Search Results (101)

Underwater exploration relies heavily on autonomous underwater vehicles and sensor platforms for sustained monitoring of marine environments, yet their operational duration is limited by energy constraints. To enhance energy efficiency, various control strategies have been proposed, including robust, optimal, and disturbance-aware approaches. Recent work introduced a variable structure controller (VSC) with a constant-amplitude control action for depth control of a platform equipped with a variable buoyancy module, achieving an average 22% reduction in energy use in comparison with conventional PID-based controllers. In a separate paper, the conditions for its closed-loop stability were proven. This study extends these works by proposing a controller with a variable-amplitude control action designed to minimize energy consumption. A formal proof of stability is provided to guarantee safe operation even under conservative assumptions. The controller is applied to a previously developed depth-regulated sensor platform using a validated physical model. Additionally, this study analyzes how the controller parameters and mission requirements affect stability regions, offering practical guidelines for parameter tuning. A method to estimate oscillation amplitude during hovering tasks is also introduced. Simulation trials validate the proposed approach, showing energy savings of up to 16% when compared to the controller using a constant-amplitude control action. Full article

This paper presents a low-cost variable buoyancy system (VBS) that can be integrated into Hybrid Aerial Underwater Vehicles (HAUVs). HAUVs are widely used in scientific research and must be lightweight and highly maneuverable to ensure sufficient endurance and operational reliability in underwater/air environments. To meet these key requirements, this study designs a low-cost and sustainable VBS that can adjust the attitude and depth of the HAUV during underwater operations. By redesigning the pipeline structure, the number of airbags, and their placement, internal gas self-circulation was achieved, thereby reducing costs. The PID controller based on depth and attitude information was further developed to ensure that the VBS operated more stably under complex underwater conditions. Field experiments on the application of the designed VBS on an HAUV based on a PID controller showed that the HAUV with the VBS could maintain the desired robot attitude and vertical speed and perform stable vertical movements. Full article

Underwater exploration is vital for advancing scientific understanding of marine ecosystems, biodiversity, and oceanic processes. Autonomous underwater vehicles and sensor platforms play a crucial role in continuous monitoring, but their operational endurance is often limited by energy constraints. Various control strategies have been proposed to enhance energy efficiency, including robust and optimal controllers, energy-optimal model predictive control, and disturbance-aware strategies. Recent work introduced a variable structure depth controller for a sensor platform with a variable buoyancy module, resulting in a 22% reduction in energy consumption. This paper extends that work by providing a formal stability proof for the proposed switching controller, ensuring safe and reliable operation in dynamic underwater environments. In contrast to the conventional approach used in controller stability proofs for switched systems—which typically relies on the existence of multiple Lyapunov functions—the method developed in this paper adopts a different strategy. Specifically, the stability proof is based on a novel analysis of the system’s trajectory in the net buoyancy force-versus-depth error plane. The findings were applied to a depth-controlled sensor platform previously developed by the authors, using a well-established system model and considering physical constraints. Despite adopting a conservative approach, the results demonstrate that the control law can be implemented while ensuring formal system stability. Moreover, the study highlights how stability regions are affected by different controller parameter choices and mission requirements, namely, by determining how these aspects affect the bounds of the switching control action. The results provide valuable guidance for selecting the appropriate controller parameters for specific mission scenarios. Full article

