Numerical Study of Shear Stress Distribution Over Sand Ripples Under Terrestrial and Martian Conditions

Ripples occur on Earth and Mars in a range of sizes. From terrestrial studies, ripple size is known to depend on grain-size frequency, wind duration, wind strength (including stronger winds that can flatten ripples), and fundamental environmental factors that differ between the two planets. Here we use computational fluid dynamics (CFD) experiments to model boundary layer shear stresses applied to aeolian ripple surfaces, to investigate how these stresses might differ between Earth and Mars. CFD experiments used ANSYS Fluent, with inlet wind speeds of 10 and 15 m/s for both planetary environments. Ripple profiles for Earth and Mars were developed using saltation and reptation properties modeled by the numerical saltation model COMSALT (Kok & Renno, 2009, https://doi.org.10.1029/2009JDO11702) to develop ripple profiles using the numerical technique of Yizhaq et al. (2004, https://doi.org.10.1016/j.physd.2004.03.015). Although the CFD experiments using these inputs could not include the effects of a saltation cloud, results are robust enough to indicate clearly that for similar modeled wind speeds on Earth and Mars, boundary layer shear stress applied to ripple surfaces is greater on Earth under conditions for which sand transport is expected. These results indicate that ripples can grow larger on Mars than on Earth, because for typical Martian wind speeds the shear velocity at the ripple crests is below the fluid threshold.