## 1 Introduction
Two-dimensional (2D) and three-dimensional (3D) CFD models provide higher accuracy in simulating transient flows during filling and emptying procedures. However, selecting the appropriate CFD modelling approach depends on the specific characteristics of the hydraulic process under study [21; 25]. Two-dimensional (2D) CFD models are characterised by the fact that they are more straightforward than 3D CFD models and allow for the fitting of a flow that predominates over a specific plane. These models assume that the flow spans mainly two directions and that the flow in the third dimension does not significantly affect the dominant flow plane. From the literature, it is essential to highlight the different relevant 2D CFD models developed in the previous research. For example, Authors such as [PERSON] and [PERSON] [26] and [PERSON] et al. [27] developed 2D CFD models with structured cells to study the air-water interaction processes through a horizontal pipe defined in profile, where the authors represented, in the two-dimensional plane, the pipe diameter (vertical axis) and its length (horizontal axis). On the other hand, [PERSON] et al. [24, 28] developed a two-dimensional CFD model using a mixed mesh (structured-non-structured) to study the air-water interactions during filling processes in a branched pipeline, with a total length of 7.3 m and an internal diameter of 51.4 mm, in which the flow presented a predominant trajectory on a vertical-horizontal plane, with the diameter being projected on the vertical dimension and the pipe axis on the horizontal axis thus forming vertical-horizontal projections. On the other hand, [PERSON] et al. [29] developed a two-dimensional CFD model using a structured mesh for studying emptying processes using a similar experimental setup. On the contrary, 3D CFD modelling contributes to a detailed representation of two-phase flows in austere and complex geometries, capturing spatial interactions and the three-dimensional effects of a fluid, especially in two-phase transient phenomena. Today, some researchers have adopted 3D CFD models, and it has been the most reliable alternative for modelling two-phase flows in water pipes with entrapped air. Authors such as [PERSON] et al. [30] studied the behaviour of the air-water interface in a horizontal-vertical water pipeline during rapid filling processes using a three-dimensional CFD model discretised through a structured mesh, comparing their numerical results obtained in a previous investigation using a 2D CFD model [31]. Furthermore, [PERSON] et al. [4] studied the dynamic behaviour of trapped air from a thermodynamics and pipe roughness approach. For this purpose, the authors used a 3D CFD model with hexahedral cells to represent the pipe layout and observe the transient and heat transfer phenomena presented in different tests. [PERSON] et al. [32] and [PERSON] et al. [33] developed 3D CFD models to study pressurised air in water pipes, in which similar air entrapment phenomena occur as in actual water pipes. In the case of [PERSON] et al. [32], a discretised CFD model in unstructured cells was used, while [PERSON] et al. [33] discretised the 3D CFD model into mixed finite elements (hexahedral and tetrahedra). [PERSON] et al. [34] studied the air-water interaction during the pressurisation of a water pipe with return air flow using three-dimensional geometry for the modelling and then discretised it in hexahedra. [PERSON] et al. [35] analysed detailed physical parameters such as velocity profile, pressure distribution, temperature, and air-water interaction during the pressurisation of a water pipeline with trapped air using a three-dimensional geometry with structured elements. Furthermore, [PERSON] et al. [6, 36] studied transient phenomena focused on emptying processes in water pipelines with/without air valves, respectively, where different phenomena were identified, such as backflow air and the aerodynamic effect of air admission.