The hydrodynamic performance of a ship with and without a stern flap was comprehensively investigated based on model tests and model- and full-scale simulations. The stern flap led to an increase in the stern pressure and improved the transom stern flow field, which resulted in a change in the ship attitude and decrease in resistance. The stern flap performance obtained using the full-scale ship simulation was associated with a larger drag reduction range and drag reduction effect than the extrapolated value. The scale effect of the stern flap was notable owing to the different effects exerted by the stern flaps with different scales on the hull hollow and rooster tail. In addition, the stern flap was observed to increase boundary-layer thickness near its installation location whilst lowering velocity across the propeller disk. However, this velocity reduction effect was over-predicted at the model scale compared to that at the full scale.

The evolution mechanisms from near to far field of propeller wakes under different inflow conditions were analyzed. Modal decomposition was performed on the propeller wakes calculated by previous numerical simulations [Wang et al., “Propeller wake instabilities under turbulent-inflow conditions,” Phys. Fluids 34, 085108 (2022)]. In particular, the Sparsity-Promoting Dynamic Mode Decomposition (SPDMD) was employed to identify the flow modes that play a dominant role in the inception mechanisms of the propeller wake destabilization. The results showed that the turbulent inflow contributes to the destabilization process of the tip vortex system under the low-turbulence inflow condition, and as a perturbation, it accelerates the interference between adjacent tip-vortex structures, intensifies the instability motion of the tip vortex, and then accelerates the breakdown of the tip vortex system. SPDMD succeeds in selecting a subset of dynamic mode decomposition modes that have the most substantial influence on the quality of approximation of the snapshot sequence. The present study further extends knowledge of propeller wake instability inception mechanisms under different inflow conditions, which can support the development and improvement of next-generation propellers.

This paper presents a numerical modeling procedure for the idealization of vortex shedding effects in the wake flow field of a NACA0009 hydrofoil. During the simulation, the lift and drag acting on the hydrofoil were monitored, and the vortex-shedding frequency of the hydrofoil was analyzed. The effects of inflow velocity, trailing-edge thickness, angle of attack, and maximum hydrofoil thickness on vortex shedding were investigated. The results indicate that an increase in the inflow velocity led to an increase in the vortex-shedding frequency and a negligible change in the Strouhal number. Furthermore, as the thickness of the trailing edge increased, the vortex-shedding frequency decreased gradually, whereas the Strouhal number first increased and then decreased. Vortex shedding and lift curve oscillations ceased altogether after the angle of attack of the hydrofoil increased beyond a certain threshold. When the maximum hydrofoil thickness was increased while keeping the thickness and chord length of the trailing edge constant, the vortex-shedding frequency decreased.

Inspired by the silent gliding feather of owl wings, the trailing edge of the duct of a pump-jet propulsor was designed with a similar serrated structure in order to reduce noise generation. Two distinct serrated structures were proposed and evaluated using the detached eddy simulation method with the shear stress transport k−ω turbulence model. The findings indicated that while the hydrodynamic efficiency changed within 1% upon the inclusion of the serrated trailing edge, a significant alteration existed in vortex structures of the wake. More horseshoe and secondary vortices were generated since large-scale vortices induced by the duct were disrupted circumferentially. This phenomenon expedited the distortion and mixing of trailing-edge vortices, causing flow instability. Furthermore, the serrated trailing-edge structure led to noise reduction. Particularly in the 0–1000 Hz range, the sound pressure level behind the duct showed a maximum reduction of 4.43 dB.

In this study, the performance of a pump-jet propulsor (PJP) with pre-swirl stator in open water is numerically investigated. Both full-scale and model-scale configurations are considered. The Reynolds-averaged Navier–Stokes equations and shear stress transport $k-\omega$ turbulence model are used in the numerical calculation. The computational domain is discretized using structured grids, and a rotating grid is affixed to the rotor to deal with the relative motion between the rotor and stationary components. The mesh quality is determined based on a grid uncertainty analysis. The numerical method is validated using model-scale experimental data. The simulation results reveal the influences of the scale size on the hydrodynamic performance and the distributions of the velocity, pressure and vorticity under three advance coefficients. With the increase in the advance coefficients, the scale influences on the efficiency become more obvious, and the efficiency of the full-scale PJP is always higher than that of the model-scale PJP. The full-scale configuration is found with a more significant instability in the gap vortex development, because it presents larger interaction between tip leakage vortex (TLV) and the inner wall of the duct. As the main velocity increases, the TLV shedding is delayed. Finally, the development process of gap vortices is analyzed for the difference operation conditions.