Simulation on Abated Effect of Nylon & Plastic Nets in Wind Tunnel

Global desertification is becoming more and more serious, the austere reality of the relative shortage of raw materials used in the fight against desertification force the researchers of wind-blown sand theory to develop new materials with sand protection function and gradually replace the previous sand prevention materials (crop straw and so on). From the application scope of sand control materials, it is mainly used for sand fixing (lattice sand barrier) and sand blocking measures (sand barrier). Sand barrier, also known as high vertical sand barrier, is one of the most important measures to prevent and control desertification at present. It has been widely used in the vast sandy areas in China as an early measure for sand fixing afforestation or an independent sand prevention measure for heavy sand disaster areas. Especially in recent decades, a great deal of research has been carried out on the role of barriers in the control of wind blown sand disasters, and some substantial progress has been made. Nylon and plastic net as a new type of sand control material, although the cost is relatively high, but can continuously improve its anti-aging properties and sand resistance. It also has the advantages of industrialized production and easy construction, etc. Especially in some areas with serious wind and sand disaster, such as industrial and mining, transportation and national defense, nylon and plastic nets are used as sand prevention materials. The protective effect is very obvious and has broad application prospects. In addition, nylon and plastic nets have longer service life than crop straw and are non perishable. Although its disposable investment cost is higher, but the total economic benefits is reasonable and effective. at the same time, it is conducive to the improvement of the ecological environment, and is much better than the general ones in the process of sand barrier. In this paper, the wind tunnel simulation of different structures of nylon and plastic net is carried out to provide the theoretical basis for the optimization design and application of field sand control project.

NPN-01: The protective effect of nylon and plastic net is very obvious.

NPN-02: The setting of nylon and plastic nets has a precise design.

• Experimental design
  The section length of this experiment is 21 m and its section is 1.2 m × 1.2 m. The selected materials include warp and weft woven nylon net, plastic net, polyester cored wire mesh and plastic warp knitting net, whose porosity (β) is 35%, 40%, 55% and 65% (porosity is estimated by projection method) and height is 10 cm. The coordinate control adopts a simple two-dimensional tunnel coordinate frame, and field sediment collecting instrument is used to measure sediment runoff at different heights before and after the net. In order to ensure the full development of sand flow, nylon and plastic nets set at a distance of 6 m at the end of free surface. Place 20-centimeter-high step sand accumulation apparatus at both 1 meter ahead the net and 1 meter behind the net, located at the place where the center of the hole is 60 cm from the side. The inlet section of the step sand accumulation apparatus is 2 cm × 2 cm. In order to prevent the free sand, in 1 m behind that place is completely fixed with cementing material. In this experiment, natural mixing sand are adopted as sand samples . At the time of each replacement of the experimental material, the location of the sediment recorder, nylon and plastic net is fixed. When each experiment of wind speed is completed, the free sand surface should be rearranged and ensure adequate source of sand. According to the common wind speed in field, 4 groups of wind speed are selected in this experiment: 8 m·s^-1, 10 m·s^-1, 12 m·s^-1, 14 m·s^-1.
• Experimental results and discussions

  • Influence of porosity on sand control effect
    Because of the different material and weave structure of the new sand protection material, the effect of porosity on the sand flow is difficult to reformed into a unified system to be compared. According to the observation and research to the role of barriers with different porosities and heights in preventing the accumulation of sand in front of the fence, it is found that the fence porosity is the most important factor affecting the protection efficiency. One is due to the effect of obstructing fence itself outside the quicksand; the other one is the effective protection of the fence porosity range or sediment accumulation range. As shown in Table 1, the maximum value in the unit time of barrier sediment (q1 + q2) occurs when the porosity is about 40%. Laboratory experiments and field observations have proved that when the porosity reaches about 40%, the effect of barrier sand is best.

