## 3 Results and Discussion
### Effect of Varying Crude Oil Grades on Slug Characteristics
#### 3.2.1 Slug Initiation Position  
Figure 9 illustrates the slug initiation position for the modelled cases. Under the constant superficial velocities of gas and crude oil, an increase in crude oil density causes the slug initiation position to decrease. Moreover, the slug initiation position for light crude oil increases by 25% when the liquid velocity increases at a constant gas velocity \(j_{G}=2.44\) (m/s). As the density of crude oil increases, the location of the slug initiation  
Figure 8: Quantitative validation of maximum induced stresses due to the effect of the slug characteristics of the air-water two-phase flow validation cases and comparison of FSI model of the present study against the FSI model of [PERSON] [19].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Gas-Liquid** & **Crude Oil Density** & **Crude Oil Viscosity** & **Natural Gas Velocity** & **Crude Oil Velocity** \\
**Two-Phase** & \((\rho_{L})\)**Kg/m\({}^{3}\)** & \((\mu_{L})\)**cP** & \((\dot{y}_{G})\) m/s & \((\dot{y}_{L})\)**m/s** \\ \hline Light crude 1 & 807.07 & 1.598 & 2.44 & 1.00 \\ Medium crude 1 & 870.84 & 6.348 & 2.44 & 1.00 \\ Heavy crude 1 & 926.91 & 64.049 & 2.44 & 1.00 \\ Very heavy crude 1 & 981.05 & 6843.76 & 2.44 & 1.00 \\ Light crude 1 & 807.07 & 1.598 & 2.44 & 0.86 \\ Medium crude 1 & 870.84 & 6.348 & 2.44 & 0.86 \\ Heavy crude 1 & 926.91 & 64.049 & 2.44 & 0.86 \\ Very heavy crude 1 & 981.05 & 6843.76 & 2.44 & 0.86 \\ Light crude 1 & 807.07 & 1.598 & 0.70 & 0.93 \\ Medium crude 1 & 870.84 & 6.348 & 0.70 & 0.93 \\ Heavy crude 1 & 926.91 & 64.049 & 0.70 & 0.93 \\ Very heavy crude 1 & 981.05 & 6843.76 & 0.70 & 0.93 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Physical properties of two-phase flow.  
position moves closer to the inlet and decreases by 84% for very heavy crude oil, 48% for the heavy crude oil and 16% for medium crude oil relative to light crude oil.  
Figure 9 also shows that the variation in slug initiation position is minimal when the crude oil grade increases and is highly noticeable for light crude oil. Meanwhile, decreasing the gas velocity to 0.7 m/s at 0.93 m/s liquid velocity results in a noticeable increase in slug initiation position for light-grade crude oil; this increase is minimal for very heavy crude oil. The contours shown in Figure 10 indicate that for light crude oil (low-viscosity crude oil), the slug initiation occurs further toward the downstream distance. As the crude oil grade increases at a constant gas-liquid velocity, the slug formation occurs close to the pipe inlet.  
Figure 10: Effect of varying crude oil grades on slug initiation position (’D’ represents the pipe diameter).  
Figure 9: Effect of varying crude oil grades on slug initiation position (’D’ represents the pipe diameter).  
Figure 11 shows that for light crude oil, 1 m/s liquid superficial velocity results in a stratified wavy flow pattern prior to slug formation. Slug formation occurs closer to the inlet under a lower liquid velocity (0.86 m/s) than under the 1 m/s liquid velocity. At \(j_{L}\) = 0.86 m/s and \(j_{L}\) = 1 m/s, the slug is captured at 48D and 64D from the inlet surface, respectively. When the gas velocity drops to \(j_{G}\) = 0.7 (m/s) at \(j_{L}\) = 0.93 (m/s), the maximum initiation position is captured for the light crude oil at 75D; this position is farther than the slug positions for Cases 1 and 2.