## 2 Background
### CFD Simulations
A more accurate approach is the large-eddy simulation (LES), which was originally developed by [PERSON] [41] to study unsteady atmospheric motions. This meteorological approach was later examined and extended for general engineering implementations by [42]. Unlike the RANS formulation that uses Reynolds averaging to reduce the computational cost of DNSs, a LES applies a low-pass filter to the flow field so that motions of large-scale eddies can be fully resolved, and vortices of small length scales are modelled. It is arguably the most accurate transient approach next to the DNS, while still being applicable to real life problems. [PERSON] and [PERSON] [43] showed that URANS, in general, cannot produce faithful results of vortex shedding in the wake region of a bluff body compared to experimental results, while 3D LESs can accurately reflect the fluctuations in velocity and pressure field of the unsteady flow. It was also shown through a comparison of 2D and 3D LESs that 2D LESs of a bluff body geometry would yield inaccurate results of fluctuating pressure and velocities, leading to large discrepancies in aerodynamic forces compared to experimental results and 3D LES results. Despite its affordability compared with DNSs, a LES is still computationally demanding as it inherits the strong grid number-dependent feature of DNSs. An appropriately configured LES normally requires finer mesh than an equivalent URANS simulation. In a comparison between URANS and LES by [PERSON] et al. [44], LES takes ten times the CPU hours of a URANS simulation with only doubled grid points. To reduce the computational cost of transient simulations, [PERSON] et al. [45] proposed the detached-eddy simulation (DES) approach which applies URANS to model the boundary layer and LES to resolve the turbulence in the trailing end. Based on the original formulation, there have been improved versions: the delayed-detached-eddy simulation (DDES) by [PERSON] et al. [46], the improved DDES (IDDES) by [PERSON] et al. [47] and the modified IDDES by [PERSON] et al. [48]. It can be summarised that the LES- and DES-based methods will be the best options for transient simulations before DNS becomes affordable. In the study of bridge engineering, CFD models are used in the study of aerostatic, aerodynamic and aeroelastic performances of bridges. Compared to wind tunnel tests, the CFD method has several advantages in this area. Firstly, there is no physical limit in the size of the testing domain and geometries, which makes full-scale simulations possible. Therefore, the consistency of the Reynolds number can be maintained. Secondly, CFD simulations provide nonintrusive data collection from every point within the testing domain. This is essential to the study of aeroelastic problems, in which a large amount of data are generated simultaneously in both the solid region and the fluid region. Finally, CFD simulations are highly flexible in terms of the cost. Based on the objectives and the budget of a task, CFD can offer wide ranges of solutions from RANS simulations that are significantly cheaper than wind tunnel tests to LESs that are computationally expensive. Despite the advantages of the CFD simulation, its implementation in bridge aerodynamics is still not as popular as the wind tunnel test because of a few limitations. Firstly, unlike the wind tunnel test that has been incorporated in bridge design for decades, the CFD method is a relatively younger technique introduced in this area. Although the application of CFDin the aeronautics provides a lot of experience for bridge CFD practice, there is no standard or guide to follow for bridge CFD simulations. Therefore, validation is still required to provide confidence in the predictions, particularly related to the choice of turbulence model and numerical settings. Secondly, although steady-state simulations are normally much cheaper than a wind tunnel test, the computational cost of high-resolution transient simulations is still high.