    Table 1 - Characteristics of sand transport rate on both sides of the fences with various porosity

    Observation content	Standard porosity β (%) batten test fence
    	10	20	30	40	50	60
    q 1 / (g · cm-1 · min-1)	0.823	0.310	0.582	2.459	1.528	1.224
    q 1 / (q 1 + q 2) / %	95.5	92.8	91.5	90.6	81.9	78.7
    V 1 / (m · s-1)	8.1	5.8	5.5	7.3	8.2	7.6
    q 2 / (g · cm-1 · min-1)	0.039	0.024	0.054	0.254	0.337	0.332
    q 2 / (q 1 + q 2) / %	4.5	7.2	8.5	9.4	18.1	21.3
    V 1 / (m · s-1)	8.7	7.4	6.5	8.3	8.3	8.1
    q 1 + q 2	0.862	0.334	0.636	2.713	1.865	1.556

    The results show that the resistance of the barrier is mainly affected by the porosity and height, moreover, the saltation lift force of sand grains is also related to the size of incoming air flow and the friction velocity of sand bed. Through researching the effect of the fence porosity, hole shape to the wind weakened, the re-circulation zone length and Reynolds shear stress, it is found that when the porosity is more than 30%, the vortex formed by the fence will disappear. In the absence of fences, the airflow accelerated along the windward slope of the dunes and separated at the top of the dunes. When a barrier with different porosities is installed, the airflow is separated at the top of the fence. Shear flow which has lower separation acts directly on the top of the dunes and forms a vortex along the windward slope of the dune behind the fence. The flow over the fence forms a more complex turbulent flow with the interaction of the laminar flow through the fence. An irregular quadrilateral turbulent flow zone with barriers, sand bed surfaces, dune windward slopes and upper separate shear layers is formed behind the barrier. It can be seen that the barrier porosity not only influences the cross flow ability of sand particles directly, but also changes the turbulence characteristics of the airflow around the fence. Finally, it has a crucial impact on its protection effectiveness. The study shows that an airtight baffle with zero porosity can also resist sand. However, the quicksand can only be blocked within twice the height of the barrier. This effect decreases with the increase of the plate height, and with the increase of porosity. At the same time, the sand blocking capacity and protection range also increase correspondingly. The inlet indicating wind speed is set at 10 m · S^-1 and the net height is 10 cm to carried out he wind tunnel simulation experiments of sand barrier with different porosities. In addition, the average net sand retention rate of nylon net fence under various wind speeds is calculated. The formula is as follows:

    
    In this formula, K is the average net sand resistance rate of the nylon netting (%); n is different wind speeds which have been measured; Q1i is the sediment accumulation at various height levels in front of the net (g); Q2i is the sediment accumulation at each height behind the net (g); i is different sand layer. The average net sediment resistivity obtained by the upper formula, combined with the average sediment transport rate before and after wind speed at 10 m S^-1 and dynamic variation of sediment transport with height in a net, the effect of nylon net with different porosity on aeolian sand flow is obtained (Table 2).

    Table 2 - Effect of nylon net with different porosity on wind-blown sand

    Porosity β (%)	Sediment transport rate q / (g · cm-2 · min-1)		Net sand resistivity K (%)	Sand control effect	Sand content and percentage of weight at different heights (cm)
    	In front of the net	Behind the net			2	4	6	8	10	12	14	16	18	20
    35	1.84	0.24	52.81	Q1i / g	97.23	47.83	29.619	20.15	9.922	7.548	3.883	2.379	1.638	0.907
    				Q1i / %	43.97	21.63	13.40	9.11	4.49	3.41	1.76	1.08	0.74	0.41
    				Q2i / g	0.401	0.934	1.452	2.059	2.853	3.186	3.839	4.479	4.75	5.23
    				Q2i / %	1.37	3.20	4.98	7.06	9.78	10.92	13.16	15.35	16.28	17.92
    				Q1i / Q2i	242.5	51.21	20.40	9.79	3.48	2.37	1.01	0.53	0.34	0.17
    40	1.47	0.15	55.42	Q1i / g	90.64	31.12	17.42	11.89	6.02	5.58	3.21	2.59	2.40	1.9
    				Q1i / %	52.46	18.01	10.08	6.88	3.48	3.23	1.86	1.50	1.39	1.10
    				Q2i / g	2.86	4.35	5.65	6.86	8.82	9.58	11.92	14.13	15.65	16.88
    				Q2i / %	2.96	4.50	5.84	7.09	9.12	9.91	12.33	14.61	16.18	17.46
    				Q1i / Q2i	31.71	7.15	3.08	1.73	0.68	0.58	0.27	0.18	0.15	0.11
    55	0.86	0.48	57.40	Q1i / g	89.71	33.49	19.79	13.30	6.58	5.33	2.96	2.12	1.68	1.20
    				Q1i / %	50.92	19.01	11.23	7.55	3.74	3.03	1.68	1.21	0.95	0.68
    				Q2i / g	2.25	2.52	2.18	1.83	1.72	1.47	1.54	1.62	1.66	1.74
    				Q2i / %	12.17	13.59	11.79	9.85	9.28	7.91	8.33	8.74	8.95	9.41
    				Q1i / Q2i	39.80	13.31	9.06	7.29	3.83	3.64	1.92	1.31	1.01	0.69
    60	1.20	0.46	47.64	Q1i / g	64.38	32.12	19.24	12.46	5.76	4.37	2.21	1.41	1.06	0.68
    				Q1i / %	44.81	22.35	13.39	80.67	4.01	3.04	1.53	0.98	0.74	0.47
    				Q2i / g	9.56	8.32	7.02	5.88	5.24	4.22	4.14	3.90	3.67	3.52
    				Q2i / %	17.23	15.01	12.66	10.60	9.44	7.61	7.47	7.03	6.61	6.34
    				Q1i / Q2i	6.74	3.86	2.74	2.12	1.10	1.03	0.53	0.36	0.29	0.19

