# Numerical Simulation of Oil Pipeline Leakage Diffusion in Dashagou Yellow River Crossing Section
## 4 Results and Discussion
### Apparent Flow Rate Analysis  
In the simulation using the resistance coefficient values measured in the laboratory, the simulation results are shown in Figure 18, the differences in the apparent flow rates of oil spills for the scenarios with a leak radius of 10 mm and 20 mm were not significant; the apparent flow rates of oil spills for the 10 mm leak radius scenario ranged from 1.04 to 1.32 m/h one hour after the spill, and the apparent flow rates of oil spills for the 20 mm leak radius scenario ranged from 1.48 to 1.87 m/h one hour after the spill 1.87 m/h. For the scenario with a 50 mm leak radius, the apparent oil leakage flow rates ranged from 2.31 to 2.93 m/h one hour after the spill, which was nearly one time higher than the other two scenarios with different leak radii. At the early stage of the leakage, the leakage flow velocities at different leakage outlets had large differences, but as the leakage continued, the apparent flow velocities of the leakage showed little difference in the X, Y and Z axes, indicating that the oil leakage diffusion process was more uniformly distributed in the space without any significant flow tendency or direction, which was possibly due to the fact that the viscous resistance coefficients and inertial resistance coefficients of the sandy chalky/powdery sandy soils were high as determined by the laboratory. The reason may be that the spilled oil cannot spread quickly under the higher resistance condition in the soil; during the oil leakage in the crossing section of the Dashagou, when the equivalent amount of spilled oil becomes extremely large, its flow trend may still be affected by gravity, and its final flow trend is still dominated by the downward movement in the vertical direction. Taking the leakage scenario with a 12 m burial depth of pipeline, lateral leakage opening and a 10 mm leakage radius as an example, the apparent velocity of Y-axis of the experimentally measured value in 30 min is 1.78 m/h, which indicates that the difference of the soil lithology on the flow of the leakage is very significant in terms of the blocking effect. Because of the pebble layer below the sandy silt layer in the Dashagou crossing section, the resistance coefficient experiment of this soil layer cannot be carried out at present due to the unavailability of specific information and pebble samples of this  
Figure 17: Comparison diagram of leakage flow patterns at the top leakage port under different leakage port sizes and different leakage duration. pebble layer. However, considering that the pebble layer has a better permeability, if a leakage occurs in the pipeline crossing this soil layer, the apparent velocity of the leakage corresponds to the leakage diffusion in the sandy soil layer, and the corresponding leakage environmental risk will be higher. Figure 18: (**a1**): The leakage port is located at the bottom, with a leakage port radius of 10 mm. (**a2**): The leakage port is located at the bottom, with a leakage port radius of 20 mm. (**a3**): A leak is present at the bottom, with a leak radius of 50 mm. (**b1**): A leak is present on the side, with a leak radius of 10 mm. (**b2**): A leak is present on the side, with a leak radius of 20 mm. (**b3**): A leakage port is present on the side, with a leakage port radius of 50 mm. (**c1**): A leak is present on top, with a leak radius of 10 mm. (**c2**): A leak is present on top, with a leak radius of 20 mm. (**c3**): A leakage port is present on top, with a leakage port radius of 50 mm. Taken together, the analysis of the simulation results about the leakage flow pattern in the crossing section of the Dashagou shows that the diffusion velocity is extremely high in the short time at the early stage of the leakage, but with the continuation of the leakage, the apparent flow velocity of the leaked oil is rapidly decaying, so the soil, as a kind of porous medium, and its blocking effect on the distribution of the flow pattern cannot be neglected.