The discovery of aerodynamic ripples in wind tunnel experiments

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.