    As can be seen from table 2, the sand blocking effectiveness decreases with the increase of porosity when the net height is determined. When porosity (β) is 35%, the average net sediment resistivity is 52.81%, and when porosity (β) increases to 65%, the average net sediment resistivity drops to 47.64%. The average sediment transport rate behind the net is proportional to porosity. When the porosity (β) is 35%, the value is 0.24 g · cm^-2 · min^-1. When porosity (β) increases to 65%, the value increases rapidly to 0.46 g · cm^-2 · min^-1. By calculating the ratio of Q_{0–10 cm} and Q_{10–20 cm}, it is found that when the porosity is between 35% and 55%, the value is less than 1, indicating that the nylon net within this porosity range intensifies the turbulence of the upper air flow. Although it can reduce the transporting amount of sand grains, there is a large amount of sand conveying downward wind with the increasing of turbulent airflow over the net. However, when porosity (β) exceeds 55%, the ratio of Q_{0–10 cm} and Q_{10–20 cm} begins to increase. That is, if the porosity is too large, the underlying sand will pass through the sand barrier directly, so that the protection effectiveness of the sand barrier will be reduced sharply. In addition, comparing the average sediment transport rate ahead and behind nylon net, when porosity (β) is 55%, the sediment transport rate ahead net is 1.84 g · cm^-2 · min^-1 and the sediment transport rate behind net is 0.24 g · cm^-2 · min^-1. When porosity (β) is 40%, the sediment transport rate ahead net is 1.47 g · cm^-2 · min^-1 and the sediment transport rate behind net is 0.15 g · cm^-2 · min^-1. It shows that the nylon net with porosity between 35% and 40% plays a very significant role in reducing the aeolian sand flow. After comprehensively analyzing the average sediment transport rate, net sediment discharge rate and the ratio of the total amount of sand to the upper and the lower of the net height Q_{0–10 cm} and Q_{10–20 cm}, it can be seen that nylon net with porosity in 35% ~ 40% not only has the good effect of sand resistance, but also weakens the role of bottom air obviously. It can also achieve the purpose of fixing sand. When the porosity is more than 65%, although the sediment deposition behind net decreased with the increase of the height of the net and weaken the turbulence performance of the airflow above the net, according to the average sediment transport rate and net sediment resistance rate, its comprehensive sand control efficiency is not very good, for a lot of sand is transported through the nylon net downward the wind direction. This phenomenon fully explains that the selection of sand barrier porosity plays an decisive role in the sand control efficiency. Because it not only affects the interception of sand particles in transit, but also affects the turbulence of the airflow in the sand movement layer.

    Table 3 - Effect of nylon net(β=40%)on wind-blown sand in different wind velocity

