Variable Buoyancy Research Articles

3D dynamics and control of a snake robot in uncertain underwater environment

Abstract The snake robot can be used to monitor and maintain underwater structures and environments. The motion of a snake robot is achieved by lateral undulation which is called the gait pattern of the snake robot. The parameters of a gait pattern need to be adjusted for compensating environmental uncertainties. In this work, 3D motion dynamics of a snake robot for the underwater environment is proposed with vertical motion using the buoyancy variation technique and horizontal motion using lateral undulation. “The neutral buoyant snake robot motion in hypothetical plane and added mass effect is negligible”, these previous assumptions are removed in this work. Two different control algorithms are designed for horizontal and vertical motions. The existing super twisting sliding mode control (STSMC) is used for the horizontal serpentine motion of the snake robot. The control law is designed on a reduced-ordered dynamic system based on virtual holonomic constraints. The vertical motion is achieved by controlling the mass variation using a pump. The water pumps are controlled using the event-based controller or Proportional Derivative (PD) controller. The results of the proposed control technique are verified with various external environmental disturbances and uncertainties to check the robustness of the control approach for various path following cases. Moreover, the results of STSMC scheme are compared with SMC scheme to check the effectiveness of STSMC. The practical implementation of the work is also performed using Simscape Multibody environment where the designed control algorithm is deployed on the virtual snake robot.

Variable Structure Controller for Energy Savings in an Underwater Sensor Platform.

This paper introduces a new variable structure controller designed for depth control of an autonomous underwater sensor platform equipped with a variable buoyancy module. To that end, the prototype linear model is presented, and a finite element-based method is used to estimate one of its parameters, the hull deformation due to pressure. To manage potential internal disturbances like hull deformation or external disturbances like weight changes, a disturbance observer is developed. An analysis of the observer steady-state estimation error in relation to input disturbances and system parameter uncertainties is developed. The locations of the observer poles according to its parameters are also identified. The variable structure controller is developed, keeping energy savings in mind. The proposed controller engages when system dynamics are unfavorable, causing the vehicle to deviate from the desired reference, and disengages when dynamics are favorable, guiding the vehicle toward the target reference. A detailed analysis determines the necessary switching control actions to ensure the system reaches the desired reference. Finally, simulations are run to compare the proposed controller's performance with that of PID-based controllers recently developed in the literature, assessing dynamic response and energy consumption under various operating conditions. Both the VBM- and propeller-actuated vehicles were evaluated. The results demonstrate that the proposed controller achieves an average energy consumption reduction of 22% compared to the next most efficient PID-based controller for the VBM-actuated vehicle, though with some impact on control performance.

Depth Control of an Underwater Sensor Platform: Comparison between Variable Buoyancy and Propeller Actuated Devices.

Underwater long-endurance platforms are crucial for continuous oceanic observation, allowing for sustained data collection from a multitude of sensors deployed across diverse underwater environments. They extend mission durations, reduce maintenance needs, and significantly improve the efficiency and cost-effectiveness of oceanographic research endeavors. This paper investigates the closed-loop depth control of actuation systems employed in underwater vehicles, focusing on the energy consumption of two different mechanisms: variable buoyancy and propeller actuated devices. Using a prototype previously developed by the authors, this paper presents a detailed model of the vehicle using both actuation solutions. The proposed model, although being a linear-based one, accounts for several nonlinearities that are present such as saturations, sensor quantization, and the actuator brake model. Also, it allows a simple estimation of the energy consumption of both actuation solutions. Based on the developed models, this study then explores the intricate interplay between energy consumption and control accuracy. To this end, several PID-based controllers are developed and tested in simulation. These controllers are used to evaluate the dynamic response and power requirements of variable buoyancy systems and propeller actuated devices under various operational conditions. Our findings contribute to the optimization of closed-loop depth control strategies, offering insights into the trade-offs between energy efficiency and system effectiveness in diverse underwater applications.

