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Developing an accurate dynamic model for an Autonomous Underwater Vehicle (AUV) is challenging due to the diverse array of forces exerted on it in an underwater environment. These forces include hydrodynamic effects such as drag, buoyancy, and added mass. Consequently, achieving precision in predicting the AUV's behavior requires a comprehensive understanding of these dynamic forces and their interplay. In our research, we have devised a linear data-driven dynamic model rooted in Koopman's theory. The cornerstone of leveraging Koopman theory lies in accurately estimating the Koopman operator. To achieve this, we employ the dynamic mode decomposition (DMD) method, which enables the generation of the Koopman operator. We have developed a Fractional Sliding Mode Control (FSMC) method to provide robustness and high tracking performance for AUV systems. The efficacy of the proposed controller has been verified through simulation results. • Linearize the nonlinear AUV dynamics by using Koopman theory. • Obtain Koopman operator using DMD method. • Apply fractional sliding mode control to the linearized model. • Reduce chattering phenomenon and improve tracking performance. [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects of ventilation parameters and motion characteristics on the drag reduction characteristics of microbubbles during the pitching motion of underwater vehicle. Based on the Population Balance Model (PBM), a numerical method is established that takes into account the phenomenon of microbubbles aggregation and breakage. The effects of the pitching frequency and ventilation volume coefficient on drag and lift are conducted in this study. The results show that the numerical method can accurately predict the distribution and force characteristics of microbubbles for the vehicle in pitching motion. Added mass force of the water phase is weakened when the vehicle is surrounded by microbubbles during the pitching motion. This causes a gradual decrease in the phase difference between the drag and lift coefficients under different pitching frequencies. Moreover, the drag rate of the microbubble is inversely related to the pitch angle. Namely, the drag reduction rate of microbubbles decreases with an increase in pitch angle. The drag coefficient decreases significantly when the vehicle is covered by the microbubbles. This leads to the phase of the drag coefficient slightly lagging behind the pitch angle. Furthermore, the lift coefficients of vehicle are similar for different ventilation flow rates. • The drag and lift coefficients' amplitudes and the phase difference increase with a higher pitching frequency. • The ventilation microbubbles have a significant drag reduction effect on the underwater vehicle with unsteady motion. • The effect of the ventilation volume coefficient on the lift coefficients is minimal. • The introduced microbubbles achieve drag reduction and provide essential normal force for the underwater navigation. [ABSTRACT FROM AUTHOR]

Effective drag-reduction technology is important for enhancing navigation speed, extending endurance, and enabling efficient energy usage of underwater vehicles. In this study, we investigated the effectiveness of a body of revolution to influence the turbulent boundary-layer properties by performing numerical simulations and varying a periodic sinusoidal force in a series of annular slots. The selected control parameters included the normalized periodic force amplitude and periodic force frequency; the effects of these parameters on drag reduction have not been demonstrated in previous studies. The surface pressure and velocity analyses revealed that frictional drag accounted for 71.7% of the total drag reduction, and it was the dominant contributor to the drag-reduction effect. Unsteady vortices were generated in the boundary layer, and a retarded fluid region was created on the surface. A clockwise vortex was generated downstream of the annular slots during blowing, reducing the pressure drag on the surface of the revolution body. The clockwise vortex weakened when suction was applied. The results of this research might provide ideas for solving the drag-reduction problem in the design of revolving-body-type underwater vehicles. • This study researched a periodic force in incompressible-fluid environment for a revolution body with tandem annular slots. • This study discussed the effect of the control positions on the drag-reduction. • A simple model was developed to correlate the periodic force amplitude and frequency with the instantaneous drag. • The drag-reduction mechanism of the periodic force was analyzed by visualizing the boundary layer and surface-drag variation. [ABSTRACT FROM AUTHOR]

