Research on velocity profiles of saltating particles based on their digital trajectories

doi:10.7522/j.issn.1000-694X.2024.00134

Journal of Desert Research ›› 2024, Vol. 44 ›› Issue (6): 231-239.DOI: 10.7522/j.issn.1000-694X.2024.00134

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Research on velocity profiles of saltating particles based on their digital trajectories

Chanwen Jiang¹^,²(), Zhibao Dong², Wanyin Luo³, Guangqiang Qian³, Zhengcai Zhang², Junfeng Lu³, Xiaoyan Wang¹

^1.College of Environment and Life Sciences / Shaanxi Key Laboratory of River Wetland Ecology and Environment，Weinan Normal University，Weinan 714000，Shaanxi，China
^2.College of Geography and Tourism，Shaanxi Normal University，Xi'an 710119，China
^3.Northwest Institute of Eco-Environment and Resources，Chinese Academy of Sciences，Lanzhou 730000，China

Received:2024-08-12 Revised:2024-09-23 Online:2024-11-20 Published:2024-12-06

Abstract

Abstract:

Velocity serves as a crucial parameter in the study of saltation motion. We utilized high-speed photography technology to acquire the vertical profiles of the average velocities of saltating sand grains on a loose sand surface during the ascending and descending phases within a wind tunnel. The outcomes reveal that the particle turbulence in the vertical direction is one to two orders of magnitude greater than that in the horizontal direction， inducing the particle velocities to conform well to a normal distribution in the horizontal direction. Meanwhile， they are more dispersed and have a narrower range （approximately 0-1 m·s^-1） in the vertical direction. At the identical height， the average horizontal velocity in the descending phase is approximately 1.6-1.8 times that in the ascending phase， whereas the vertical velocity in the descending phase is marginally smaller than that in the ascending phase. The relative average horizontal velocities （v_x / $g d$ ， where g represents the gravitational acceleration and d denotes the particle diameter） in both phases are proportional to the square root of the relative height （z/Z， where z is the height from the bed surface and Z is the thickness of the boundary layer） and the relative wind speed （u_*/u_*_ti， where u_* stands for the friction wind speed and u_*_ti is the impact threshold）. The average vertical velocities in both the ascending and descending phases typically exhibit a slightly ascending trend with height. The average vertical velocity in the ascending phase is slightly higher than that in the descending phase. The underlying mechanisms of the average vertical velocity distribution patterns and the characteristics of variation with height necessitate further investigation for clarification.

Key words: aeolian saltation, average velocity, vertical variation, digital trajectory, high-speed photography technology

CLC Number:

X169

Chanwen Jiang, Zhibao Dong, Wanyin Luo, Guangqiang Qian, Zhengcai Zhang, Junfeng Lu, Xiaoyan Wang. Research on velocity profiles of saltating particles based on their digital trajectories[J]. Journal of Desert Research, 2024, 44(6): 231-239.

Figures/Tables 10

Fig.1 Layout of the experiment

Fig.2 Velocity profiles for the approaching ?ow as a function of height above the surface

Table 1 Experimental conditions

实验组	沙样	d/mm	u_*/（m·s^-1）	u_*_ti /（m·s^-1）	z₀/mm
1	人造石英沙	0.1~0.2	0.26	0.16	0.0243
2	人造石英沙	0.2~0.3	0.28	0.20	0.0260
3	人造石英沙	0.3~0.4	0.31	0.24	0.0258
4	人造石英沙	0.4~0.5	0.33	0.27	0.0261

Fig.3 Relationship between particle turbulence intensity and height above the sand-bed surface

Fig.4 Typical probability distribution of horizontal particle velocity

Fig.5 Typical probability distribution of vertical particle velocity

Fig.6 Variation of the dimensionless horizontal particle velocity （vx /gd） as a function of relative height （z/Z）

Table 2 The fitting results of each experimental group in Figures 6A-B using equation （5）

实验组	d/mm	上升段		降落段
实验组	d/mm	$a u p$	R²	$a d o w n$	R²
1	0.1~0.2	142.0090	0.9429	233.5624	0.9658
2	0.2~0.3	86.1428	0.9700	148.7638	0.9809
3	0.3~0.4	78.3712	0.9477	131.5276	0.9275
4	0.4~0.5	67.2285	0.9455	109.9196	0.9606

Table 2 The fitting results of each experimental group in Figures 6A-B using equation （5）

实验组	d/mm	上升段		降落段
实验组	d/mm	$a u p$	R²	$a d o w n$	R²
1	0.1~0.2	142.0090	0.9429	233.5624	0.9658
2	0.2~0.3	86.1428	0.9700	148.7638	0.9809
3	0.3~0.4	78.3712	0.9477	131.5276	0.9275
4	0.4~0.5	67.2285	0.9455	109.9196	0.9606

Fig.7 Variation of the constant a with dimensionless wind speed velocity （u*/u*ti ）

Fig.8 Variation of the vertical particle velocity as a function of height

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Research on velocity profiles of saltating particles based on their digital trajectories

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