Non-negligible factors in low-pressure sprinkler irrigation: droplet impact angle and shear stress

Abstract

Droplet shear stress is considered as an important indicator that reflects soil erosion in sprinkler irrigation more accurately than kinetic energy, and the effect of droplet impact angle on the shear stress cannot be ignored. In this study, radial distribution of droplet impact angles, velocities, and shear stresses were investigated using a two-dimensional video disdrometer with three types of low-pressure sprinkler (Nelson D3000, R3000, and Komet KPT) under two operating pressures (103 and 138 kPa) and three nozzle diameters (3.97, 5.95, and 7.94 mm). Furthermore, the relationships among these characteristical parameters of droplet were analyzed, and their influencing factors were comprehensively evaluated. For various types of sprinkler, operating pressures, and nozzle diameters, the smaller impact angles and larger velocities of droplets were found to occur closer to the sprinkler, resulting in relatively low droplet shear stresses. The increase in distance from the sprinkler caused the droplet impact angle to decrease and velocity to increase, which contributed to a significant increase in the shear stress that reached the peak value at the end of the jet. Therefore, the end of the jet was the most prone to soil erosion in the radial direction, and the soil erosion in sprinkler irrigation could not only be attributed to the droplet kinetic energy, but also needed to be combined with the analysis of its shear stress. Through comparing the radial distributions of average droplet shear stresses among the three types of sprinklers, D3000 exhibited the largest value (26.94-3313.51 N/m 2 ), followed by R3000 (33.34-2650.80 N/m 2 ), and KPT (16.15-2485.69 N/m 2 ). From the perspective of minimizing the risk of soil erosion, KPT sprinkler was more suitable for low-pressure sprinkler irrigation than D3000 and R3000 sprinklers. In addition to selecting the appropriate sprinkler type to reduce the droplet shear stress, a suitable sprinkler spacing could also provide acceptable results, because the distance from the sprinkler exhibited a highly significant (P<0.01) effect on the shear stress. This study results provide a new reference for the design of low-pressure sprinkler irrigation system.

Sprinkler Irrigation on Sloping Land: Distribution Characteristics of Droplet Impact Angle and Shear Stress

“…Water 2024, 16, x FOR PEER REVIEW 10 of 15 In addition, it is important to note from the regression analysis that droplet shear stresses and impact angles under three slopes and two directions had obvious exponential relationships, which could be expressed by = e d s S c  . These results do not coincide with the polynomial relationship between the two indicators obtained by Hui et al [33] using a low-pressure sprinkler on flat land. This also implies a stronger regularity between these two indicators under the influence of the slope, and it could be observed from Table 2 that the average R 2 value under various treatments was 0.879.…”

“…In addition, it was not difficult to find that the farther distance from the sprinkler on sloping land was associated with a more dispersed droplet impact angle, which suggested that the spray jet end might also be the area where droplets collided most violently with each other. This result is consistent with the droplet diameter distributions obtained by Hui et al [21] using a ball-driven sprinkler and Hui et al [33] using low-pressure sprinklers. Furthermore, it can be observed from Figure 4 that the droplet impact angle distribution varies with slope and direction.…”

“…This result indicates that the impact angles of most droplets near the sprinkler were close to perpendicular to the slope surface, thus resulting in relatively weak soil erosion. Similar outcomes in sprinkler irrigation experiments on flat land have been observed by other researchers [18,21,33]. This was expected because small diameter droplets predominate near the sprinkler [16,34,35].…”

“…Overall, larger droplet impact angles corresponded to weaker droplet shear stresses, and smaller impact angles corresponded to stronger shear stresses. This result has been confirmed in many studies on flat land [20,21,33]. Nonetheless, the shear stresses of some droplets were basically unchanged, and their values were very small.…”

“…As the distance increased to 5 m, the relative frequency of droplets above 70 • under three slopes and two directions tapered off by an average of 6.9% on the basis of 3 m. Until the distance reached the spray jet end, a large number of droplet impact angles began to appear in the range of 30-50 • , and the droplet relative frequency in this range even accounted for 23.4% under the downhill with a slope of 20%. These findings signified that the distance from the sprinkler on sloping land had a significant negative effect on the distribution of droplet impact angles [33]. Meanwhile, it can be seen from Figure 5 that the relationship between the droplet impact angle and velocity was close, which indicates that the droplet impact angle decreased with the increase in the velocity.…”