# A Review: Factors Affecting Internal Two-Phase Flow-Induced Vibrations
## 1 Introduction  
Fluid flow is an important phenomenon in many engineering systems. In the literature, primarily two different flows are discussed, namely single-phase and multiphase flows. Two-phase flow is a simplified form of multiphase flow, which is further classified into gas-liquid [1], liquid-liquid [2; 3], gas-solid [4; 5], and liquid-solid flow [6; 7]. It is widely present in various engineering systems, such as petroleum transportation and production, power plants, chemical industry, and heat exchangers. Moreover, two-phase flow is very unstable due to its fluctuating density, pressure, velocity, and momentum [8; 9]. This fluctuating nature of two-phase flow, when combined with bends used to change the flow direction, can produce gravitational, centrifugal, and buoyant forces [10; 11]. This leads to a complex fluid-structure interaction (FSI), which causes severe vibrations. These vibrations have recently attracted considerable attention and are known as flow-induced vibrations (FIV).  
Fluid-related vibrations were discussed in the two conferences [12; 13] held in 1972 and 1979 in Germany. Several practical problems in the field of fluid-related vibrations and their results were presented. The term, FIV, became well-known after it was coined by [PERSON] [14] in his book. Flow-induced vibrations can be explained as a form of sequential interaction between hydrodynamic forces and structural dynamics. The fluid exerts force on the surface of structure, causing the structure to deform. The deformation of the structure depends on the mechanical properties of the structure. In response, the deformed structure will react and apply the opposite force against the fluid. Consequently, flow-induced vibration is generated due to the interaction between these two forces, accordingto [PERSON] [15]. Furthermore, for the first time, [PERSON] classified FIV based on two types of flows, namely steady and unsteady flows. Later, the types of FIV already known were classified based on the type of flow involved [16], as shown in Figure 1. In steady flow, the interaction between fluid and structure is the main cause of vibration forces, whereas in unsteady flow, turbulent forces are the main source of vibrations.  
The focus of this review is FIV due to the two-phase flow. Two-phase FIV can be divided into three categories [16], as illustrated in Figure 2. The first category is the vibration of piping system by the two-phase flow, which includes FIV generated due to the impact of flow or change in flow direction as a result of piping components, such as bends and T-junctions. The second category, bubble-induced vibration, is primarily due to the sloshing effect. The third category, thermal-hydraulic vibration associated with phase change, induced from the main sources of the FIV process, involves phase change due to the energy transfer between the interface, such as the instability caused by boiling and condensation. Examples of thermal-hydraulic vibration include flow in suppression pool and feedwater piping of nuclear power plant. Moreover, vibrations generated by the two-phase flow can be categorized based on flow orientations, namely internal and external flow. Flow-induced vibrations due to internal and external flows are discussed in the next section.