THE EFFECT OF DENSITY RATIO ON SALTATION THRESHOLD: REDERIVING THE CURVE

The aeolian saltation threshold is the minimum wind speed required to entrain sediment into saltation. A general expression for saltation threshold is needed to predict threshold speeds on Earth and on other planetary bodies where aeolian activity is present. Threshold is a function of the density ratio, the ratio of the density of the sediment being entrained to the density of the fluid (or wind) entraining the sediment. Previous work resulted in an expression for the dimensionless threshold parameter, A, as a function of the density ratio [Iversen et al. 1987], although this expression contained a mistaken value for γ [Nield, 2017]. This expression was based on data from experiments in water, high-density air, and low-density air, which gave density ratios that cover five orders of magnitude. Data from the Venus Wind Tunnel, filtered to include only data for grain sizes greater than 200 µm and particle Reynolds numbers greater than 10, constrain the central portion of the curve. However, this transitional portion of the curve, between low (<10) and high (>1000) density ratios, does not fit recent threshold data from the Titan Wind Tunnel (TWT) subject to the same filtering conditions. We investigated the validity of the density ratio term and its inclusion in the Iversen et al. threshold model. This investigation entailed running new threshold experiments in the TWT under a range of density ratios conditions. We derived a new expression for the density ratio curve to fit the previous and newly collected TWT threshold data. This new expression increases the value of A in the transitional portion of the curve. This result is consistent with the use of previous TWT threshold data that were found to be higher than predicted [Burr et al., 2015] by the Iversen et al. model. Potential issues with these results include the definition of threshold, which was not consistent throughout previous data collection, and unexpectedly low z_o values derived from boundary layer profiles. The on-going exploration and resolution of these issues will yield a new threshold curve that will increase the accuracy of predicting saltation threshold friction speeds, improving planetary models and terrestrial ones as well.