3.1.3. Velocity along the Four Featured Edges

Figure 6 shows a velocity diagram that indicates the variation in velocity along the four edges of pipeline. Near the inlet, the velocity along four edges drops from 30 m/s to about 13 m/s, and then a small reduction in speed occurs in the first straight section. Near the elbow section, the flow velocity on the interior edge was subject to small fluctuations. Although the velocity magnitude of interior edge in the elbow section has increased dramatically (up to 28 m/s) and then undergoes a drastic reduction (low as 1 m/s), the flow velocity on other three edges is increased. In the second straight section, the speeds on the top and bottom edges decrease slightly and stay within a certain value, while the speed on the exterior edge still maintains an increase, and then reduced and kept consistent. On the interior edge, speed has been gradually increased to reach steady state.

**Figure 6.** Line graph of velocity magnitude along four edges of pipeline.

#### *3.2. Electrochemical Corrosion Behavior*

#### 3.2.1. Species Concentration Distribution Characteristic

Figure 7 shows the concentration distribution of chemical substances in the pipe. The concentration of substances is accumulated at the entrance of the elbow, especially near the intrados surface. Carbonic acid concentration reaches 443.84 mol/m<sup>3</sup> . Hydrogen ion concentration is maintained at a constant value of 9.70 mol/m<sup>3</sup> . The highest concentration of carbonate ions reaches 9.71 mol/m<sup>3</sup> . The maximum concentration of bicarbonate ion is 5.55 mol/m<sup>3</sup> . However, near the extrados surface, the accumulation of substance concentration is relatively small. All simulations were performed by the experimental parameters in Table 1.

**Figure 7.** A three-dimensional diagram of material concentration distribution. (**a**) Carbonic acid concentration distribution; (**b**) bicarbonate concentration distribution; (**c**) carbonate ion concentration distribution; (**d**) hydrogen ion concentration distribution.
