The discovery of aerodynamic ripples in wind tunnel experiments
Aeolian sand ripples formed due to the interaction between wind and loose sand and they are ubiquitous both on Earth and Mars. Terrestrial normal ripples forming in unimodal fine sand are quite small with wavelengths smaller than 30 cm and height in the order of 1 cm. Surprisingly, on Mars, these ripples are much larger with wavelengths of an order of 1-3 m and height of a few cm with smaller decimeter superimposed ripples. Since the discovery of these large martian ripples, there is an ongoing scientific debate about their formation and two main theories have been suggested to explain their formation. The first hypothesis views the large martian ripples as impact ripples that grew larger due to the lower dynamic pressure on Mars. This hypothesis can explain the observed coexistence of small and large ripples but not their simultaneous formation.
According to the second theory, the large martian ripples are wind drag ripples or aerodynamic ('hydrodynamic') ripples that are similar to subaqueous ripples that form due to the large kinematic viscosity of the martian atmosphere. This hypothesis argues that these two ripple sizes have distinct size distributions and lack bedforms in the ∼20–80 cm range indicating two different formative mechanisms that can overlap. The large ripples form due to hydrodynamic instability and their size scale with the thickness of viscous sublayer ν/u* where ν is the kinematic viscosity and u* is the shear velocity.
Here we present a detailed experimental study with different glass bead sizes that show the coevolving of two scale ripples at the Ben Gurion University boundary layer wind tunnel and at the low-pressure wind tunnel in Aarhus University in Denmark (Fig. 1). The small scale ripples (~cm) are interpreted as impact ripples, whereas the large scale ripples (~10 cm) interpreted as aerodynamic ripples that developed due to hydrodynamic instability like aeolian dunes or subaqueous ripples. Fig. 1b shows the incipient wavelengths of the two scale ripples for different grain sizes in a series of wind tunnel experiments close to the fluid threshold. For natural dune sand the observation of the two scale ripples is less clear indicated that grain shape and the exact grain size distribution play a role in the formation of the aerodynamic ripples. We further discuss the conditions that favor the formation of the aerodynamic ripples. These new results can shed light on the formation of the large martian ripples.
The theory behind the formation of the aerodynamic ripples will be presented in separate abstracts by Orencio Durán and Katharina Tholen (see also Yizhaq et al., 2024).
Fig. 1 (a) Coevolving of aerodynamic ripples and impact ripples for glass beads μm in the Ben Gurion University wind tunnel. (b) Incipient wavelengths of both aerodynamic ripples (fluid drag ripples) and impact ripples for different glass bead sizes and for different wind velocities.
How to cite: Yizhaq, H., Saban, L., Vinent Durán, O., Kroy, K., Tholen, K., Pähtz, T., Silvestro, S., Franzese, G., Merrison, J., Iversen, J., Rasmussen, K., and Katra, I.: The discovery of aerodynamic ripples in wind tunnel experiments , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4780, https://doi.org/10.5194/egusphere-egu24-4780, 2024.
Crescent-shaped dunes called Barchans are found in areas with constant wind direction and little sand. They often cluster together, so we need to consider the interaction between each of the barchans. In previous research, two models represent the movement of barchan: one is the ABCDE model, which describes the collision of barchan by similar triangles in the cross-section, and the other is the single crest line model, which describes the movement of a single barchan by a single line. However, the single crest line model has the problems that the conservation of sand is not strictly considered and that the shape and height of the dune do not reach a steady state. Therefore, in this research, we take into account the conservation of sand and improve the single crest line model. As a result, we confirm that even near the boundary of the barchan, where the sand height is very small, the sand balance is kept and the shape of the barchan reaches a steady state. Numerical simulations of this single crest line show it could reproduce various phenomena that had been confirmed with the cellular model, such as the situation when sand was supplied to the barchan. In addition, to handle the behavior of multiple barchans in 3D space, a model that extends the single crest line model to multiple lines is effective. Hikosaka (master thesis, Hiroshima University, 2019) tried to derive a ”multiple crest lines model” by adding the idea of the ABCDE model to the single crest line model (Fig. 1). However, it is still in the process of completion. By using the sand balance matching method proposed in this research, we can handle the influence of sand at both ends of the barchan more accurately. Then we can make the model of dune collision phenomena with multiple crest lines. In our model, we also include a method for calculating the time evolution of a valley between two dunes created during their collisions. We compare the result of numerical simulation with the results of water tank experiments in previous research to analyze whether the collision patterns of recombination, breakup and coalescence appear or not. By using our proposal way, we can calculate the multiple dunes collision (Fig. 2). This means we get a step toward theoretical analysis of the dynamics of multiple dunes.
How to cite: Navarro Yabe, S., Shiraishi, M., and Nishimori, H.: Barchan collision using multiple crest lines model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-497, https://doi.org/10.5194/egusphere-egu24-497, 2024.