Accurately identifying sources of flow loss and hydroacoustics and clarifying the mechanism of their generation are crucial for directing the optimal design of efficient and quiet pump-jet propulsors (PJPs). In this paper, numerical simulations of steady and unsteady flow are performed for a PJP equipped with pre-swirl stationary vanes, based on which both sources of flow loss and hydroacoustics are investigated at multi-level granularity. Analyses of flow efficiency and entropy generation rate are performed to identify the sources of flow loss, and analyses of thrust fluctuation and wall pressure fluctuation are conduced to identify the sources of hydroacoustics. The results indicate that the pressure drag accounts for 76% of the total drag and is mainly contributed from the stator and the duct, but the flow efficiency of the rotor is much smaller than that of the stator and the sources of the flow loss are mainly located at three regions of the rotating blades: the leading edge, the tip, and the corner of the suction surface. The hydroacoustic sources are mainly located at the leading edge and the tip of the rotating blades due to stator–rotor and duct–rotor interactions, respectively, but the Taylor's frozen turbulence hypothesis is inappropriate to describe the wake evolution of the stationary vanes owing to the potential interaction caused by the blade rotation.

Porous materials fixed on and downstream the cylinder can reach a much better effect in suppressing wall pressure fluctuations. In the present paper, numerical comparative studies have been conducted to investigate passive control of flow past a cylinder surface, in which three schemes with different porous treatments are applied to compare their pros and cons. The results show all of the three schemes of porous materials increase the time-averaged flow drag and reduce fluctuations of lift and drag forces. It can be concluded the velocity gradient reduction inside the boundary layer and the vortex shedding delay through porous coating, as well as reverse transition from turbulent vortex shedding into laminar through porous treatment downstream the cylinder, are main flow control mechanisms of porous materials. These mechanisms all reduce fluctuations of lift and drag fluctuations, but have a distinct effect on the features of wake evolution, such as the wake width and length as well as the fluctuating components of the flow velocity. In addition, the wake evolution is highly affected by the location of porous materials.

Previous numerical studies hold inconsistent viewpoints of the porous coating on a circular cylinder reducing or increasing drag, and the present experimental study aims to clarify this argument. Additionally, a common viewpoint is that the porous coatings can suppress both the near-field wall pressure fluctuations and far-field acoustic pressure, but optimal parameters of the porous materials are still unclear. In this paper, the experimental studies reveal effects of three parameters, i.e., incoming flow velocity, pores per inch, and thickness of metal foams, on the pressure drag, wall pressure fluctuations, and associated noise level.

Proper orthogonal decomposition (POD) is a widely used data-driven, reduced-order method for feature extraction and field reconstruction of unsteady flows. The number of modes required in the POD varies greatly with the flow characteristics, and a high number of modes is a significant challenge to effective multiscale flow analysis. The first part of this study shows that the number of modes required for flow field reconstruction increases almost linearly with the energy proportion of the stochastic fluctuation, making it costly to accurately reconstruct the stochastic fluctuation in unsteady flows. To address this issue, the second part of this study proposes a frequency domain proper orthogonal decomposition (FDPOD) method, which reduces the number of modes required for accurate reconstruction while preserving key spectral characteristics of the unsteady flow. Three case studies, i.e., cylinder flow, jet flow, and fan flow, are employed to illustrate the advantage of the developed FDPOD method, which provides a more accurate and efficient tool for turbulence analysis, especially for multiscale flows dominated by stochastic fluctuations.

With the increase of photovoltaic penetration rate, the fluctuation of photovoltaic power generation affects the reliability of ship power grids. Marine PV grid-connected systems with high penetration rates should generally have a low voltage ride-through capability. In the present paper, a strategy in which super capacitors are applied for energy storage in a marine photovoltaic grid-connected system is proposed, and an inverter adopts independent decoupling control of active and reactive currents to improve the LVRT capability of photovoltaic grid-connected systems. In addition, a comprehensive control strategy is also designed to control the supercapacitor, to regulate the active power through the control method of the voltage outer loop and the current inner loop, in order to maintain the DC bus voltage stability. At the same time, the inverter can increase the reactive power output to support the grid voltage. The advantage of this system is in smoothing the power imbalance in a short time, enhancing the low voltage ride-through capability of the photovoltaic grid-connected system, improving the power quality, and ensuring the safety and stability of the ship’s power grid. MATLAB/Simulink were employed to establish a ro-ro ship super capacitor–marine photovoltaic grid-connected power system model and to carry out simulation experiments by setting the grid voltage drop. The results show that when the grid voltage drops, the inverter adjusts the distribution of active and reactive power. The power factor drops from 1 to 0.77, and the effective value of the voltage drop increases from 150 V to 156 V, which proves that this strategy effectively reduces the depth of the grid voltage drop and improves the low voltage ride-through capability of the photovoltaic grid-connected system.