## 1 Introduction  
Flow of fluids is an important phenomenon in many industrial and physical processes. In the literature, two main kinds of flows are discussed, namely single-phase and multi-phase flow. Two-phase flow is a simpler form of multi-phase flow, which is further classified into gas-liquid [1], liquid-liquid [2], gas-solid [3], and liquid-solid flow [4]. Gas-liquid flow is the most common multiphase flow encountered in engineering systems like refrigeration, petroleum transportation, chemical processes, and heat exchangers [5, 6]. Two-phase flow can be further sub-divided into different regimes based on their appearance. An investigation on issues related to flow regime identification is important because almost every relation/mechanism for two-phase fluid models depends on the flow regime. Multi-phase flow measurements like void fraction, flowrate, and fluid velocity depend on the flow regime [7, 8]. The first step to design, analyze, and operate any gas-liquid two-phase flow system is the correct identification of the flow regime. Different flow regime identification methods have been presented in the literature but there is no agreement on the universality of the methods. Traditionally, subjective methods, like Close Visual Inspection (CVI), were used to identify the flow regime [9, 10] in industry. This method requires the analysis to be performed within arm's length and optical/visual access, which is not practical and feasible for site/field deployment. Void fraction, defined as the ratio of gas volume to total volume, is a parameter that is widely used to identify flow regime/patterns. [PERSON] and [PERSON] [11] measured void fraction fluctuations in air-water flow in a rectangular section using a linearized X-ray void measurement system. It was concluded that the Probabilistic Density Function (PDF) of void fraction fluctuation can be used to differentiate slug, bubbly, and annular flow. They concluded that slug flow is simply a transitional, periodic time combination of annular and bubbly flow. The method by [PERSON] and [PERSON] faces some difficulties in implementation due to the use of radioactivity and complicated construction. [PERSON] et al. [12] used rotating electric field conductance gauge with six electrodes to measure void fraction in circular cross section for air-water two-phase flow in vertical and horizontal flow. They also used the statistical concept of PDF to study the behavior of different flow patterns. Electrical capacitance tomography [13] and conductance probes [14] are also used to identify flow regimes. All these methods, even though capable of identifying flow regimes, possess some intrinsic limitations on the range of flow regimes that can be identified. The intruv-siveness in nature of some of these methods due to the placement of sensors within the flow stream could potentially alter the flow regime. Radiation absorption techniques can provide excellent results, but the use and handling of radiation sources can cause health and safety related issues, which completely defies the original purpose. The Electrical Capacitance Tomography (ECT) [15] technique is also a good alternative for flow regime identification because it is relatively inexpensive. However, its deployment is rarely seen in industries because of the cumbersome processes involved in setting up the ECT base reference for tomography. The use of a transducer to obtain pressure signals is a more sensible approach considering pressure readings are often needed in the process train safety and their data is easily available. That is why pressure fluctuation signals are commonly used to identify two-phase flow regimes [16; 17; 18; 19]. This method is simple, inexpensive, and requires small modification to the existing piping setup. It is suitable for both laboratory and industrial deployment. Based on literature, pressure fluctuations combined with post signal processing techniques can be used to identify different flow patterns.