The usage of composite material helps reducing structure’s weight in many conditions. In this paper, The Composite Ring Stiffened Cylindrical Hulls used by Unmanned Underwater Vehicles is optimized in various depths. The optimization aims to reducing the weight of the composite ring stiffened cylindrical hulls. Two candidate composite materials, T700/Epoxy and B(4)5505/Epoxy, are chosen to form the outer shell and stiffeners. Number and shape of the stiffeners, ply angles, and the number of the plies are considered. The Multi-Island Genetic Algorithm (MIGA) is performed in MATLAB and a new two-step method is proposed in crossover operations. ANSYS parametric design language is integrated in optimization procedure to ensure the acceptance of the optimal result. The results reveal that the proposed two-step method for multi-island genetic algorithm is efficiency and robust in reducing the weight of the composite ring stiffened cylindrical hulls. The weight for optimized composite ring stiffened cylindrical hulls with T700/Epoxy shells and B(4)5505/Epoxy stiffeners is 10.5% lighter than that with single composite material when the operating depth is 200 m.

Flapping foils are studied to achieve an efficient propeller. The performance of the flapping foil is influenced by many factors such as oncoming vortices, heaving amplitude, and geometrical parameters. In this paper, investigations are performed on flapping foils to assess its performance in the wake of a D-section cylinder located half a diameter in front of the foil. The effects of heaving amplitude and foil thickness are examined. The results indicate that oncoming vortices facilitate the flapping motion. Although the thrust increases with the increasing heaving amplitude, the propelling efficiency decreases with it. Moreover, increasing thickness results in higher efficiency. The highest propelling efficiency is achieved when the heaving amplitude equals ten percent of the chord length with a symmetric foil type of NACA0050 foil. When the heaving amplitude is small, the influence of the thickness tends to be more remarkable. The propelling efficiency exceeds 100% and the heaving amplitude is 10% of the chord length when the commonly used equation is adopted. This result demonstrates that the flapping motion extracts some energy from the oncoming vortices. Based on the numerical results, a new parameter, the energy transforming ratio (R_ET), is applied to explicate the energy transforming procedure. The R_ET represents that the flapping foil is driven by the engine or both the engines and the oncoming vortices with the range of R_ET being (0, Infini) and (−1, 0), respectively. With what has been discussed in this paper, the oncoming wake of the D-section cylinder benefits the flapping motion which indicates that the macro underwater vehicle performs better following a bluff body.

Multi-fidelity surrogate-based methods play an important role in modern engineering design applications, aiming to improve model accuracy while reducing computational cost. One of the widely adopted approaches is the calibration-based method, which calibrates the low-fidelity model through a discrepancy model between low-fidelity and high-fidelity models. Since discrepancies between models exhibit varying characteristics across different problems, using a single surrogate for discrepancy approximation may lack stability. In practical engineering design problems, it is often hard for designers to select optimal surrogate models. To this end, a hybrid-surrogate-calibration-assisted multi-fidelity modeling (HSC-MFM) approach is proposed in this paper. Specifically, this approach integrates three representative surrogate models, including the polynomial response surface, Kriging model, and radial basis function, to comprehensively capture the discrepancy characteristics between different fidelity models. Furthermore, an adaptive weight calculation method is developed to improve the modeling accuracy. Testing results demonstrate that HSC-MFM achieves enhanced stability compared to most existing methods while maintaining good prediction accuracy. Finally, the proposed method is applied to predict the strength of the frame for a blended-wing-body underwater glider, which verifies its engineering applicability.