Transport mass of creeping sand grains and their movement velocities

Abstract

[1] Aeolian sand transport is an important component of material circulation above terrestrial surfaces and can include processes of creep, saltation, and suspension. The complex movement of material and energy during aeolian transport has meant that these processes have previously been examined in isolation. Although a significant amount of research has been conducted on aeolian sand transport, this focused primarily on saltation. As a result, there are few data available on sand grain creep, primarily due to a lack of theoretical models and the difficulty of direct measurements. In this study, we present novel methods and instrumentations to accurately measure the transport mass and the velocity of creeping sand grains. Using data collected with above instrumentations and a ladder sampler at four friction velocities (u * = 0.26, 0.35, 0.47, and 0.56 m s À1 ) in a wind tunnel, we studied the transport mass of creeping sand grains and their movement velocities, as well as other key parameters of aeolian sand transport. Four major conclusions can be drawn from this study: (1) The transport mass (q 0 ) of creeping sand grains increases with increasing frictional wind velocity (u * ). The relationship between these variables is represented by the power function q 0 = À0.053 + 9.195u * 2.800 . (2) The contribution of creep to total aeolian sand transport decreases with increasing frictional wind velocity. Creep contributed 57% of total aeolian transport at the lowest frictional wind velocity but only 19% at the highest velocity. (3) The threshold frictional wind velocity for entrainment was 0.158 m s À1. (4) The movement velocity of creeping sand grains ranged from 0 to 0.14 m s À1, but more than 70% of recorded velocities were less than 0.02 m s À1. Although the results of this study require further validation, they provide a strong basis for future research and help deepen our understanding of aeolian sand transport.

Aeolian creeping mass of different grain sizes over sand beds of varying length

“…To reduce the effect of measurement error on q c and q s , six traps with the same length (L = 20 mm) and different width (D i = 1, 2, 3, 4, 5, and 6 mm) were used to measure M i . The six bed traps in a line perpendicular to the wind direction were located at a position 0.10 m away from the edge of the sand bed; their structure is listed in Cheng et al [2013].…”

“…It was also very difficult to assess the true creeping transport rates because there were variable entrance widths of bed traps: 1-3 mm [Bagnold, 1941], 20 mm [Wu et al, 2011], or not provided [Qi et al, 2001;Wu, 2003]. For the entrance width of 20 mm, Journal of Geophysical Research: Earth Surface 10.1002/2014JF003367 Cheng et al [2013] thought that the saltating mass captured by the bed trap resulted in estimates 2.91-3.89 times the actual creeping mass. Second, the creeping transport rate estimation by inverting the vertical distribution of the flux of blown sand flow to zero height [Dong et al, 2004] also overestimated the creeping mass because there were plenty of saltating sand grains at zero height.…”

“…Bagnold [1941] first studied aeolian creep by using a bed trap with a 1-3 mm entrance in a wind tunnel and noted that the creeping proportion of the total sand flux was 25%. However, the creeping proportion greatly differed in existing literatures, such as 25% [Bagnold, 1941;Willetts and Rice, 1985], 16% [Chepil, 1945], 7-17% [Horikawa, 1960], 20% [Namikas, 2003], 3-6% [Nickling and McKenna Neuman, 1997], 4-29% [Dong et al, 2004], and 19-57% [Cheng et al, 2013]. Furthermore, the creeping proportion decreased with increasing wind velocity [Chepil, 1959;Horikawa, 1960;Wu, 2003].…”

Sidewall effects and sand trap efficiency in a large wind tunnel

“…Wind speeds were measured with pitot tubes at a location 5.1 m from the entrance of the test section and at elevations of 0.01, 0.02, 0.05, 0.10, 0.15, and 0.20 m above the sand surface. Each wind profile satisfied, statistically, the logarithmical law: u = a ln(z) + b (where u is wind velocity, z is height, and a and b are the fitting coefficients) and estimates of shear velocity, u *, were found using u * = ak (where k is von Kármán's constant of~0.4; Dong et al, 2004;Cheng et al, 2013). The shear velocities used for these efficiency tests were 0.33, 0.38, 0.43, 0.47, and 0.52 m/s.…”

An improved numerical model suggests potential differences of wind‐blown sand between on Earth and Mars

“…The incident speed V imp and angle θ imp can be obtained through calculating the trajectory of sand particles. The speed of the impacted particles V surf could be set to zero roughly, because the speed is small enough to ignore (about 75% < 0.02 m/s) based on wind tunnel measurements [Hong et al, 2013]. The key of the problem is transferred to determine the restitution coefficients in normal (k 1 ) and tangential (k 2 ) direction, as shown in Figure 1b.…”