## 4 Wind Tunnel Testing  
In Section 3.2, it was mentioned that a subset of three deck designs among the considered set of samples for the surrogate model training were selected to be tested in the wind tunnel. In this manner the CFD simulations can be validated with experimental data. The wind tunnel campaign was conducted at the aerodynamic wind tunnel of the University of A Coruna, which is a blown-type open circuit class facility, featuring a 1 by 1 m\({}^{2}\) test cross-section. The sectional models were manufactured at a geometric scale of 1/70, obtaining the required stiffness by placing one aluminium bar at the core of each box girder. In order to prevent end-effects, large circular end-plates were affixed to the ends of the sectional models. The wind tunnel tests were conducted in smooth flow. For the evaluation of the force coefficients, the sectional model was connected to the force balances by means of stiff bars, measuring the aerodynamic forces. In Figure 5, general and more detailed views of the sectional models in the test chamber are provided. The force coefficients were obtained for the fixed models at a Reynolds number, Re, of 1.2 \(\times\) 10\({}^{5}\), after checking the insensitivity of the results for Re > 8.0 \(\times\) 10\({}^{4}\). The experimental values in the range (\(-\)6\({}^{\prime}\), 6\({}^{\prime}\)) are reported in Figure 7 for cases g01, g04 and g07. For the evaluation of the flutter derivatives, the sectional model was elastically supported by springs, behaving as a three degree-of-freedom dynamical system. For the identification of the flutter derivatives, the sectional model was released out of the balance position at different flow speeds. The Iterative Least Squares method (ILS), proposed by [35], was adopted for obtaining the experimental flutter derivatives from the model's displacement time-histories at different reduced velocities. The experimental flutter derivatives obtained for cases g01, g04 and g07 are reported in Figures 8-10.  
distribution) for evaluating the aerodynamic and aero-lastic responses of the considered set of 15 samples. The Reynolds number for the CFD simulations is the same as the one considered for the experiments performed in the wind tunnel for validation: \(\text{Re}=1.2\times 10^{5}\). For all 15 sample points, the CFD simulations are first simulated for the static deck at \(0^{\circ}\) AoA. To be computationally efficient, these simulations are later used as the initialization for the forced oscillation simulation that enables the evaluation of the flutter derivatives at different reduced velocities. It should be noted that a subset of three samples, namely cases g01, g04 and g07, were tested in the wind tunnel to provide validation for the numerical results (see Section 5). The flow domain adopted for the CFD simulations has a length of \(40C_{\text{ref}}+B\) in the streamwise direction and of \(30C_{\text{ref}}+D\) along the directional orthogonal to the undisturbed flow. The windward corner of the twin-box deck is placed at \(15C_{\text{ref}}\) from the inlet at mid height in the flow domain. Higher density mesh regions are created near the twin-deck box and in the wake region, while, away from the deck, coarser meshes have been used by creating different blocks. The meshing strategy adopted in the work is similar to the research group's previous work [23]. A uniform inlet speed with a turbulence intensity of 1% and a length scale of \(0.1C_{\text{ref}}\) has been specified as the inlet boundary condition, whereas the outlet boundary condition is set as an atmospheric pressure outlet. It should be noted that corners \(b\), \(d\) and \(e\) in Figure 1 are considered sharp in the CFD model, while corner \(a\) is modelled as slightly rounded, with a radius relative to the reference box width \(R/C_{\text{ref}}=0.008\), and corner \(c\) is modelled adopting a degree of sharpness ratio, \(R/C_{\text{ref}}\), of 0.015.