Numerical investigation on the impact of circumferentially varying temperature on the buoyancy and heat transfer characteristics of supercritical CO2 natural circulation loop

The non-uniform heating in the supercritical natural circulation loop (SCNCL) and its associated heat transfer behaviour and buoyancy variation are not well understood. Since buoyancy is the primary force that drives fluid motion in the SCNCL, any factor affecting buoyancy will have an impact on overall performance. The present study investigated the circumferentially varying temperature (CVT) at the source, compared it to constant uniform temperature conditions, and examined the associated variation in buoyancy, and heat transfer characteristics. This study establishes a numerical model for a three-dimensional circular cross-section rectangular SCNCL. The SCNCL was examined at different pressures ranging from 8 MPa to 12 MPa, with 1 MPa pressure increments, to cover a wide range of applications. The results indicated that at 12 MPa pressure, the heat transfer rate and heat transfer coefficient decreased by a maximum of 14 % and 50 %, respectively, compared to that of constant uniform temperature. In contrast, the mass flow rate shows minimal variation between the CVT and a constant temperature condition. The fluid density increases significantly at higher pressure, resulting in decreased buoyancy generation in the loop. Therefore, for all cases, velocity increases up to 10 MPa and then begins to decrease. Thus, the peak velocity was observed at 10 MPa pressure. The CVT negatively impacts both horizontal and vertical heater configurations, leading to a reduction in overall performance. Additionally, non-uniform heating has a greater impact on the horizontal heater configuration compared to the vertical heater configuration.

Design and extensibility analysis of a variable buoyancy system for small autonomous underwater vehicles

PurposeA switching depth controller based on a variable buoyancy system (VBS) is proposed to improve the performance of small autonomous underwater vehicles (AUVs). First, the requirements of VBS for small AUVs are analyzed. Second, a modular VBS with high extensibility and easy integration is proposed based on the concepts of generality and interchangeability. Subsequently, a depth-switching controller is proposed based on the modular VBS, which combines the best features of the linear active disturbance rejection controller and the nonlinear active disturbance rejection controller.Design/methodology/approachThe controller design and endurance of tiny AUVs are challenging because of their low environmental adaptation, limited energy resources and nonlinear dynamics. Traditional and single linear controllers cannot solve these problems efficiently. Although the VBS can improve the endurance of AUVs, the current VBS is not extensible for small AUVs in terms of the differences in individuals and operating environments.FindingsThe switching controller’s performance was examined using simulation with water flow and external disturbances, and the controller’s performance was compared in pool experiments. The results show that switching controllers have greater effectiveness, disturbance rejection capability and robustness even in the face of various disturbances.Practical implicationsA high degree of standardization and integration of VBS significantly enhances the performance of small AUVs. This will help expand the market for small AUV applications.Originality/valueThis solution improves the extensibility of the VBS, making it easier to integrate into different models of small AUVs. The device enhances the endurance and maneuverability of the small AUVs by adjusting buoyancy and center of gravity for low-power hovering and pitch angle control.

Use of VEGA data to analyse balloon options for possible subsequent long endurance Venus cloud layer missions

Abstract The vertical motions and buoyancy variations of the two VEGA super-pressure balloons, flown in the middle cloud layer of Venus, are discussed. Using data derived from these 1985 nightside flights, estimates are made of the energy required to operate some alternative balloon platform schemes under consideration for future-proposed Venus-atmosphere in situ science missions. Despite the dissimilarity of these alternative platform schemes, the energy inputs required to operate each scheme on the Venus nightside are found to be similar. Estimates of the associated mass penalties of the associated energy sources are also made. Further investigation of a vertical propulsive assist scheme is recommended.

