A wind tunnel study of the effect of intermediate density ratio on saltation threshold

Abstract

An expression for aeolian saltation threshold-the minimum wind speed required to initially entrain sediment particles into saltation-is used in modeling aeolian processes and the formation of aeolian landforms on Earth and other planetary bodies. Previous experiments under high-fluid-density conditions in the Venus Wind Tunnel (VWT) concluded that this threshold is a function of the ratio of the density of the particle to the density of the entraining fluid (ρp/ρ). A curve for the dimensionless threshold parameter, A, as a function of (ρp/ρ), was derived [Iversen et al. 1987]. In this work, we revisit this curve of A(ρp/ρ) using data from the Titan Wind Tunnel (TWT) [Burr et al., 2015b]. We use both previous and newly collected TWT data of freestream threshold wind speed spanning a range of density ratio conditions to derive friction wind speeds and then values for A. The TWT-derived data generally overly the VWT data used to derive the density ratio curve, although showing greater variability and an apparent offset to slightly higher values. We derive a new density ratio curve from the aggregated threshold data using the same format as the previous expression but with new values for the two exponential parameters. These TWT data provide additional constraints on the transitional portion of the density ratio curve between the low density ratio conditions of flume data and the high density ratio conditions of Mars Surface Wind Tunnel data.

A lower-than-expected saltation threshold at Martian pressure and below

“…Over several decades, sediment transport thresholds were measured experimentally in aeolian [64,80,77,78,81,92,93,63,85,97], subaqueous [72,101,60,84,87,75] or intermediate [61,62] conditions. In those experiments, the dynamic or cessation threshold is defined in most papers as the extrapolation to vanishing flux of the relation between the average sediment flux and the imposed constant, shear velocity u * .…”

Unified Model of Sediment Transport Threshold and Rate Across Weak and Intense Subaqueous Bedload, Windblown Sand, and Windblown Snow

“…If it was indeed generally true, it would indicate a conceptual problem in most, if not all, existing aeolian transport threshold models. In fact, existing threshold models, regardless of whether they model transport initiation (e.g., Burr et al., 2015, 2020; Iversen & White, 1982; Lu et al., 2005; Shao & Lu, 2000) or transport cessation (e.g., Andreotti et al., 2021; Berzi et al., 2016; Claudin & Andreotti, 2006; Kok, 2010; Pähtz et al., 2012), usually incorporate expressions that describe the entrainment of bed surface grains by the flow and/or grain‐bed impacts, and both entrainment mechanisms are strongly hindered by cohesion. Furthermore, even those few existing models that do not consider bed sediment entrainment explicitly account for cohesive forces (Berzi et al., 2017; Pähtz & Durán, 2018a), which increase the calculated transport threshold.…”

Scaling laws for planetary sediment transport from grain scale-resolved numerical simulations

“…The numerical model (21) resolves both grain-grain and fluid-grain interactions and the resulting change in the average flow field, and successfully reproduces subaqueous and aeolian transport (18,19) (Materials and Methods). Numerical simulations overcome a common problem with wind tunnel experiments of extraterrestrial aeolian transport: they lack sufficient feeding of sediment at the leading edge of the bed (22)(23)(24)(25)(26)(27)(28)(29), which is required for sustaining equilibrium transport conditions near the cessation threshold because of the limited fetch of such wind tunnels. In that regard, the simulations capture natural field conditions, where long fetches, along with a variety of mechanisms to generate airborne grains, can plausibly lead to equilibrium transport near the cessation threshold (7,17,30) (see Supplementary Materials for details).…”