## 4 Conclusions and Challenges  
In this paper, recent wind tunnel tests and CFD simulations in the study of aeroelastic and aerodynamic instabilities of long-span cable-supported bridges have been reviewed. These two methods have made tremendous contributions to the research on flutter, VIVs and RWIVs, yet both have benefits and drawbacks. Some summarising comments are listed as follows:  
1. In general, the wind tunnel test is undoubtedly the most popular method in the study of bridge aerodynamics because of its great development in both theories and apparatus for almost a century. 2. The application of CFD simulations in bridge engineering is still at its \"young age\" due to a variety of reasons. Firstly, the accuracy of CFD simulations depends on the level of complexity of the numerical model which varies the cost. Although CFD simulations are significantly more affordable than wind tunnel tests for steady-state cases, the cost of transient and FSI simulations is higher than that of a wind tunnel test. Therefore, to save computational power, most studies use basic configurations including simplified geometries, 2D mesh, smooth-flow conditions and small scales. As a result, the advantages of CFD are not fully utilised. Secondly, the best that CFD can deliver is an accurate solution to mathematical models that reflect only a part of the underlying physics of a fluid-related phenomenon. Therefore, currently, validation with experiments is still necessary. However, challenges exist in the validation of cases where complex geometries, highly turbulent flows or high wind velocities are involved, where wind tunnel tests can be more prone to uncertainty and errors. 3. In the study of flutter instability, both free vibration and forced vibration wind tunnel tests have shown great performances in low-velocity cases. In the study of high-velocity cases, the forced vibration wind tunnel test normally yields better results but requires more complex equipment and configurations. However, neither test can guarantee accurate measurement of flutter derivatives in turbulent flow conditions. Since controlling the turbulence is challenging in a wind tunnel, developing and applying new turbulence controlling techniques in wind tunnel tests will add some interesting insights to this area. Most CFD simulations in the study of flutter instability employ the forced vibration method because it demands less computational effort. Existing studies also favour URANS simulations due to the limitation of computational power. Although it is not practical to adopt LESs in every study, it is recommended that better transient approaches be applied in the study of flutter instability, such as DESs, DDESs and IIDESs, which have been shown to be more affordable than LESs. Moreover, it has been shown that flutter is sensitive to the bridge girder geometry and turbulence conditions. Hence, it is also recommended that a comprehensive comparison be made with existing turbulence modelling strategies with a focus on compatibility with different types of bridge decks. 4. The consistency of the Reynolds number is important in the study of VIVs. Therefore, most state-of-the-art wind tunnel tests of VIVs are performed on large-scale models, which makes the experiment costly, whereas CFD simulations in this area have shown great performances and high efficiencies. It has been shown that most studies favour the free oscillation simulation which makes it easier to identify the onset of VIV, although it is computationally demanding. Recent studies have demonstrated the feasibility of using forced oscillation simulations to study VIVs, which is more computationally affordable than free oscillation simulations. Further studies to improve the accuracy of results derived from forced oscillation simulations will add interesting insights to this area. 5. The wind tunnel test has enabled researchers to reproduce RWIVs in the laboratory, which helps to form the current water rivulet theory of the mechanism.