## 6 Mechanical Systems--Case Study Application
### Buffeting Simulations and Verifications  
The size of pressure waves generated by the passage of HSV depends on the size, speed, and aerodynamic properties of the vehicle, as well as the geometry of the gantry structure and clearance between the structure and vehicle. The dynamic response of a gantry structure to a pressure wave can be evaluated using full-scale field measurement. Alternatively, a carefully-setup downscaled wind tunnel test can also provide data for the size and trend of dynamic pressure exerted on the gantry structure during the passage. Figure 7: CAD model of HSV (moving zone), portal gantry (high resolution stationary zone), and zonal evolution track (lane in a carriageway). Neither of these methods are a financially feasible technique for design data generation considering a wide variety of vehicle types/sizes (truck, buses, double-decker buses, and so on), their aerodynamic properties (with/without air deflector, among others), and speeds. In addition, after calculating the pressure waves, dynamic and fatigue analyses of the large gantry structures have to be carried out using solid mechanics techniques. Traditionally, it has been accomplished using the history-based FE method, where time transient linear/nonlinear simulation of the structure is carried out at sequential time steps followed by a fatigue assessment analyses. Although the application of the FE method to structural dynamics and fatigue assessment is popular and successful, long computational times are expected for transient simulation of large structures under pressure-history loading. Hence, in the research work herein, a software code written based on the complex damped spectral Eigen technique (CDSET) scheme, which has been presented in the previous sections, is employed. The technique is concerned with the synthesis of waveforms from the super-position of many frequency components. It is based on the exact wave solution of the governing differential equation for the dynamic motion of structures. However, it differs from the classical FE method because it uses fast Fourier transform for conversion purposes. To start the vehicle buffeting investigation using the finite zone technique herein, it is essential to carry out a verification study to validate the reliability and accuracy of the results. A 3D verification case study was proposed to verify the results of developed multi-zone CFD simulation compared with the results of full-scale field studies as well as basic physical principals of fluid-structure interactions. In the study herein, the reported measurements for full-scale field study of a HSV under cantilever mast [38] were simulated using the multi-zone CFD technique. A HSV vehicle-zone model along with a model of a signal box as well as atmospheric environment (main domain) were developed and exported to the solver for vehicle buffeting analyses. Figure 8 shows the field-study details including a cantilever mast, signal box, and HSV vehicle. The identical speed of the vehicle and the positions of measurement points above the centerline of the vehicle are chosen to simulate the replica of the field study data. A  
Figure 8: (**a**) Filed study setup (HSV and signal box cantilever gantry, [38]); (**b**) simplified HSV model with its signal box; and (**c**) snapshots of HSV passage simulation under the signal box. three-dimensional geometry model of the simulated test is also shown in Figure 8, where the geometry of the cantilever is neglected because of its minor effects on the generated pressure wave. The overall dimensions of the HSV are taken into account for the 3D geometry model, although the detailed shape is not identical to the real HSV.