DESIGN AND ANALYSIS OF BUOYANCY ENGINE SYSTEMS ON CHANGES IN BUOYANCY FORCES IN UNDERWATER GLIDER

The vast and deep conditions of the ocean still contain many objects that have not been explored. Based on the type of autonomous underwater vehicle (AUV) device design, it consists of types that can drift, glide, and can propel itself in water. Types of AUV without propellers (nonpropelled, drifter). or glider), can float without thrusters or glide up and down the water by changing its buoyancy. This AUV uses little or no battery power compared to propeller-driven AUVs, but cannot perform as many maneuvers. Variable Buoyancy System used in different types of underwater craft, designed and built for specific rides and applications. Therefore, each system is completely different from the others. This research focuses on systems that raise and lower volume, using components such as diaphragms, bladders, and bellows. It is possible to use a diaphragm for a buoyancy engine to make the vehicle easier to fabricate. The aim of this research is to determine the magnitude of deformation of the diaphragm made from silicon rubber and EPDM (ethylene propylene diene monomer) with varying thickness of 0.5 mm; 1mm and 2mm. Through the Finite Element Method approach to obtain deformation analysis modeling, EPDM material provides higher deformation than using silicon material. However, for pressures of no more than 0.5 bar, using a rubber thickness of 0.5 mm is not suitable. So the appropriate application for Gliders is 1 mm thick.

Light-powered sustained chaotic jumping of a liquid crystal elastomer balloon

Chaotic systems comprising of active materials have the capability to absorb energy directly from the external environment to preserve their own sustained motion, holding tremendous potential for utilization in diverse fields, for instance, controllable sensors, energy harvesting, motion behavior control of smart materials and more. In this paper, a sustained jumping system consisting of photosensitive liquid crystal elastomer (LCE) balloon is developed innovatively, in which the LCE balloon can transition from a static state to a sustained jumping state under periodic illumination. Given the dynamic constitutive model of LCE and Hertz contact theory, the nonlinear dynamics model of the sustained jumping LCE balloon system is established. Numerical calculations demonstrate the existence of two jumping modes for the LCE balloon, i.e., periodic jumping and chaotic jumping. The mechanism of sustained jumping that counterbalances the energy dissipation by the net work combining contact expansion and buoyancy variation, as well as the mechanism of chaotic phenomena, are revealed. In addition, the influences of vital parameters on the jumping mode of the balloon are examined, and the conversion of the jumping modes with parameters is illustrated by bifurcation diagrams. This work can broaden the knowledge of the dynamic behavior of smart materials, which is of guiding significance for the research on chaos control and chaotic system design.

Analytical Solutions to Problems of Wave Dynamics of Stratified Media with Variable Buoyancy Frequency

Analytical Solutions to Problems of Wave Dynamics of Stratified Media with Variable Buoyancy Frequency

Electrohydraulic and Electromechanical Buoyancy Change Device Unified Vertical Motion Model

Depth control is crucial for underwater vehicles, not only to perform certain tasks that require the vehicle to be still at a given depth but also because most propeller-driven vehicles waste a considerable amount of energy to counteract the passively tuned positive buoyancy. The use of a variable buoyancy system (VBS) can effectively address these items, increasing the energetic efficiency and thus mission length. Achieving accurate depth controllers is, however, a complex task, since experimental controller development in sea or even in test pools is unpractical and the use of simulation requires accurate vertical motion models whose parameters might be difficult to obtain or measure. The development of simple, yet comprehensive, dynamic models for devices incorporating VBS is therefore of upmost importance, as well as developing procedures that allow a simple determination of their parameters. This work contributes to this field by deriving a unified model for the vertical motion of a VBS actuated device, irrespective of the specific technological actuation solution employed, whether it be electromechanical or electrohydraulic. A concise analysis of the open-loop stability of the unified model is presented and a straightforward yet efficient procedure for identifying several of its parameters is introduced. This identification procedure is designed to be convenient and can be carried out in shallow waters, such as test pools, while its results are applicable to the deeper water model as well. To validate the procedure, experimental values obtained from an electromechanical VBS actuated device are used. Closed-loop control of the electromechanical VBS actuated device is conducted through simulation and experimental tests. The results confirm the effectiveness of the proposed unified model and the parameter identification methodology.

