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This study investigates the energy-harvesting behavior and configuration effects of two semi-active flapping foils through numerical simulations, considering both tandem and unaligned configurations, as well as the influence of Reynolds number and pitch amplitude. The tandem configuration shows the greatest efficiency improvement, maximizing the utilization of the upstream foil's wake. In contrast, the unaligned configuration allows the downstream foil to surpass the energy-harvesting efficiency of the upstream foil, which is advantageous for clustered systems. The phase difference impacts wake structures, with the downstream foil recapturing the wake, though its efficiency is limited by the lower wake speed compared to the incoming flow velocity. The Reynolds number effect does not appear to influence the peak efficiency ratio, suggesting that the efficiency relationship between the foils remains consistent across different Reynolds numbers, despite potential differences in the optimal phase difference. Furthermore, reducing the pitch amplitude of the upstream foil optimally enhances the efficiency of the downstream foil, a factor that has often been overlooked. Lowering the pitch amplitude increases the wake speed, providing the downstream foil with a larger energy utilization space. This study provides valuable insights into the configuration design of flapping turbines, with the optimal two-foil system achieving an impressive efficiency of up to 46.3%. In the unaligned configuration, the lower the pitch amplitude of the upstream foil, the higher the efficiency ratio, which can reach up to ∼1.8. With different pitch amplitudes, the efficiency ratio consistently exceeds 1, indicating that the downstream foil benefits from the wake of the upstream foil. Combining more foils might further enhance efficiency in the unaligned configuration, as the higher efficiency in the tandem configuration is due to the repeated utilization of the sweep area by the two-foil system. [ABSTRACT FROM AUTHOR]

Excellent self-righting performance is important to guarantee the normal navigation of Unmanned Surface Vehicles (USVs) after overturning, and the restoring time is an important index in design requirements. Traditionally, the static stability method and experiments on full-scale vehicles were used to analyze the large-angle stability of the USV. However, when it comes to the analysis of self-righting performance, the traditional static stability method will cause improper integration, and experiments are not convenient. To solve these problems, an improved static stability method was proposed, and a whole self-righting process simulation of a physical model was finished. The numerical simulation method was used to predict the self-righting process of a USV under four working conditions. Firstly, a midpoint average method based on the static stability theory was adopted to compute the static restoring time, and the results were compared with the results of the references, which verifies the effectiveness of the midpoint average method. Also, the midpoint average method is convenient because it only needs the restoring arm curve, the width and the gravity center height. Then, a numerical simulation of a physical model in static water was finished, and an experiment for a physical model in a towing tank was conducted. Comparing the restoring time of the midpoint average method, the numerical simulation and the experiment, the results show that the numerical simulation has high accuracy. Moreover, the numerical simulation was used to predict the self-righting process and analyze the self-righting performance of a USV under four working conditions. [ABSTRACT FROM AUTHOR]

The interaction between a fluid and a wall is described with a certain boundary condition for the fluid velocity at the wall. To understand how fluids behave neara rough wall in a completely laminar flow regime, the fluid velocity at every point on the rough surface may be provided. This approach requires detailed knowledge of, and likely depends strongly on the roughness. Another approach of modelling the boundary conditions of a rough wall is to coarse grain and extract a penetration depth over which on average the fluid penetrates into the roughness. In this work, we examine the impact of well-defined patterned surfaces on the fluid flow behaviour. We considered two extreme cases: one with horizontal ridges and another with vertical ridges on the wall and an intermediate case with ridges at an angle on the wall. We show that for a broad range of periodic roughness patterns and relative flow velocities, a universal penetration depth function can be obtained. We obtain these results with experiments and complementary numerical simulations. We evaluate how this penetration depth depends on the various roughness parameters such as ridge depth, ridge spacing and ridge angle. Our results present a novel approach to investigating wall roughness boundary conditions by considering the penetration depth δ that captures the spatially averaged behaviour of the decaying velocity profile between the asperities. We find that this penetration depth δ can be rescaled into a simple exponential master curve δ = δM(1 -- e-kD/S) for horizontal ridges with varying depth D and spacing S. A similar variation of δ with D and S is observed for vertical ridges, but with a smaller magnitude δM, while for ridges at an angle, the penetration depth lies between the two extreme cases. [ABSTRACT FROM AUTHOR]

In this paper, an improved axisymmetric interfacial lattice Boltzmann flux solver abandoning the previous predictor-corrector scheme is proposed. Unlike the previous model starting from the two-dimensional standard lattice Boltzmann method (LBM), the present method is developed using the axisymmetric LBM, which directly incorporates the axisymmetric effects into the distribution functions. As a result, the proposed solver does not need the corrector step involving complicated space derivatives. It makes this method simpler and more computationally efficient. In the present solver, the resultant governing equation is globally resolved by the finite volume method, while the fluxes are reconstructed by local application of the axisymmetric LBM. Therefore, the inconsistency between the local reconstruction and the global governing equation can be eliminated because the global equation can be strictly derived from the axisymmetric LBM, which holds stronger physical basis than the previous method. Numerical experiments about the interface capturing and the multiphase flows are conducted to test the proposed model. Results show that the present method is superior to the fractional step solver in terms of the accuracy, stability, and computational efficiency. In addition, this solver has the capacity of simulating large-density-ratio and complex interfacial change. [ABSTRACT FROM AUTHOR]