Discovery of Hydrodynamic Ripples in Wind Tunnel Experiments

A detailed and targeted experimental study shows at the first time the formation of small impact ripples and larger hydrodynamic ripples in a boundary wind tunnel for different grain sizes of glass beads. The theory behind the formation of these two ripple types will be presented in a separate presentation in this conference by Orencio Duran.

The results can shed light on the formation of the debated large martian ripples. The wind (aeolian) experiments were performed using an open circuit wind tunnel at the Ben-Gurion University consisting. Glass beads of three fractions have been used in the experiments with grain size distributions of d50=90, d50=170 and d50=430 micrometer. The amplitude and the wavelength of the sand ripple were measured, and their spatial patterns were analysed by 3D laser scanner. The sand flux at different heights was measured by saltation traps.

For the first two fine fractions (d50=90, d50=170 micrometer), two scales ripples were formed due to different instabilities for the same wind velocities as shown in Fig.1. The small ripples interpreted as impact ripples and the larger ripples were interpreted as hydrodynamic ripples. However, only one-size impact ripples were developed for d50=430 micrometer. For higher wind velocities the hydrodynamic ripples developed to a more complex wavy pattern as shown in Fig. 1c, and the impact ripples disappeared. A CFD was applied to simulate the distribution of the shear velocities over the ripples during their development. Our new experimental results can dramatically change our understanding of the physics of sand transport at the mesoscale, thus opening up a whole new field of research.

Figure 1. Hydrodynamic ripples and impact ripples in the wind tunnel for d50=90 micrometer (a and c) and for d50=170 micrometer (b and d). For 9 m/s wind velocity, the hydrodynamic ripples show a complex pattern like subaqueous ripples, and even avalanches at specific locations. For lower wind velocities, there is a coexistence of impact and hydrodynamic ripples. (e) Coexistence of small impact ripples after 40 minutes (λ≈1cm) and larger hydrodynamic ripples (λ≈12cm). The wind velocity was 4.5 m/s from left to right (d50=170 micrometer ). For larger wind velocities only the hydrodynamic ripples exist.