    Wind speed (m·s-1)	Sediment transport rate (g · min-1 ·cm-2)	Sand control effect	Sand content and percentage of weight at different heights (cm)
    			2	4	6	8	10	12	14	16	18	20	Average
    8	0.16	Q1i / g	43.27	10.65	5.08	2.73	0.98	0.79	0.41	0.32	0.49	0.24
    		Q1i / %	66.58	16.38	7.81	4.20	1.51	1.18	0.63	0.60	0.75	0.36
    		Q2i / g	0.75	0.91	1.22	1.43	2.00	2.23	2.74	3.22	3.41	3.36
    		Q2i / %	3.51	4.30	5.73	6.71	9.42	10.49	12.85	15.15	16.02	15.82
    		Q1i / Q2i	57.93	11.65	4.16	1.91	0.49	0.34	0.15	0.12	0.14	0.07	7.70
    10	0.86	Q1i / g	90.64	31.12	17.42	11.89	6.02	5.58	3.21	2.59	2.40	1.90
    		Q1i / %	52.46	18.01	10.08	6.88	3.48	3.32	1.86	1.49	1.39	1.10
    		Q2i / g	2.86	4.35	5.65	6.86	8.82	9.58	11.92	14.13	15.65	16.88
    		Q2i / %	2.96	4.50	5.84	7.09	9.12	9.91	12.33	14.61	16.18	17.46
    		Q1i / Q2i	31.71	7.15	3.08	1.73	0.68	0.58	0.27	0.18	0.15	0.11	4.57
    12	5.79	Q1i / g	100.3	44.41	28.43	19.55	11.9	9.81	5.85	4.51	3.91	2.95
    		Q1i / %	43.29	19.16	12.17	8.44	5.18	4.23	2.53	1.95	1.69	1.26
    		Q2i / g	5.10	7.01	8.09	8.61	9.72	10.49	12.28	13.94	14.19	14.81
    		Q2i / %	4.89	6.72	7.76	8.26	9.33	10.06	11.78	13.37	13.61	14.22
    		Q1i / Q2i	19.69	6.34	3.51	2.27	1.23	0.94	0.48	0.32	0.28	0.20	3.53
    14	13.86	Q1i / g	111.42	50.91 2.623	34.59	24.27	16.2	13.48	8.75	7.12	5.83	4.54
    		Q1i / %	40.21	18.38	12.48	8.76	5.85	4.86	3.16	2.57	2.10	1.63
    		Q2i / g	5.38	6.06	8.10	8.68	9.67	9.72	10.99	12.40	13.21	14.05
    		Q2i / %	5.48	6.17	8.24	8.83	9.84	9.89	11.18	12.62	13.44	14.31
    		Q1i / Q2i	20.71	8.40	4.27	2.80	1.68	1.39	0.80	0.57	0.44	0.32	4.14

  • Influence of wind speed on sand control effect
    Under different wind speeds, the attenuation effect of nylon net on wind sand flow is different. As can be seen from table 3, the ahead sediment accumulation of nylon net decrease as height increases, while the behind sediment accumulation of nylon net increase constantly as height increases. The maximum sediment accumulation before the net is 0–2 cm away from the sand bed. In addition, it is not difficult to find that the value of sand control effectiveness Q_1i / Q_2i, it decreases sharply with increasing height. With the increase of wind speed, the attenuation effect of nylon net on aeolian sand flow is decreasing. When the wind speed is 8m · s^-1, the average value of Q_1i / Q_2i is 7.70; when the wind speed increases to 14 m · s^-1, the average value of Q_1i / Q_2i drops to 4.14. 0–2 cm ahead the net, the percentage of sand weight in the high layer is inversely proportional to the wind speed. When the wind speed is 8m · s^-1, the percentage of sand weight in the high layer is 66.58%. When the wind speed increases to 14 m · s^-1, the percentage of sand weight in the high layer drops sharply to 40.21%. The percentage of sand weight in the 2–4 cm height layer is relatively stable, accounting for about 18% of the total sediment discharge. The percentage of sand weight in the height layer above 4 cm is in direct proportion to wind speed. That is, with the increase of wind speed, the amount of sand in the lower air stream is relatively less, a corresponding increase in the amount of sediment transport in the upper air, while the amount of sand transported in the upper air flow is accordingly increased.
• Conclusion
  Through this experiment and field observation, it is found that nylon and plastic nets with different structures and porosity has quite obvious attenuation of aeolian sand flow. It can ultimately affect the protection effectiveness of sand barriers by changing the turbulence characteristics of air flow. Take the average sediment transport rate, net sediment discharge rate and the ratio of the total amount of sand to the upper and the lower of the net height Q_{0–10 cm} and Q_{10–20 cm} as the reference indicators, this article proves that nylon and plastic net as a new material for sand control are completely feasible to replace crop straw and branches. Its protection benefit is also very obvious and it has broad application prospects.