An experimental study of multiple grain‐size ejecta produced by collisions of saltating grains with a flat bed

Abstract

Collision data are presented from coloured high‐speed films of three size fractions of sand grains saltating over a bed of the total grain population. Each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded. The results confirm earlier findings that V3/V1≅0.5–0.6, Vn/V1≅.08 and the rebound angle increases with decreasing grain size. Ejected grains are examined in relation to their size, the impactor size, ejection speed and angle and the number of ejecta per collision. In addition, changes in grain parameters are observed with time. For fine impactors, ejection speeds generally increase with a decrease in ejecta size, but the fine fraction does not follow this trend for the coarse and medium impactors. Ejection angles are typically between 40° and 60°, with coarse grains having shallower mean angles than fine ejecta. The number of ejections per collision increases with a decrease in particle size for each impactor size. The general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes. There is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains. Much of the forward momentum of the saltating grains is transfered to creeping grains. The composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud.

An improved parameterization of wind‐blown sand flux on Mars that includes the effect of hysteresis

“…The parameter values a = 0.020, B = 0.96, and g = 1 s/m were determined by the numerical study of Kok and Renno [2009b] and are consistent with laboratory and numerical experiments [e.g., Anderson and Haff, 1991;Rice et al, 1995].…”

“…where ej = ej;0 ffiffiffiffiffiffiffiffiffiffiffiffi g=g eff p (see Text S1), and where ej;0 ≈ 0.15 is the average fraction of impacting momentum that is contained in the splash ejecta for collisions of cohesionless sand [Rice et al, 1995]. Substituting equation (15) into equation (14) and evaluating the integral yields…”

“…where R ≈ 0.55 [Rice et al, 1995] is the average fraction of momentum retained by a particle upon colliding with the soil surface, and f ej is the fraction of particles lifting off from the surface that are splash ejecta. I obtain this fraction by combining equations (9a), (11), and (12), which yields…”

“…[9] Since particles strike the soil at angles of only ∼5-15°f rom horizontal [Rice et al, 1995], the vertical component of the impact speed is much smaller than the horizontal component. The horizontal speed gained by the particle over its trajectory can thus be expressed as…”

Experimental study of aeolian sand ripples in a wind tunnel

“…It has been verified that particle size distributions play an important role in the formation and evolution of sand ripples (Ling et al, 2003). The collision angle between sand grains and the bed increases with grain size (Jensen and Sørensen, 1986;Willetts and Rice, 1989;Rice et al, 1995;Fu et al, 2013) and decreases with wind velocity (Mitha et al, 1986;Cheng et al, 2009) as well as shadow zone length (Cooke and Warren, 1973). Furthermore, the latter increases with ripple wavelength (Schmerler et al, 2016), indicating that the wavelength of sand ripples decreases with collision angle (Hoyle and Woods, 1997).…”