## 1 Introduction  
Transmission pipelines are widely used in the petrochemical, nuclear and petroleum industries as they are reliable, efficient and cost-effective methods for connecting supply and consumers [1]. Specifically, in the petroleum industry, hydrocarbon mixtures are produced from natural oil and gas reservoirs, and they are composed of two or more fluids of different phases. The gas-liquid two-phase flow is the most common type of multiphase flow in most oil and gas systems [2; 3; 4; 5]. One of the main challenges in piping design is the occurrence of slug flow because its extensive encounter in most oil and gas production systems, chaotic behaviour and instability cause undesirable consequences [6; 7]. Hydrodynamic slug flow could be developed from a stratified flow regime by the natural growth of small random disturbance in the gas-liquid flow interface or by liquid accumulation due to the instantaneous imbalance caused by hilly terrain pipes. Moreover, the high superficial velocity of the gas phase promotes the coalescence of kinematic waves which acts as an important mechanism for the formation of liquid slugs [8; 9]. The occurrence of slug flow is not limited to a specific piping geometry or configuration and may occur in vertical, inclined and horizontal piping [10]. Slug flow is characterised as the alternating flow of liquid slugs and gas pockets. This alternating phenomenon exerts forces on pipe walls and causes stresses and vibration which are known to lead to serious pipeline damage. Therefore, predicting slug behaviour and quantifying slug characteristics and the consequent stresses are essential steps in flow assurance and piping design. They should be predetermined in the design stage of any production system. Researchers have investigated slug characteristics analytically, experimentally, and numerically. However, most existing research focused on air-water or gas-low viscous liquid two-phase flows and on determining slug characteristics by varying operating conditions. Nevertheless, computational fluid dynamics (CFD) modelling has become a viable option to predict the behaviour of slug flow and enable researchers to investigate different fluid and gas flow behaviours, including slug flow [11]. Laboratory-sized experiments are widely used for air-water two-phase flows in transparent piping to visualise the multiphase flow behaviour, including slug flow. However, natural gas-crude oil two-phase flows require a highly sophisticated setup and instrumentation for data acquisition to determine multiphase flow characteristics. Hence, CFD modelling techniques are widely used to predict multiphase slug flow behaviour and characteristics. [PERSON] [12] performed the CFD modelling of slug flow by using the commercial CFX software to explore the slug characteristics whilst varying the piping length, inlet and outlet conditions and free surface flow agitation. [PERSON] validated his model against the experimental work of [PERSON] [13]. A reasonable agreement was achieved between the numerical cases of [PERSON] [12] and the experimental work of [PERSON] [13]. [PERSON] and [PERSON] [14] attempted to determine the effect of the liquid Froude number on the formation of liquid slugs in air-water flow through horizontal piping by identifying the liquid holdup at multiple locations. They stated that for air superficial velocity between 1 and 4 m/s, slugs were not formed before 40D (D represents the pipe diameter) from the piping inlet. [PERSON] et al. [15] investigated experimentally the effect of the liquid holdup and variation of gas and liquid superficial velocities on slug initiation. They used the air-water two-phase flow in a rectangular channel and concluded that for liquid holdup \(H_{L}\) = 0.25, the slug initiation position occurred farther toward the downstream direction with the increase of the superficial velocities of air and water. For \(H_{L}\) = 0.5, it exerted no significant impact on the slug initiation position. Recently, [PERSON] et al.