Turbulent Shear Flow Over Large Martian Ripples

Abstract

Aeolian ripples are abundant in arid regions on Earth and on the surface of Mars. On Earth, wind of sufficient strength can begin mobilizing sand grains into "saltation" (bouncing) trajectories that disturb other grains on the bed with every rebound, quickly leading to a cascading mobilization of many other grains. In fully developed saltation, shear stress at the bed is reduced by momentum exchange between the boundary layer and numerous grains in flight, so that continued grain mobilization from the bed is due primarily to impact effects from high-speed saltating grains (e.g., Bagnold, 1941, pp. 31-33). Ripples formed during this process are referred to as "impact ripples" or "ballistic ripples" (Figure 1) to distinguish them from ripples forming underwater by a different, fluid drag process. Impact ripples in relatively well-sorted desert sands Abstract On Mars, large aeolian ripples with wavelengths typically 1-3 m but lacking very coarse sand at crests have been encountered by rovers and observed from orbit. These bedforms have no terrestrial counterpart and several hypotheses for origins have been proposed. This work reports results of Computational Fluid Dynamics (CFD) experiments with ANSYS Fluent under terrestrial and Martian boundary layer conditions, using the k − ω SST turbulence model to evaluate shear stress along a topographic profile of large Martian ripples at different boundary layer wind speeds. Results indicate that, compared with Earth conditions: (1) boundary-layer flow along large ripples under Martian conditions is less turbulent due to higher kinematic viscosity; (2) reverse-flow vortex regions from crests at ripple lee flanks are larger; and (3) shear stresses at crests of large ripples are relatively low, so ripple flattening is less likely at high wind speeds. These results indicate Martian ripples formed by the saltation impact splash mechanism should be less constrained by shear stress effects limiting growth of exposed ripple crests, because the low-density Martian atmosphere applies relatively low wind-related shear stress to ripple surfaces. Other origins for the large Martian ripples are not excluded, however. On Earth, very large ripples with crests unprotected by very coarse grains do not develop due to higher wind-related shear stresses. Plain Language Summary Ripples of wind-blown sand are ubiquitous in terrestrial sandy deserts, and also on the arid surface of Mars. On Earth, aeolian ripples in typical dune sands have wavelengths <30 cm and develop when surface grains are disturbed by numerous rebounding impacts of high-speed, wind-driven grains that bounce rapidly downwind. On Mars, rovers have observed aeolian ripples in similar sands but of much greater size, including numerous examples with wavelengths >2 m. There is no consensus explanation why much larger ripples develop on Mars than on Earth. Work reported here describes Computational Fluid Dynamics (CFD) numerical flow experiments using a ground profile modeled after large Martian ripples. Experiments with this model...

Re‐Evaluation of Large Martian Ripples in Gale Crater: Granulometric Evidence for an Impact Mechanism and Terrestrial Analogues

“…On Earth, the relatively potent fluid force from the wind limits ripple size. Here, our findings are consistent with the mechanisms suggested by Sullivan and Kok (2017), Sullivan et al (2020), Siminovich et al (2019) and Yizhaq et al (2021) where the lower fluid force of the thin Martian atmosphere does not provide the same height-capping mechanism as on Earth and allows a broader range of ripple scales. There is clear separation in the threshold of motion for grains from adjacent large and small ripples (Figure 5).…”

An Evolving Understanding of Enigmatic Large Ripples on Mars

“…(2016). Further supporting the latter speculation, recent Computational Fluid Dynamics (CFD) simulations also demonstrated that higher wind kinematic viscosities decrease turbulence within the boundary layer along a range of ripple sizes, such that lower atmospheric densities translate into lower shear stresses at ripple crests, possibly allowing them to grow larger and longer (Siminovich et al., 2019; Yizhaq et al., 2021).…”

Planetary Aeolian Landforms: An Introduction to the Fifth Planetary Dunes Workshop Special Issue

“…Together with the longitudinal ripples, the impact mechanism in a low wind dynamic pressure environment can also explain the presence of megaripples and transverse aeolian ridges (TARs-bedforms having width/wavelength of ∼10 m which can be either small dunes or megaripples (Bourke et al, 2010;Zimbelman et al, 2013)), which are not explained by the fluid-drag hypothesis (Lapotre et al, 2016a(Lapotre et al, , 2016bSullivan et al, 2020). The Martian low density boundary layer also imposes lower (compared to Earth) shear velocities over the ripple slopes (Siminovich et al, 2019;Yizhaq, Siminovich, et al, 2021). CFD simulations have been used to investigate if and where shear velocities reach the fluid threshold for sand motion over a synthetic and a more realistic ripple profile (Siminovich et al, 2019;Yizhaq, Siminovich, et al, 2021).…”