The accurate and efficient prediction of the flow and temperature fields on water surface modulated by internal waves induced by underwater vehicles in a density-stratified ocean can serve as the basis for indirect detection. The disturbance of an underwater object on density-stratified water can be considered as the mixing of two incompressible fluids: freshwater and saltwater. Furthermore, the modulation effect of internal waves on the convergence and divergence of flow field and thermal skin layer is captured at the air–water interface. Therefore, in this study, a CFD solver called "interMixing_skin_Foam" based on OpenFOAM is proposed. It can manage three incompressible fluids, two of which disperse, and the interface between the immiscible fluids (air–water) is captured using the VOF technique. In addition, the energy equation is implemented to investigate the thermal signature variations at the air–water interface. Comparison analysis of experimental results and those obtained from the CFD solver "twoLiquidMixing_skin_Foam" developed in a previous study, wherein only two incompressible fluids were considered, is performed for standard cases in literature. Subsequently, the influence of the air–water interface on the flow and temperature fields is studied for a submerged sphere travelling horizontally in linearly and strongly density-stratified water. • The thermal signature variations of internal waves by underwater target at the air–water interface were investigated. • A solver based on OpenFOAM platform for flow and temperature fields of three incompressible fluids was developed. • The viscosity drag effect, surface tension, and movable free surface have significant influence on thermal signatures. • In strongly density-stratified water, the upper fresh-water layer could suppress the thermal signatures. [ABSTRACT FROM AUTHOR]

Undulating propulsion is promising solution for underwater vehicles used in subsea security systems, especially due to its low noise, better energetic efficiency, low water turbulence and living animals mimicry. The design of the biomimetic autonomous underwater vehicle (BAUV) for intelligence surveillance and reconnaissance (ISR), built in the SABUVIS project, is presented in the paper. The shape of the vehicle hull was inspired by living fish. Key aspects of the design and construction of BAUV needed to achieve tasks required to be performed by the robot are described. Two research questions are stated in the paper: how big drag forces act on a biomimetic underwater vehicle and how mechanical design influences the noise level generated by the vehicle. To answer these questions vehicle forward velocity, hull drag, added mass and generated noise were investigated. Forces acting on the BAUV hull while being pulled forward and sideways were measured. Noise generated by the vehicle was also measured for two types of propulsors' drives in order to select those with the lower sound pressure level. Quantitative results of drag and noise obtained in the research are important for designers of mechanical structures of useful, full scale biomimetic underwater robots and their control subsystems. • The hull of underwater vehicle design based on the shape of a real fish 3D scan. • Fish-like shaped hull of the vehicle is characterized by good drag coefficient. • The obtained drag coefficient values are smaller than for other fish-like vehicles. • Added mass can be as high as the mass of the biomimetic vehicle. • Noise of the biomimetic vehicle can be greatly reduced by proper design. [ABSTRACT FROM AUTHOR]

As humans accelerate the pace of marine development, autonomous underwater vehicles (AUVs) are increasingly attracting worldwide attention. Due to the limitations of carrying energy and battery technology, AUV's endurance is nonideal. Therefore, designers usually make AUVs more streamlined to reduce drag. Here we show that when a layer of porous material is attached to an AUV's surface, the AUV's drag changes significantly. In this paper, simulations of the basic body of a REMUS100 and SUBOFF submarine model were carried out under multiple conditions. It is found that the drag increases as the porous viscosity coefficient or the thickness of the porous material increases. When REMUS100 and SUBOFF models are attached to the porous material with suitable porous viscosity coefficient, their drag becomes smaller. Boundary layer theory is also used to explain and analyze the phenomenon of the proportional increase of viscous pressure drag when using porous material, which is verified by vertical plate numerical simulations. Finally, we tested the mechanical properties of porous nickel and aluminum alloy 6061, and found that the porous material does have an effect of drag reduction, and can reduce the fluctuation range of the drag during the movement. • Simulations of the basic body of a REMUS100 and SUBOFF submarine model were carried out under multiple conditions, including different porous viscosity coefficients and different porous material thicknesses. When the porous material with the viscosity coefficient of 100 kg/m3s was used as the pressure shell, the drag was reduced by 58.3% and 63%, respectively. • Boundary layer theory is used to explain and analyze the phenomenon of the proportional increase of differential pressure drag which is verified by vertical plate numerical simulation. • Porous nickel and aluminum alloy 6061 were selected as test objects to analyze their mechanical properties during motion. At the same velocity, the average drag of porous nickel in the constant speed stage and the maximum drag in the acceleration stage were about half of that of aluminum alloy, and porous nickel can reduces the fluctuation range of the drag. [ABSTRACT FROM AUTHOR]