No abstract

Recent studies indicate that nature-inspired thrusters based on flexible oscillating foils show enhanced propulsive performance. However, understanding the underlying physics of the fluid–structure interaction (FSI) is essential to improve the efficiency of existing devices and pave the way for novel energy-efficient marine thrusters. In the present work, we investigate the effect of chord-wise flexibility on the propulsive performance of flapping-foil thrusters. For this purpose, a numerical method has been developed to simulate the time-dependent structural response of the flexible foil that undergoes prescribed large general motions. The fluid flow model is based on potential theory, whereas the elastic response of the foil is approximated by means of the classical Kirchhoff–Love theory for thin plates under cylindrical bending. The fully coupled FSI problem is treated numerically with a non-linear BEM–FEM scheme. The validity of the proposed scheme is established through comparisons against existing works. The performance of the flapping-foil thrusters over a range of design parameters, including flexural rigidity, Strouhal number, heaving and pitching amplitudes is also studied. The results show a propulsive efficiency enhancement of up to 6% for such systems with moderate loss in thrust, compared to rigid foils. Finally, the present model after enhancement could serve as a useful tool in the design, assessment and control of flexible biomimetic flapping-foil thrusters.

Much work has been done over the past years to obtain a better understanding, predict and alleviate the effects of cavitation on the performance of lifting surfaces for hydrokinetic turbines and marine propellers. Lifting-surface sheet cavitation, when addressed as a free-streamline problem, can be predicted up to a desirable degree of accuracy using numerical methods under the assumptions of ideal flow. Typically, a potential solver is used in conjunction with geometric criteria to determine the cavity shape, while an iterative scheme ensures that all boundary conditions are satisfied. In this work, we propose a new prediction model for the case of partially cavitating hydrofoils in a steady flow that treats the free-streamline problem as an inverse problem. The objective function is based on the assumption that on the cavity boundary, the pressure remains constant and is evaluated at each optimization cycle using a source-vorticity BEM solver. The attached cavity is parametrized using B-splines, and the control points are included in the design variables along with the cavitation number. The sensitivities required for the gradient-based optimization are derived using the continuous adjoint method. The proposed numerical scheme is compared against other methods for the NACA 16-series hydrofoils and is found to predict well both the cavity shape and cavitation number for a given cavity length.

Bio-inspired thruster designs based on flapping-foils have the potential to achieve high efficiency and stealth, thus allowing for an extension of the overall operational capabilities of autonomous underwater vehicles (AUVs) propelled solely using foils. In this work, we produce thruster designs with enhanced propulsive performance by introducing prescribed chordwise and spanwise changes in the geometry during each flapping-cycle, i.e. active morphing, with optimally tuned parameters to further mimic aquatic locomotion. The reference design performs a thrust-producing combination of out-of-phase heaving and pitching motions, whereas for the evaluation of each candidate design, a cost-effective GPU-accelerated boundary element solver (BEM) is proposed.

Bio-inspired thrusters designed to mimic the propulsive capabilities and mechanisms of fish locomotion pose an alternative to the conventional autonomous underwater vehicle (AUV) propeller propulsion. In this work, we examine a particular configuration of a flapping-foil thruster, employed for AUV propulsion, operating at a constant speed that undergoes prescribed heaving and pitching oscillations about its pivot axis. An optimization study is performed to determine optimal kinematic parameters and flexural rigidity distribution. The performance assessment of the device is carried out using a fluid-structure interaction (FSI) model based on ideal fluid flow assumptions and Kirchhoff-Love plate theory for cylindrical bending. The proposed coupled boundary element and finite element method (BEM-FEM) has been compared against experimental data and has been found to be suitable for the prediction of the hydroelastic response of such systems. The solver predicts the hydrodynamic forces and the flexural response of the system by treating the non-linear FSI problem. A We comparative analysis between the rigid and the passively deforming flapping-foil thruster designs deduced during the proposed optimization process is performed to illustrate that incorporating bio-inspired features leads to considerable performance enhancement.