The saltation model for roughness surface incorporating effect of free wind velocity over boundary layer

doi:10.7522/j.issn.1000-694X.2024.00053

Journal of Desert Research ›› 2024, Vol. 44 ›› Issue (6): 79-86.DOI: 10.7522/j.issn.1000-694X.2024.00053

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The saltation model for roughness surface incorporating effect of free wind velocity over boundary layer

Yang Xu¹(), Fanmin Mei¹(), Haibo Zhu²

^1.School of Environmental and Chemical Engineering，Xi'an Polytechnic University，X'an 710048，China
^2.Yangling Vocational and Technical College，Yangling 712100，Shaaxi，China

Received:2024-02-23 Revised:2024-05-14 Online:2024-11-20 Published:2024-12-06
Contact: Fanmin Mei

Abstract

Abstract:

Two previous saltation models for roughness surface focus on effect of aerodynamic roughness length on sand transport rate， disagreement with recent investigations which find that such the factor as free wind velocity over boundary layer also affects significant saltation flux. Thus， saltation fluxes under various free velocities over 9 roughness surfaces were observed in a blown-sand wind tunnel to develop new model， including the 6 sand surfaces covered with compact roughness elements， and the 3 sand surfaces with porous elements. We find that there exists a significant gap between the observed and computed saltation fluxes by the traditional models， and that the new saltation model is better since it incorporates the ensemble effects of roughness length， free wind velocity and the modified wind friction （function of wind friction threshold velocity and wind friction velocity） on saltation. The effect of free wind velocity on saltation flux could be related with the variation of total shear stress over roughness surface with height.

Key words: saltation flux, free wind velocity, roughness element, wind friction threshold velocity, wind friction velocity

CLC Number:

U418.5+6

Yang Xu, Fanmin Mei, Haibo Zhu. The saltation model for roughness surface incorporating effect of free wind velocity over boundary layer[J]. Journal of Desert Research, 2024, 44(6): 79-86.

Figures/Tables 11

Table 1 Geometric parameters of the roughness surfaces

编号	d/mm	h/mm	行×列	λ	AR	S_h
11	5	60	11×12	0.0396	12	0.3
12	5	60	21×12	0.0756	12	0.6
13	5	60	41×12	0.1476	12	1.2
14	5	60	41×25	0.3075	12	1.2
26	40	20	21×5	0.0840	0.5	0.2
28	40	40	21×5	0.1680	1	0.4

Table 2 Geometric parameters of the roughness surfaces with porous elements

编号	h_s/mm	D_s/mm	n_s	H/mm	D_k/mm	N_s/(n·m^-2)	λ	P
36	50	2	17	50	40	1 780	0.1780	0.15
37	50	2	9	50	40	945	0.0945	0.55
38	50	2	5	50	30	525	0.0525	0.75

Fig.1 Probability of the sand particle size distributions for this wind erosion experiment based on two- population log normal model

Fig.2 Schematic illustration for porous elements （hs-porous elements' height， ds-diameter of individual element， H-height of modelled compact element， Dk-diameter of modelled compact element）

Fig.3 The specific details for this wind tunnel experiment

Fig.4 Comparisons between saltation fluxes computed by Marticorena model and the observed fluxes in this wind experiment （NO. of the roughness beds in （A） and （B） can be seen in Tables 2 and 3）

Fig.5 Comparisons between saltation fluxes computed by Alfaro model and the observed fluxes in this wind experiment （NO. of the roughness beds in （A） and （B） can be seen in Tables 2 and 3）

Fig.6 Drops of normalized saltation fluxes NFh over various roughness beds under various wind free velocities with the increased roughness element density （A）， and with the aerodynamic roughness lengths （B） respectively.

Table 3 Variations of the empirical parameters in the equations （5） and （6） with the increased wind free velocity

U_f /（m·s^-1）	m	n	m'	n'
12	0.5970	20.4860	0.3040	0.5462
14	0.7965	17.1600	0.5026	0.4917
16	0.7792	14.9250	0.5090	0.4194
18	0.7372	13.0810	0.5042	0.3654
20	0.6573	12.3100	0.4587	0.3431

Fig.7 Decreases of γ values under various wind free velocities with the increased roughness element density （A）， and with the aerodynamic roughness lengths （B） respectively

Table 4 Variations of the empirical parameters in the equations （9） and （10） with the increased wind free velocity

U_f/（m·s^-1）	χ	ω	$χ'$	$ω'$
12	32.7260	33.2530	12.1940	0.9082
14	132.6300	28.5800	56.0490	0.7766
16	120.2500	23.8010	66.2490	0.6844
18	114.7000	20.4810	70.1790	0.5957
20	137.7100	19.5640	81.4900	0.5518

Table 4 Variations of the empirical parameters in the equations （9） and （10） with the increased wind free velocity

U_f/（m·s^-1）	χ	ω	$χ'$	$ω'$
12	32.7260	33.2530	12.1940	0.9082
14	132.6300	28.5800	56.0490	0.7766
16	120.2500	23.8010	66.2490	0.6844
18	114.7000	20.4810	70.1790	0.5957
20	137.7100	19.5640	81.4900	0.5518

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The saltation model for roughness surface incorporating effect of free wind velocity over boundary layer

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