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

An expression for saltation threshold – the minimum wind speed required to initially saltate particles – is necessary for modeling aeolian processes on Earth and other bodies. Analysis of a compilation of experimental data led to the conclusion that this threshold is a function of the ratio of the density of the particle to that of the entraining fluid (ρ_p/ρ), and to a curve for the dimensionless threshold parameter, A(ρ_p/ρ). Whereas data of low-density ratio and of high-density ratio conditions show constant A values, the single dataset used to define the transitional region of the curve shows a range of values. To revisit this transitional region, we collect new freestream threshold data at 1–20 bars (1–20 × 10⁵ Pa) with particles 150–1000 µm in diameter and having densities 400–3300 kg/m³ using the Titan Wind Tunnel. From these new data spanning a range of intermediate density ratios, we calculate friction wind speeds and values for A(ρ_p/ρ). We filter our threshold data for the same conditions (particle diameter > 200 µm, particle Reynolds number > 10) as in previous work and combine them with previously published data to derive a new density ratio curve with the same form as the previous expression. This new curve of A(ρ_p/ρ), with different parameter values and including uncertainties, confirms the slope in the transitional region between low- and high-density ratios, though giving slightly higher values for A. This work offers improved prediction of threshold wind speeds under thicker-than-terrestrial atmospheres on other solar or extrasolar planets, while also suggesting current challenges to accurate experimental simulation of aeolian transport under such conditions.