# A Review: Factors Affecting Internal Two-Phase Flow-Induced Vibrations
## 6 Conclusions  
In conclusion, this review article briefly introduces the background of flow-induced vibrations and its classification, followed by an in-depth review of FIV in internal two-phase flow. It is evident from the literature that for a successful investigation of FIV, the vibration of experimental flow loop should be diminished, and the natural frequency of experimental structure must be determined to avoid resonance. In addition, while evaluating the natural frequency of pipe structure, the effect of added mass due to the two-phase flow should not be ignored. It should also be noted that eliminating the risk of resonance does not remove the risk of fatigue failure or excessive stress and stress calculations should be performed in detail. Flow-induced vibration phenomenon is flow regime dependent and most of the literature agree that forces due to slug and churn flow regimes are more intense. The effect of flow regimes should be explored further, particularly annular flow regime, which has received the least attention by researchers. Moreover, for analysis of FIV in multiphase flow, researchers are still utilizing flow regime maps developed in the 1970s and 1980s. Furthermore, additional advanced approaches are needed, such as machine learning algorithms or flow regimes maps developed for specific experimental conditions and geometries. Geometry is considered as the major factor while considering FIV in multiphase flow. As suggested by [PERSON] et al. [68], flow behavior in small and large diameter pipes is different. It can be concluded from the literature that the physical mechanism of phase distribution would be different in small and large diameter pipes and between vertical and horizontal flows. Therefore, it would be inappropriate to derive conclusions regarding FIV behavior in large pipes based on conclusions derived for small pipes and vice versa. In addition, this remark is true when comparing horizontal and vertical two-phase flows. Figure 6: Comparison of dimensionless forces plotted against the Weber number [8; 48; 63; 64; 65; 66; 73; 76; 77]. As an example, [PERSON] et al. [103] concluded that the effect of void fraction on FIV is the same for different diameter pipes. However, the authors' observations were based on small diameter pipes and may not hold true for large diameter pipes with bend in a vertically upward orientation. Several predictive models are available in literature to predict dominant frequency and magnitude of forces due to the two-phase flow. For example, the model of [PERSON] et al. [66] and [PERSON] et al. [62] can predict the magnitude and dominant frequency of forces with an average error of 30%. This model with its relatively high margin of error also needs void fraction data as an input from experiments to determine force. Therefore, to use this model to solve problems, experiments must be conducted to extract void fraction data, for specific geometry and flow conditions. The correlation by [PERSON] et al. [48] can also be used to calculate RMS of force fluctuations. It is recommended for void fraction value of 55% to 98% and the error margin reported is 50%. An important point to note here is that these models were developed for small diameter pipes only and the error and limitations can further increase if an attempt is made to predict for large diameter pipes. An attempt was made [72] to fit the RMS of force fluctuations for large diameter pipe (6 inch) and the prediction was further outside the original 50% error. This suggests that FIV in large diameter pipe needs to be investigated, considering there is a negligible database of FIV on large diameter pipes. Due to the inherent complexities in interface and turbulent behavior of two-phase flow, in the past, CFD was not preferred as a tool to investigate FIV in multiphase flow. Recently, it is gaining more recognition and some previously mentioned researchers have implemented and validated their CFD results [60; 65], against experimental results, to incorporate more confidence in the use of CFD for research.