Accelerating turbulence in heated micron tubes at supercritical pressure

The purpose of this research was to provide further understanding of turbulent dynamics and heat transfer mechanisms in accelerating flows with thermophysical variations and pressure drops in micron tubes. Direct numerical simulations were conducted to investigate the turbulence to supercritical pressure ${\rm CO}_2$ in heated micron tubes with inner diameter $99.2~\mathrm {\mu }$ m. In general, the turbulent heat transfer enhancement/deterioration at supercritical pressure is dominated by variations in thermophysical properties, buoyancy and thermal acceleration; however, the mechanism differs in micron tubes ( $d^* < 100\ \mathrm {\mu }$ m). The results showed that the pressure drop and scale effect made significant contributions to the development of turbulence flows heated at supercritical pressure in micron tubes, leading to the prominent property change and flow acceleration in the inlet fully developed turbulent flow. The deviation on temperature distribution because of pressure changes was non-negligible. The primary contribution of the acceleration was the decay of a boundary layer, which significantly suppressed the production of turbulence and decreased heat transfer. The acceleration had stabilizing effects on the ejection and sweep motions of the turbulent flow. The high-speed fluid contributed to a new disturbance scenario of the flow with a larger spanwise wavenumber superimposed on existing perturbations. The high-speed streak width in the quasilaminar region was approximately $150$ – $160\nu /u_\tau$ in accelerating flow. In the micron tubes, the Reynolds stress events of quadrant Q4 contributed 60 % of the Reynolds stress, greater than those of quadrant Q2.

Quantifying Self-Noise of the Seaglider AUV Using a Passive Acoustic Monitor

Abstract The Seaglider, a type of underwater glider, is a relatively quiet vehicle in comparison with propelled autonomous underwater vehicles, making it a desirable acoustic receiving platform. Vehicle operations, such as pumping/bleeding oil to change buoyancy, shifting/rotating the battery to change pitch/roll, and oceanographic data collection, do, however, produce some self-noise. This system performance study analyzes the prevalence, frequency content, duration, and levels of self-noise associated with vehicle operations using data collected with a passive acoustic monitoring system mounted on the body of a Seaglider vehicle. Guidance and control functions, including pitch, roll, and buoyancy changes, were the major source of platform noise, producing broadband noise ranging from less than a second to over 3 min in duration, with sound pressure levels of 120‐145.5 dB re 1μPa. Frequencies below 10 kHz were the most impacted by self-noise, with a maximum 1/3 octave level of 137.5 dB re 1μPa in the 2.5kHz band caused by the pumping of oil in the variable buoyancy device. Guidance and control changes occurred during 4%‐13% of the dive for dives greater than 500 m. The bulk of these operations, however, were performed near the surface and apogee of the dive and typically affected only about 6% of the dive cycle duration for deep dives.

A composite phase change material with large volume change rate for thermal underwater glider

The thermal underwater glider (TUG) is an important device currently used for exploring ocean resources and monitoring ocean temperature. The TUG harvests ocean thermal energy (OTE) through working medium and converts into net buoyancy variation for propulsion. The phase change material (PCM) filled in the thermal engine is the key working fluid for the TUG to harvest OTE. A large volume change rate of PCM at ocean temperature difference is important for improving the TUG's efficiency and performance. Previous studies focus on N-hexadecane for OTE conversion with limited performance. This paper proposes N-hexadecane (C16) based composite PCM for an improved thermal-to-mechanical conversion process. The experimental test results show that the composite PCM (C16:C24 = 9:1 wt%) exhibits a similar phase transition temperature (17.8–18.4℃) with C16 (18.2–18.6℃). As compared to pure C16, the volume shrinkage of this composite PCM was 26.67% higher than that of C16 and the volume expansion rate was 30.74% higher than that of C16, indicating that the composite PCM based TUG could output more net buoyancy variation and propulsion power. Following experimental test, molecular dynamics simulation of the composite PCM is carried out. The reason for this volume expansion/shrinkage improvement was revealed as the larger C–C bond length and H–C–C bond angle variations. The composite materials provided in this paper could potentially improve the propulsion performance of TUG.

Variable buoyancy anchor deployment analysis for floating wind applications using a Marine Simulator

To study the feasibility of deploying a novel type of anchor with variable buoyancy for mooring floating offshore wind turbines, a set of detailed modelling studies was performed in the state of-the-art Marine Simulator at the National Decommissioning Centre. The aim of the multiphysics simulations is to assess fully a proposed craneless deployment method that involves towing the anchor from the harbour to the installation site, pumping liquid ballast to overcome anchor’s buoyancy and lowering it to the seabed using only a winch, thereby simplifying the process, and reducing installation costs. As a test case, a novel shape of the floating anchor is considered, to establish the feasibility of its deployment in conjunction with the variable buoyancy technology and installation sequence. The analysis is divided into three sections: characterisation of the anchor buoyancy, positioning the anchor under the stern of the vessel and the controlled descent of the anchor to the seabed, under varying weather and operational conditions (e.g significant wave height, current, winch velocity, liquid ballast mass, ballast pump rate). The analysis allows assessment of the importance of the different factors affecting the proposed deployment scenario of variable buoyancy anchors, such as the winch velocity, the ballast mass and the pump rate.

