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

Abstract

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.

Turbulent Shear Flow Over Large Martian Ripples

“…Knowledge of shear stress along the profiles of these large Martian ripples, particularly at crests, can inform further discussions about possible origins of these bedforms. In a previous work, we used Computational Fluid Dynamics (CFD) simulations in Martian conditions to study the distribution of shear stress over modeled ripples with 30‐cm wavelength, which is relatively small compared to the meter‐scale large Martian ripples (Siminovich et al., 2019). Nevertheless, results predicted that shear velocity at the ripple crest at typical Martian wind speeds would be significantly lower than in terrestrial boundary layer conditions (Siminovich et al., 2019).…”

“…In a previous work, we used Computational Fluid Dynamics (CFD) simulations in Martian conditions to study the distribution of shear stress over modeled ripples with 30‐cm wavelength, which is relatively small compared to the meter‐scale large Martian ripples (Siminovich et al., 2019). Nevertheless, results predicted that shear velocity at the ripple crest at typical Martian wind speeds would be significantly lower than in terrestrial boundary layer conditions (Siminovich et al., 2019). The main goal of the current work is to utilize CFD simulations to evaluate shear stress distribution and flow characteristics along a larger two‐dimensional ripple profile more representative of the largest ripples encountered by Mars rovers and observed from orbit.…”

Dingo Gap: Curiosity Went Up a Small Transverse Aeolian Ridge and Came Down a Megaripple

“…These ripples result from the interaction of saltating sand grains and a monodispersive sand surface; here we refer to these as “ sand ripples .” Sand ripples develop from even smaller ripples formed by the splashing of sand grains when a saltating grain impacts a sand surface. The splashed grains move in what is called “reptation” rather than saltation, forming ripples approximately 1/6th the wavelength of sand ripples (Anderson, 1987; Anderson & Haff, 1988); these early stage wind ripples smaller than normal sand ripples have also been called “impact ripples.” Continuum analytical models utilizing both saltation and reptation of windblown sand have been developed to study aeolian sand deposits on Earth (Momiji et al., 2000; Yizhaq et al., 2004) and Mars (Siminovich et al., 2019; Yizhaq, 2005). Rovers on Mars have documented both sand ripples and reptation ripples, although both are an order of magnitude larger on Mars than they are on Earth (Bridges & Ehlemann, 2017; Lapotre et al., 2016; Sullivan et al., 2008, 2020; White, 1979).…”

Aeolian Ripple Migration and Associated Creep Transport Rates

“…Aspects of these themes have been reviewed or described extensively elsewhere, and we offer only a few examples here. Many papers have addressed the nature of ripples and the dynamics of ripple formation [8,[26][27][28][29][30]. Aspects of ripple geometry, especially key relationships between length, height, sand grain size, and migration rates, have also received considerable attention [31][32][33][34][35][36][37][38], as have sedimentary structures caused by aeolian ripple migration [39][40][41].…”