The imbalance between the radial pressure gradient and centrifugal force in curved pipe flow produces a secondary flow, resulting in a non-uniform distribution of streamwise velocity across the pipe cross section. These phenomena are believed to exhibit higher fluid resistance than straight pipes with similar flow rates, thereby motivating research into the mechanical energy losses in curved pipes. First, to ensure the accuracy and efficiency of the calculations, the results of various turbulence models were compared with direct numerical simulation to select the most appropriate turbulence model. Based on the momentum conservation equation, the mechanical influencing factors of secondary flow and streamwise velocity stratification in toroidal flow field were theoretically studied. Computational fluid dynamics method was employed to explore the spatiotemporal evolution characteristics of the mechanics and velocity distribution in transient flow fields to explain the formation mechanism of the secondary flow and the coupling relationship between the streamwise and radial directions. Then, the typical energy components of the toroidal flow field were analyzed using the energy equation, and the energy conservation and distribution characteristics were numerically studied. Furthermore, the influence of typical parameters (Reynolds number and curvature ratio) on the velocity distributions and mechanical properties as well as the percentage and distribution of various energy components were analyzed. Finally, the calculation results were statistically presented to quantify the variation of the energy components with typical parameters.

A new type of anti-rolling device denoted as a fluid momentum wheel (FMW) is proposed to address the limitations of traditional gyrostabilizers in reducing the roll responses of floating platforms in waves. The proposed device is based on the same gyroscope theorem, which differs from a rigid gyrostabilizer in that the internal fluid generates secondary flow in the cross-section under the combined effects of inertial centrifugal force and a radial pressure gradient, and the streamwise velocity exhibits a non-uniform distribution. These instability phenomena may cause mechanical energy loss in the flow field, which is critical for selecting the driving device and the anti-roll control performance of offshore platforms. In the study, different turbulence models are compared with the results of a Direct Numerical Simulation (DNS) and experiments to ensure the accuracy of the numerical method, and the spatiotemporal distribution characteristics of the flow field in FMW are analyzed. Therein, the SST k-ω model accurately verifies the flow instability phenomenon of the FMW observed in the Particle Image Velocimetry (PIV) experiment. Next, this paper proposes corresponding evaluation parameters to assess the impact of typical parameters on the flow field instability. The results show that the flow instability increases with an increase in the typical parameters of FMWs (such as the pipe diameter, curvature radius, and velocity). Furthermore, the paper discusses the relationship between dimensionless mechanical factors (Reynolds number, curvature ratio) and the spatiotemporal instability of the flow field, revealing the essential effects of the curvature ratio and Reynolds number on the loss coefficient.

For the new sandglass-type FPSO, the unique shape of its floating body with oblique side and external expansion can significantly improve the motion performance, but meanwhile may result in specific slamming characteristics in waves. On this basis, this paper establishes a CFD method including numerical wave-tank technique based on the Open FOAM platform. Therein, the velocity-inlet boundary method and the active absorption method are applied for numerical wave-making and wave-absorption. Compared with experimental results, the numerical method can be validated to be accurate enough to simulate wave slamming on floating ocean platforms. Then, the specific slamming phenomena on the sandglass-type floating body under a classic long wave can be investigated by comparing with the cylindrical model, including nonlinear wave rollover and breaking, water cushion, rooster-tail wave, side wave, water tongue, and so on. The mechanism of these phenomena and their effects on slamming pressure are studied. The essences of typical peaks in the time-history curve of the slamming pressure are mainly discussed. More interestingly, the main peak can be found to be related to the small peak due the amount of the broken water and the thickness of the water cushion. Finally, the slamming characteristics of the sandglass-type model in a classic short-wave condition are comparatively discussed.

For a fish tank of breeding vessels with perforated broadsides, in order to analyze its fluid characteristics under wave conditions, this paper presents a numerical method and predicts the fluid field in a fish tank with opening holes. Therein, three critical problems were solved, which include wave generation, the fluid–structure interaction of waves and vessel motion, and the sloshing phenomenon in the tank. The feasibility of the method was validated through a comparison with theoretical and experimental results. On this basis, the fluid characteristics in the fish tank of a fixed breeding vessel with opening holes on side walls were discussed. Firstly, the effects of wave direction, wave height, and wave period on the flow field in the tank were mainly analyzed. Next, the fluid characteristics in the tank of a floating vessel with opening holes equipping a multi-point mooring system were investigated. Finally, the influence of motion responses of breeding vessels on the fluid field in the fish tank was examined.

Large-eddy simulation was conducted on turbulent flow in toroidal pipes of various lengths to study the impact of streamwise fluctuation structures on the secondary motion and explore the origin of the swirl-switching phenomenon. Proper orthogonal decomposition analysis was carried out to identify the primary coherent structures in the flow field and determine the oscillation frequencies of the swirl switching. The results show that high-frequency switching (St > 0.05) is closely related to the spatial scale of very-large-scale motion (VLSM), and that its streamwise extension is approximately the characteristic length corresponding to the switching frequency. Conversely, the characteristic length of low-frequency switching (St ≤ 0.05) is significantly larger than the physical model, which is not attributed to the spatial scale of streamwise fluctuations but rather to the temporal oscillations of the VLSM. Due to the sensitivity of the VLSM structure, the alignment mechanism of VLSM changes under vertical excitation, leading to a reduction in the streamwise characteristic length and an increase in the frequency of swirl switching. The spatiotemporal variations of the VLSM feedback into the frequency of swirl switching, thus confirming the significant effect of streamwise fluctuation. Additionally, the wall shear stress exhibits oscillation frequencies associated with swirl switching and VLSM, indicating a correlation between flow field fluctuations and wall shear stress, which could potentially contribute to material erosion and local resonance.