Hydrodynamic Performance Enhancement of Unmanned Underwater Gliders with Soft Robotic Morphing Wings for Agility Improvement
Authors: A. Giordano, G. De Meurichy, V. Telazzi, C. Mucignat, I. Lunati, D. A. L. M. Louchard, M. Iovieno, S. F. Armanini, M. Kovac
Organizations: Laboratory of Sustainability Robotics, Empa, 8600 Dübendorf, Switzerland. · Department of Aeronautics, Imperial College London, SW7 2AZ London, United Kingdom. · Laboratory for Computational Engineering, Empa, 8600 Dübendorf, Switzerland. · Dipartimento di Ingegneria Meccanica e Aerospaziale (DIMEAS), Politecnico di Torino, 10129 Torino, Italy. · EPFL, 1005 Lausanne, Switzerland.
This work assesses the hydrodynamic efficiency of Underwater Unmanned Vehicles (UUVs) equipped with soft morphing wings compared to conventional rigid wings. Unlike rigid wings, deformable counterparts can alter their aerodynamic properties on demand. Improvements in hydrodynamic efficiency extend a UUV's operational range and may determine mission feasibility. Structural and Computational Fluid Dynamics (CFD) simulations were conducted for both a soft morphing wing and a UUV incorporating it. The results show that a UUV employing soft wings achieves 9.75 percent higher overall efficiency than an equivalent vehicle with traditional rigid wings. These findings confirm the potential of soft robotics to enhance underwater vehicle performance, particularly in applications requiring pressure-agnostic operation.
Figures & tables
Fig. 1: Cross-sectional view of the soft morphing wing showing its profile and internal structure for all deformation levels tested in [ Giordano2024 ] . From top to bottom: 0ml , 30ml , 60ml , 90ml , and 120ml . All profiles shown are the results of the simulations performed for the present work.
Property
Aluminium
Ecoflex 00-50
PLA
Density [kg m -3 ]
2.70\times103
1.07\times103
1250
Young’s Mod. [MPa]
6.90\times104
–
3.50\times103
Poisson’s Ratio
3.30\times10−1
–
2.00\times10−1
Bulk Mod. [MPa]
6.96\times104
–
1.94\times103
Shear Mod. [MPa]
2.60\times104
–
1.46\times103
C10 [MPa]
–
4.76\times10−2
–
TABLE I: Material Properties of Aluminium, Ecoflex 00-50, and PLA
Fig. 2: Correlation between the inflation levels of the soft wing [ Giordano2024 ] and the isotropic pressure field applied in the simulations to reproduce equivalent deformations. A second-order polynomial fit P(I)=−0.025I2+0.7273I−0.1543 was applied to the four experimental data points, enabling the extrapolation of 3 additional inflation levels (yellow).
Fig. 3: Camber profiles of the soft wing from the experiments performed in [ Giordano2024 ] , Fig. 5 (solid black lines), and corresponding profiles as an output of the structural simulations performed for the present work (coloured dots).
Inflation
RMS Error
Max Error
Pressure
[ mL]
[mm]
[mm]
[kPa]
0
0.921
1.60
0.00
30
0.835
1.26
18.8
60
0.632
1.05
35.5
90
0.301
0.543
44.3
120
1.07
1.60
51.2
TABLE II: Comparison between inflation, input pressure, root mean square (RMS) and maximum (max) error.
Fig. 4: Hydrodynamic characterisation of the soft wing at Re2≃9.20\text{\times}{10}^{4} (free-stream velocity $U_{2}=$0.40\text{\,}\mathrm{m}\text{/}\mathrm{s} ). Solid lines represent the water tunnel measurements, as reported in [ Giordano2024 ] , while the dotted lines correspond to the force values obtained from the CFD simulations performed for the present work.
Fig. 5: Domain of all CFD simulation performed by the authors and involving an underwater glider. The cylinder represents the UUV and has a total length of IJ=LUUV . The parallelepiped of vertices A, B, C, D, E, F, G, and H delimits the water volume. La=Lb=Lh=1.5⋅LUUV , Lf=5⋅LUUV .
Fig. 6: Two views of the 3D rendering of the simulated Underwater Unmanned Vehicle (UUV) implementing the soft wing.
Fig. 7: CFD validation of the simulations using the Underwater Unmanned Vehicle (UUV) presented in [ singh2017cfd ] and [ ichihashi2008development ] .
Fig. 8: Left hand side : simulation of pitching moments q for each inflation tested experimentally at a flow-speed of 0.26ms−1 . Right hand side : simulation of rolling moments p for all tested inflations at α=0° . Each inflation shown in this plot is given under the assumption that the two wings are inflated in opposite directions ( 30 and −30 mL, 60 and −60 mL, 90 and −90 mL, and 120 and −120 mL).
