (low-energy) and saltating (high-energy) particles. The saltation flow rate is slightly larger, whereas the number of saltating particles is one order of magnitude lower than that of the creeping ones. The velocity field of the saltating particles is comparable to the velocity field of the driving fluid. It can be observed that the spatial profile of the shear stress reaches its maximum value upstream of the crest, while its minimum lies at the downstream foot of the dune. The particle tracking method reveals that the deposition of entrained particles occurs primarily in the region between these two extrema of the shear stress. Moreover, it is demonstrated that the initial triangular heap evolves to a steady state with constant mass, shape, velocity and packing fraction after one turnover time has elapsed. Within that time the mean distance between particles initially in contact reaches a value of approximately one quarter of the dune basis length.
Highlights
S Online supplementary data available from stacks.iop.org/NJP/12/063025/ mmedia ‘We are right in the open desert,’ said the doctor
The crescent-shaped barchan dune was chosen as a suitable object because of its relatively fast dynamics
-called ‘minimal models’ were established to describe the basic dynamics of barchan dunes [8,9,10]. These two-dimensional (2D) models deal with dune slices along the direction of the driving wind. They combine an analytical description of the turbulent shear flow over low elevations [11, 12] with a continuum description, which models the saltation on the surface of the dune [13]
Summary
S Online supplementary data available from stacks.iop.org/NJP/12/063025/ mmedia ‘We are right in the open desert,’ said the doctor. Only particles from the surface are in motion, the center of mass of the dune migrates downstream with a velocity vcom. We define five characteristic areas (A–E), as shown, to compare the vertical velocity profiles of the grains and the water flow in figure 9(b).
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