4.2.1. Static Scouring Experiment
Firstly, the simulation and experimental results are compared from two aspects of the scour pit shape and depth to verify the reliability of the simulation results. According to the numerical simulation results, experimental restriction conditions are set as shown in
Table 3.
Table 4 compares the experimental and simulation results of the scour pit depth and width, and the simulation data refer to the scaling criterion to scale the original simulation results.
The development trend of the scour pit depth and width of the simulation results is close to the results from the experiment. However, the scour depth from the simulation is relatively small compared with the experimental value, which is mainly caused by the wall effect of the glass tank [
14,
15]. On the other hand, the simulated scouring width is larger than the experimental result, which is due to the certain deviation in the smoothness of the sand surface and the viscosity of the glass wall to the fine sand, causing the width of the scouring pit on the upper part to narrow during the scouring.
(a) Influence of jet standoff distance on scour effect
According to the curve trend in
Figure 7 of the single-nozzle simulation, once the standoff distance exceeds 600 mm, the scouring depth will further decrease, even not exceeding 200 mm, which is not in line with our ideal situation. Therefore, in the experiment, 8 standoff distances are set to verify the changes in scour pit depth and width at different standoff distances of 0–70 mm, where the experiment is repeated twice for each scour condition.
As shown in
Table 5 and
Figure 19, the scour pit depth decreases with the increase in the jet standoff distance, while the scour pit width increases. Here, the jet is submerged, leading to an entrainment flow during the spraying process. Currently, the flow rate increases while the average velocity decreases. With the increase in the jet standoff distance, the hydraulic jet flow expands along the direction of the jet. Although the shear generated by the flow velocity decreases, the scour flow rate and effective area increase, resulting in a decrease in the scour pit depth and increase in the scour width [
30,
33,
34].
(b) Influence of jet angle on scouring effect
According to the results of the single-nozzle simulation, when the jet inclination angle is 60–90°, the scour depth is relatively deep, with little influence on the side wall. When the jet inclination angle is less than 60°, the jet depth decreases sharply. Therefore, 4 scour conditions are set in this experiment, where the jet angle ranges from 60° to 90°. The experiment is repeated twice for each scour condition.
As shown in
Table 6 and
Figure 20, the maximum scour depth can be achieved at the jet angle of 90°. The scour pit width is the largest when the scour angle is 60°, while the scour pit widths are similar at other different scour angles.
(c) Influence of jet flow rate on scour effect
In order to verify the changes in the scour pit depth and width at different jet flow rates, 6 scour conditions are set, and the experiment is repeated twice for each scour condition.
Table 7 and
Figure 21 show the scour pit depths and widths under different jet flow rate conditions.
The scouring depth does not increase with the increasing flow rate within the experimental range. In working conditions 6 and 7, the scour pit depth is basically the same, but the scour pit width in working condition 7 is larger. Moreover, compared with conditions 5 and 6, the scour pit depth changes slowly, but the scour pit width changes at a higher rate. This is because at a low flow rate, the scour is mainly achieved by the friction between the flow and the sediment surface, so, within this range, the greater the flow rate, the greater the scour pit depth. Once the velocity exceeds a certain value, the intensity of the hydraulic jet flow penetrating the water is enough to generate a turbulence vortex [
25,
35], thus decreasing the scour pit depth.
4.2.2. Dynamic Scouring Experiment
The dynamic scouring experiment mainly analyzes the influence of different scour angles and moving speeds of the scour platform on the scour effect. The maximum working speed of the laying machine is set as 150 m/h—that is, the maximum moving speed is 41.7 mm/s. Thus, 4 scour conditions are set in the experiment, as shown in
Table 8, where the experiment is repeated twice for each group.
Figure 22 shows the change in the scour shape in the experiment at working condition 2. A deep scour pit is observed at the early stage of the experiment. Later, the overall scour depth decreases, and it is basically the same in the nozzle moving path. At the beginning of the scour, it can be approximated as static scour, forming a deeper scour pit. As the nozzle moves, the tilting nozzle flushes the sediment to the scour pits previously formed, resulting in a decrease in the scour depth [
16].
Compared with the working condition 4 in the nozzle inclination variable experiment, the two working conditions only differ in the moving speed. It is obvious that the depth of the scour pit in the dynamic scouring condition is less than that in the static scouring condition. In the static scouring experiment, there is a lot of suspended sand in the scour pit during the scour process, and most of the suspended sand is settled during the mobile scour measurement, so the depth of the scour pit is smaller during dynamic scouring.
By comparing the results of working condition 3 and 4, it is found that the scour depth increases slightly when the scour angle increases. According to the results, the scour in all directions is equally difficult during the static scour, so the inclined scour has no advantage in the static scour experiment. On the contrary, it is easiest to scour the sediment backward for the dynamic scour experiment, as the inclined nozzle can flush the sediment into the scour pit formed by the previous period. Therefore, in the mobile scour experiment, increasing the jet inclination angle within a certain range can increase the scour sludge discharge effect.
Compared with the working conditions 1, 2, 3, and 5, the scour depth decreases slightly when the moving speed of the scour platform increases. In other words, for the substrate used in the experiment, the traveling speed change within a certain range has little influence on the trench depth and width.