Fig. 9: Hydrodynamic characterization of the selected Underwater Unmanned Vehicle (UUV) implementing the soft wing at all inflations tested in [ Giordano2024 ] and the interpolated 15ml . Free-flow speed U1=0.26\text{,}\mathrm{m}\text{/}\mathrm{s}$$ .
Underwater robotics develops autonomous and remotely operated machines for navigation, inspection, manipulation, and exploration in aquatic environments.
Miniature hybrid underwater gliders have attracted increasing attention for long-endurance ocean observation and confined-space inspection. Large-range wing reconfiguration offers a promising yet largely unexplored approach for simultaneously enhancing maneuverability and shape adaptability in constrained underwater environments. However, such morphing introduces substantial challenges in mechanical integration, dynamic modeling, and hydrodynamic characterization. This paper presents FoDeGlider, a miniature hybrid underwater glider equipped with two independently actuated wings capable of large-range folding and deflection. To capture configuration-dependent variations in mass distribution, center-of-geometry location, and hydrodynamic loading, a multibody dynamics model is developed by treating wing configuration as a structural variable. A composite rigid body algorithm (CRBA)-based projection formulates the composite inertia, wrench transformations, and component-level hydrodynamics into a unified Fossen-form dynamic model applicable to arbitrary wing configurations. A sequential parameter-identification framework is further proposed to estimate fuselage and wing hydrodynamic coefficients, resulting in an open benchmark dataset for model identification and validation. Extensive experiments are conducted, the results of which demonstrate accurate dynamic modeling and parameter identification across diverse morphing configurations. Gate traversal experiments further validate FoDeGlider's ability to actively reconfigure its morphology during locomotion, enabling enhanced navigation in confined underwater environments.
Yongjian Zhu, Yusen Tao, Feitian Zhang
Robotics and Control Laboratory, School of Advanced Manufacturing and Robotics, and the State Key Laboratory of Turbulence and Complex Systems, Peking University, Beijing 100871, China
Underwater robots are widely deployed for ocean exploration and manipulation. Underactuated mechanisms are advantageous in aquatic environments because reducing actuator count lowers motor-leakage risk while introducing inherent mechanical compliance. However, accurate modeling of underwater underactuated and soft robotic systems remains challenging, as it requires identifying high-dimensional structural and hydrodynamic parameters. In this work, we propose a trajectory-driven global optimization framework for unified structural-hydrodynamic modeling of underwater multibody systems. Inspired by the Covariance Matrix Adaptation Evolution Strategy (CMA-ES), the proposed approach simultaneously identifies coupled elastic, damping, and distributed hydrodynamic parameters through trajectory-level matching between simulated and experimental motion. This enables high-fidelity reproduction of underactuated mechanisms and compliant soft robotic systems in underwater environments, using as little as a single video recording. We first validate the framework on a link-by-link underactuated multibody mechanism, demonstrating accurate identification of distributed hydrodynamic coefficients, with normalized end-effector position error below 5% across multiple trajectories, initial conditions, and both active-passive and fully passive configurations. The modeling strategy is further validated on an asymmetric octopus-inspired soft arm, confirming its effectiveness for compliant soft robotic systems. Finally, eight identified arms are assembled into a swimming octopus robot, where the unified parameter set enables realistic whole-body behavior without additional retuning. These results demonstrate the scalability and transferability of the proposed structural-hydrodynamic modeling framework across underwater underactuated and soft robotic systems.
Chenrui Zhang, Yiyuan Zhang, Yunfei Ye +3
Department of Mechanical Engineering and Advanced Robotics Centre, National University of Singapore, Singapore. · Singapore-MIT Alliance for Research and Technology (SMART) Centre, 138602, Singapore.
In this paper, we present a generalized, comprehensive nonlinear mathematical model and conceptual design for the MetaMorpher, a metamorphic Unmanned Aerial Vehicle (UAV) designed to bridge the gap between vertical takeoff and landing agility and fixed-wing cruising efficiency. Building on the successful design of the spincopter platform, this work introduces a simplified mechanical architecture using lightweight materials and a novel wing-folding strategy. Unlike traditional rigid-body approximations, we derive a nonlinear flight dynamics model that enables arbitrary force distributions across a segmented wing structure. This modularity allows for testing different airfoils, mass distributions, and chord lengths in a single environment. As part of this work, various flight modes were specifically tested and analyzed in the Simulink environment. The results show that the model behaves predictably under different structural configurations, demonstrating its reliability as a tool for rapid design evaluation.
Anja Bosak, Dorian Erić, Ana Milas +1
All authors are with the University of Zagreb, Faculty of Electrical Engineering and Computing, LARICS Laboratory for Robotics and Intelligent Control Systems, Unska 3, 10000 Zagreb, Croatia