# A Review: Factors Affecting Internal Two-Phase Flow-Induced Vibrations
## 7 Future Research Prospects and Challenges  
The following recommendations are made to overcome the challenges in internal two-phase FIV research.  
* Predictive models used to estimate FIV have many limitations, such as high margin of error, developed for a specific range of parameters and, in some cases, require experiments to collect input data for models. A study should be conducted to improve predictive models.
* Flow-induced vibrations due to multiphase flow is a flow regime dependent phenomenon. Among two-phase flow regimes, the annular flow regime needs to be explored further.
* Flow regime maps developed in the 1970s and 1980s should be replaced and new approaches should be developed.
* Research on large diameter pipes is limited and research is encouraged to widen the database.
* The effect of physical parameter, such as surface tension, viscosity, and gravity should be explored further to develop better understanding. In addition, predictive models should consider physical parameters and its effect on overall model performance.
* The effect of added mass due to two-phase flow on the natural frequency of system needs more attention.
* Developing better CFD models to study FIV. Additional comparisons of CFD and experimental results are encouraged to provide confidence to CFD users for studying multiphase FIV.  
Conceptualization, U.K., W.P.; writing--original draft preparation, U.K.; writing--review and editing, W.P. and N.S.; data collection, U.K.; formal analysis, U.K.; visualization, U.K.; supervision, W.P. and N.S.; data analysis, U.K. and W.P.; methodology, U.K.; project administration, U.K. and W.P.; funding acquisition W.P. and N.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The first and second author wish to specially acknowledge the funding for this study from Malaysia Ministry of Higher Education through Fundamental Research Grant Scheme FRGS/1/2019/TK03/UTP/02/10. All the researchers wish to thank Yayasan Universiti Teknologi PETRONAS under YUTP-15 LC0-456 and YUTP-015 LC0-237 for supplementary funding.  
**Institutional Review Board Statement:** Not applicable.  
**Data Availability Statement:** The authors declare that all data can be obtained from the corresponding author upon request.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclature**  
\begin{tabular}{l l} \(A\) & Area (m\({}^{2}\)) \\ \(D\) & Diameter (m) \\ \(F\) & Force (N) \\ \(f\) & frequency (Hz) \\ \(f\) & Dimensionless frequency \\ \(G\) & Gravitational acceleration (m/s\({}^{2}\)) \\ \(P\) & Pressure (Pa) \\ \(t\) & Time (s) \\ \(V\) & Fluid Velocity (m/s) \\ \(A_{g}\) & Area occupied by gas phase (m) \\ \(A_{l}\) & Area occupied by liquid phase (m) \\ \(V_{M}\) & Mixture Velocity (m/s) \\ \(V_{sg}\) & Superficial Velocity of gas(m/s) \\ \(V_{sl}\) & Superficial Velocity of liquid (m/s) \\ \(We\) & Weber number \\ \(Re\) & Reynolds number \\ \(Fr\) & Froude number \\ \(\beta\) & Void fraction \\ \(\mu\) & dynamic viscosity (Pa s) \\ \(\sigma\) & Surface tension (N/m) \\ \(\rho_{l}\) & Liquid phase density (kg/m\({}^{3}\)) \\ \(\rho_{g}\) & Gas phase density (kg/m\({}^{3}\)) \\ Subscripts & Gas Phase \\ \(l\) & Liquid Phase \\ \(RMS\) & Root Mean Square \\ \(m\) & Liquid-Gas Mixture \\ \(sg\) & Superficial Gas \\ \(sl\) & Superficial Liquid \\ \end{tabular}