EXPERIMENTAL STUDY OF A VARIABLE BUOYANCY SYSTEM FOR LOW DEPTH OPERATION

Autonomous Underwater Vehicles (AUVs) are used for the underwater survey both in coastal areas and deep sea without any human intervention during their operations. However, high energy consumptions required by the thrusters of the AUV’s adversely affect their performance such as range and endurance, etc. Herein, we have presented a novel design and development of a standalone variable buoyancy system for autonomous underwater vehicle and investigated the numerical and experimental analysis of Variable Buoyancy System (VBS) in standalone mode. Design idea is based upon the ‘Pump Driven Variable Buoyancy System (PDVBS)’ using ‘Water Hydraulic Variable Buoyancy System (WHVBS)’ method to control the buoyancy using a diaphragm type positive displacement pump (PDP) with capacity of the maximum buoyancy change rate of 4.5 kg/min. Detail experimental and simulation performance of the developed VBS has been investigated. Developed VBS is tested for maximum depth of 5 m and compared the performance of the experimental and simulation results. Presented results demonstrate that designed VBS is effective in changing buoyancy and controlling heave velocities and that will result to achieve the higher range and endurance and better performance in rescue/attack operations.

Inclination‐Enabled Generalized Microfluid Rectifiers via Anisotropic Slippery Hollow Tracks

The power of spontaneous maneuvering of microfluids that allow gas or liquid to penetrate along a single direction, is usually endowed with the wettability gradient and the Laplace pressure difference through tailoring special solid‒gas interfacial geographies. Herein, inspired by the structures on rice leaves and the slippery surfaces of the pitcher plant, anisotropic slippery hollow tracks (ASHTs) are fabricated and thus propose a class of unique inclination‐enabled bubble “diode”, on which one‐way bubble penetration can be reversibly switched on/off. The minimum transition tilt angle is subject to the tract spacing and the infused oil amount. The underlying mechanism of bubble penetration is attributed to the competition between the variable buoyancy component and the interfacial energy gradient perpendicular to the samples. A wide spectrum of generalized microfluid maneuvering capabilities is further demonstrated, including heavy oil selective penetration, underwater CO2 gas collection, and even dry/wet environment management in smart buildings, which are challenging for conventional counterparts. This framework should facilitate the development of microfluid control with favorable retainability and wide applicability in multiple fluids.

Nezha-F: Design and Analysis of a Foldable and Self-Deployable HAUV

This letter introduces a small hybrid aerial underwater vehicle (HAUV), which we named Nezha-F, that can fly in the air, perform vertical profiling underwater, and vertically take off and land from both the water surface and ground. A foldable and self-deployable arm mechanism linked to and driven by a piston variable buoyancy system (PVBS) is proposed to reduce the excessive underwater drag caused by aerial structures. By having a compact size and successfully balanced aerial and underwater performance without adding excessive actuators, this design provides a feasible idea for the miniaturization of amphibious floats. The dynamic characteristics of the small PVBS are linear fitted, and modeled. The originally nonlinear actuator performance is linearized by the post-fitting mapping. Asymmetric dead zones of the actuator are removed by adding compensation to the algorithm. During a 10-day field test, the vehicle showed good aerial performance and underwater control effect. Several full mission cycle tests proved the vehicle's ability in semi-autonomous operation and robust domain crossing and verified the vehicle's endurance during each mission stage.

