# Design and Evaluation of Novel Submerged Floating Tunnel Models Based on Dynamic Similarity
## 1 Introduction  
Submerged Floating Tunnels (SFTs) represent a promising structural form similar to bridges, designed for future transportation across wide waters. They offer several advantages over traditional bridges and tunnels, including enhanced environmental adaptability, reduced construction costs, and longer spanning ability. Despite these advantages, the practical engineering application of SFTs has been hindered by many technical challenges that are deserving of further research [1, 2]. SFTs operate within complex and severe marine environments over long periods [3, 4]. Given the complexity of the fluid-structure interaction within the SFT, employing numerical simulations and hydrodynamic experiments allows for a more comprehensive and in-depth investigation of its hydrodynamic characteristics. This effort contributes to the optimization of the SFT's structural design and facilitates subsequent maintenance and monitoring during the operation phase. Numerical simulation is a valuable and essential tool in the study of SFTs. The numerical model usually needs to be calibrated to prove it can accurately reflect the inherent dynamic characteristics of SFTs, and the subsequent parameter analysis based on the numerical model is valuable [5, 6, 7, 8]. The simplified numerical model is widely favoreddue to its rapid computation speed and acceptable accuracy. These models generally incorporate certain simplifications and assumptions [9; 10; 11; 12], such as ignoring the elastic deformation of the tunnel body and simplifying it into a rigid body; limiting the freedom of motion of the model; the calculation of the wave load using semi-empirical and semi-theoretical formula; maintaining a constant mooring cable length; and ignoring the wave force on the mooring cable. In contrast, the high-fidelity simulation model demands a longer calculation time and greater computer performance, but it can more accurately simulate the coupled dynamic response of SFTs, such as the crossflow vortex-induced vibration [13; 14], fluid-structure interaction [15], the impact of wave-flow coupling [16], and even the tunnel-cable-train interaction [17]. Experimenting with a model of an SFT can directly reflect its dynamic response under load, and provide valuable data for validating the predictions of theoretical and numerical models [18; 19]. However, due to the lack of relevant equipment for hydrodynamic experiments, many studies of SFTs have only carried out theoretical derivation and numerical simulation. Most of the completed SFT model experiments have many assumptions and simplifications due to the limitations of experimental facilities. [PERSON] et al. [20] used a 98 cm long rigid tube in a wave flume to investigate the dynamic response of a tunnel segment under regular waves. [PERSON] et al. [21] experimentally investigated the dynamic behavior of an SFT subjected to the combined action of an earthquake, waves, and current. [PERSON] et al. [22] adopted an aluminum tube model anchored in a static water channel to research the impact of local cable failures. The rigid model cannot reflect the elastic deformation of the tunnel body, so it cannot accurately simulate the dynamic characteristics of SFTs. The ability of the scale model to accurately simulate the dynamic characteristics of the prototype is important to ensure the accuracy of the experimental results. Given the substantial length-to-diameter ratio of SFTs, the behavior of SFTs would be like that of a slender hollow cylinder constrained by tethers [23]. Therefore, the tunnel body shows clear deformation, and the impact on the dynamic characteristics of the structure and the surrounding fluid flow is non-negligible. Many studies have attempted to simulate the elastic deformation of the tunnel body in hydrodynamic experiments [24]. A widely adopted approach is utilizing a rigid segment model connected with springs and dampers to simulate the middle segment of the SFT.