Contra-rotating propellers (CRPs) are widely used in different kinds of underwater vessels, owing to their excellent stability, propulsive efficiency, and balanced torque distribution characteristics. However, the cavitation that occurs due to their high rotational speeds also generates high levels of cavitation noise. In this study, CRP noise radiations were predicted via detached-eddy simulations with the shear stress transfer k-ω turbulence model, the Ffowcs Williams–Hawkings (FW–H) acoustics model, and the Schnerr–Sauer cavitation model. The cavitation noise of the CRP was predicted using sound radiation theory for spherical bubbles and compared to non-cavitation noise levels from the FW–H equation. It was found that the non-cavitation noise of the CRP is significantly louder in the axial direction than in the radial direction. Furthermore, the noise sound-pressure levels (SPLs) of the flow field generally decrease with increasing frequency. When the effects of cavitation on the flow are taken into account, the SPLs of the CRP noise become significantly higher at all equidistant positions from the center of the propeller. The anisotropy in sound pressure also becomes insignificant when cavitation is present.

The Large Eddy Simulation (LES) method was used to study the interaction between propeller wake and a rudder. The INSEAN E779A propeller and the NACA0020 rudder were used, and the $Q-Criterion$ method was employed to analyse the vortices. The evolution of the propeller-wake in oblique flows and the effect of oblique flows on the propeller wake vortex were studied. The pulsating pressures on the rudder surface for different oblique-flow angles were compared. The results show that the hydrodynamic coefficients of the propeller and the rudder increased under oblique flows. The vortex of the propeller shifted as the angle changed, but it did not completely coincide with the oblique-flow angle. Under the effects of the propeller-wake, the rudder surface generated a pulsating pressure. The frequency of this pulsating pressure relates to the blade frequency of the propeller, and the amplitude of the pulsating pressure increased with the oblique-flow angle.

Severe wake-structure interaction induces intense vibrations and noises. In this study, the rudder is simplified into a plate fixed at two ends. The vibrations of the plate operating in the propeller wake are analyzed. Detached eddy simulation is employed to simulate the turbulence in the flow field and propeller wake. The structural deformation equation is solved via the finite volume method. The pressure fluctuations in the propeller wake and the vibrations on the plate are investigated. The results show that the excited vibrations coexist with natural vibrations on the plate. The natural vibration mode can be occupied by the excited vibration. The lock-in regime between the excited vibrations and natural vibrations leads to weaker vibration at excitation frequencies. The vibration mode induced by the hub vortex transfers to the first natural vibration mode when the shaft frequency approaches the first natural frequency. The vibrations on the plate are more dominant at the first natural frequency in the approach of the shaft frequency to the first natural frequency. This investigation of plate vibrations induced by the propeller wake contributes to the structural design of the ship.

The propeller wake can cause vibrations on the rudder surface, which worsen the noise and reliability. The vibration monitoring of the rudder operating in the propeller wake with fluid-structure interaction (FSI) method is still challenging. In the present study, the structural response during propeller-rudder interaction is investigated using detached eddy simulation. Three-dimensional distributions of loads, stresses, and deformations are discussed. The leading and trailing edges exhibit the strongest deformations in opposite directions, which are S-shaped. The strongest lateral deformation occurs between the tip vortex and hub vortex regions. In the tip vortex region, the dominant lateral vibrations fluctuate at the blade passing frequency (BPF) and shaft frequency (SF). However, the 75 Hz-fluctuation becomes significant at the trailing edge of the rudder. In the hub vortex region, the lateral deformation fluctuates mainly at 75 Hz except the area near the leading edge. There are weak vibrations occurring at the natural frequencies of the rudder when the natural frequencies of the rudder are much higher than the SF and BPF. However, the plate in the propeller suffers intense vibrations at the frequencies near the natural frequencies, where the natural frequencies of the plate are close to SF and BPF.

The deformation of the cylinder has been proved to greatly reduce the fluctuation of lift and the vortex-induced vibration. In this article, a new form of deformation mode for the smooth cylinder is proposed in order to reduce the vortex-induced vibrations, which can be applied to marine risers and submarine pipelines to ensure the working performance and safety of offshore platforms. Large eddy simulation (LES) is adopted to simulate the turbulent flow over wavy cylinders with three different twisted angles at a subcritical Reynolds number Re = 28,712. Comparing with the results of smooth cylinder, the maximum drag and lift reduction of wavy cylinder A3 with α = 40° can reach 17% and 84%, respectively, and the corresponding vortex formation length increases significantly, while the turbulence intensity decreases relatively. Meanwhile, the circumferential minimum pressure coefficient is greater than that of the smooth cylinder, which also provides a greater drag reduction for the cylinder. The surface separation line, turbulent kinetic energy distribution, and wake vortex structure indicate that the elongation of separated shear layer and wake shedding position is larger than that of the smooth cylinder, and the vorticity value in the near wake region decreases. A periodic vortex structure is generated along the spanwise direction, and a weaker and more stable Karman vortex street is reformed at a further downstream position, which ultimately leads to the reduction of drag and fluctuating lift of the wavy cylinder.

The working principle of a cycloidal propeller is complicated given that its blades are engaged in multi- degrees of freedom (DOF) motions. The instantaneous forces developed on the blades were studied to analyse the hydrodynamic characteristics of the propeller. The results of this study show that the oscillating centre distributed in the chordwise direction will exert significant effects on the fluctuation of the thrust produced by the cycloidal propeller. It will also significantly affect the average value of the thrust and efficiency. In addition, the effect of eccentricity on the thrust produced by the cycloidal propeller was also analysed. A higher eccentricity will considerably increase the thrust, and will make the curve of hydrodynamic characteristics steeper. Therefore, for the design of the cycloidal propeller, it is crucial to choose an appropriate eccentric point and an eccentricity value so as to balance the magnitude of thrust and its fluctuation.