Numerical investigation on regulation and suppression of heat and mass transfer by varying thermal and solutal buoyancy force

In this study, a three-dimensional numerical analysis is presented for regulation and suppression of heat and mass transfer by varying thermal and solutal buoyancy force with a rotating cylinder placed at the center of the cavity. The energy and concentration equations are coupled by Dufour and Soret parameters to have a mutual effect of concentration and temperature on heat and mass transfer. The thermal buoyancy in the flow is adjusted by varying Rayleigh number of Ra = 104, 105, 106 and the mixed convection in the flow is regulated by varying Richardson number of Ri = 0.5, 1, 1.5 at unity buoyancy ratio (N = 1), Soret number, Lewis number and Dufour number. The present heat and mass transfer solver is developed and validated using the open-source computational fluid dynamics (CFD) package OpenFOAM 5.0. The two vertical opposite sides of the cavity are maintained as isothermal and isosolutal (iso-concentration), and the remaining four surfaces with rotating cylinder are kept as adiabatic. The present analysis reveals the impact of the mutual coupling of heat and mass transfer with the presence of thermal buoyancy, solutal buoyancy including mixed convection flow. The increase in the forced circulation at fixed thermal and solutal buoyancy force increases the heat and mass transfer. The variation of Nu avg and Sh avg is observed to be steady even with increasing Ra, then changes to periodic and chaotic with the implementation of a rotating cylinder. The detailed analysis on the variation of thermal buoyancy at unity buoyancy ratio, forced convection is reported by plotting streamlines, temperature and concentration contours, average Nusselt and Sherwood number.

A Revised Estimate of Early Pliocene Global Mean Sea Level Using Geodynamic Models of the Patagonian Slab Window

AbstractPaleoshorelines serve as measures of ancient sea level and ice volume but are affected by solid Earth deformation including processes such as glacial isostatic adjustment (GIA) and mantle dynamic topography (DT). The early Pliocene Epoch is an important target for sea‐level reconstructions as it contains information about the stability of ice sheets during a climate warmer than today. Along the southeastern passive margin of Argentina, three paleoshorelines date to early Pliocene times (4.8–5.5 Ma), and their variable present‐day elevations (36–180 m) reflect a unique topographic deformation signature. We use a mantle convection model to back‐advect present‐day buoyancy variations, including those that correspond to the Patagonian slab window. Varying the viscosity and initial tomography‐derived mantle buoyancy structures allows us to compute a suite of predictions of DT change that, when compared to GIA‐corrected shoreline elevations, makes it possible to identify both the most likely convection parameters and the most likely DT change. Our simulations illuminate an interplay of upwelling asthenosphere through the Patagonian slab window and coincident downwelling of the subducted Nazca slab in the mantle transition zone. This flow leads to differential upwarping of the southern Patagonian foreland since early Pliocene times, in line with the observations. Using our most likely DT change leads to an estimate of global mean sea level of 17.5 ± 6.4 m (1σ) in the early Pliocene Epoch. This confirms that sea level was significantly higher than present and can be used to calibrate ice sheet models.

On the Development of a Mid-Depth Lagrangian Float for Littoral Deployment

This study presents the complete, detailed development process of an enhanced one-man portable Lagrangian float designed for littoral deployment to depths of up to 300 m. The design focused on maximization of the Lagrangian characteristics of the hull, minimization of the noise emission and energy efficiency of the propulsion system, and the versatility of the platform for various scientific missions. The platform is propelled by a variable buoyancy engine that is actuated by an oil-submerged, gas-pressure assisted micro gear pump. The pressure assistance lowers the pressure differential across the pump ports at depth, resulting in quieter and more efficient operation. An enhanced proportional–integral–differential control scheme is employed to pilot the platform. To enhance diving safety, a software safety agent was incorporated. If the software safety agent detects a major failure, a drop weight is released. To eliminate the chance of water ingress through dynamic hull penetration, the drop weight is actuated by an in-house developed magnetic coupling mechanism. An onboard installed hydrophone continuously records and monitors ambient sounds for phenomena of interest and enables commands and mission updates from the surface. For surface recovery, the platform is equipped with GPS and an Iridium beacon for long-range localization, and an RF beacon and strobe for short-range localization and as a backup. The performance of the platform is demonstrated in a simulation and in an actual real sea mission conducted in the eastern Mediterranean at a depth of 10 and 12 m.