Few studies have examined Hybrid Aquatic–Aerial Vehicles (HAAVs), autonomous vehicles designed to operate in both air and water, especially those that are aircraft-launched and recovered, with a variable-sweep design to free dive into a body of water and breach under buoyant and propulsive force to re-achieve flight. The novel design research examines the viability of a recoverable sonar-search child aircraft for maritime patrol, one which can overcome the prohibitive sea state limitations of all current HAAV designs in the research literature. This paper reports on the analysis from computational fluid dynamic (CFD) simulations of such an HAAV diving into static seawater at low speeds due to the reverse thrust of two retractable electric-ducted fans (EDFs) and its subsequent breach back into flight initially using a fast buoyancy engine developed for deep-sea research vessels. The HAAV model entered the water column at speeds around 10 ms⁻¹ and exited at 5 ms⁻¹ under various buoyancy cases, normal to the surface. Results revealed that impact force magnitudes varied with entry speed and were more acute according to vehicle mass, while a sufficient portion of the fuselage was able to clear typical wave heights during its breach for its EDF propulsors and wings to protract unhindered. Examining the medium transition dynamics of such a novel HAAV has provided insight into the structural, propulsive, buoyancy, and control requirements for future conceptual design iterations. Research is now focused on validating these unperturbed CFD dive and breach cases with pool experiments before then parametrically and numerically examining the effects of realistic ocean sea states. Full article

A barrier to the adoption of floating offshore wind turbines is their high cost relative to conventional fixed-bottom wind turbines. The largest contributor to this cost disparity is generally the floating substructure, due to its large size and complexity. Typically, a primary driver of the geometry and size of a floating substructure is the extreme environmental load case of Region 4, where platform loads are the greatest due to the impact of extreme wind and waves. To address this cost issue, a new concept for a floating offshore wind turbine’s substructure, its moorings, and anchors was optimized for a reference 10-MW turbine under extreme load conditions using OpenFAST. The levelized cost of energy was minimized by fixing the above-water turbine design and minimizing the equivalent substructure mass, which is based on the mass of all substructure components (stem, legs, buoyancy cans, mooring, and anchoring system) and associated costs of their materials, manufacturing, and installation. A stepped optimization scheme was used to allow an understanding of their influence on both the system cost and system dynamic responses for the extreme parked load case. The design variables investigated include the length and tautness ratio of the mooring lines, length and draft of the cans, and lengths of the legs and the stem. The dynamic responses investigated include the platform pitch, platform roll, nacelle horizontal acceleration, and can submergence. Some constraints were imposed on the dynamic responses of interest, and the metacentric height of the floating system was used to ensure static stability. The results offer insight into the parametric influence on turbine motion and on the potential savings that can be achieved through optimization of individual substructure components. A 36% reduction in substructure costs was achieved while slightly improving the hydrodynamic stability in pitch and yielding a somewhat large surge motion and slight roll increase. Full article

This study validates a three-zone lumped-parameter airflow model for Urban Building Energy Modeling, focusing on its accuracy in estimating air change rates caused by natural ventilation, referred to here as air change rate. The model incorporates urban-scale variables like canyon geometry and roughness elements for the accurate prediction of building infiltration, which is an important variable in building energy consumption. Air change rate predictions from the three-zone lumped-parameter model are compared against results from a three-zone CONTAM model across a range of weather scenarios. The study also examines the impact of building level of detail on air change rates. Results demonstrate that the three-zone lumped-parameter model achieves reasonable accuracy, with a maximum Mean Absolute Error of 0.1 h⁻¹ in winter and 0.03 h⁻¹ in summer compared to three-zone CONTAM model, while maintaining computational efficiency for urban-scale energy consumption simulations. However, its applicability is limited to buildings within urban canyons rather than detached structures, due to the assumptions made in the methodology of the three-zone lumped-parameter model. The results also showed that the model had lower errors for low to mid-rise buildings since the simplification of a detailed high-rise building into a three-zone model alters the buoyancy effect; a 4-story building showed Mean Absolute Percentage Error of 7% and 5% for a typical winter and summer day respectively when a detailed and simplified three-zone models are compared, while the error for a 16-story building were 18% and 12%. The results of building air change rates are used as input data in an hourly energy consumption model at urban scale and validated against measured hourly consumption to test the effect of the calculated urban-scale hourly air change rates. Full article

A pervasive and significant academic challenge confronted by students on a global scale is the phenomenon of test anxiety. This phenomenon is exacerbated in China, especially among final-year high school students who face college entrance exams. Perceived social support is widely regarded as the most prevalent protective factor against test anxiety. Academic buoyancy also demonstrates a significant correlation with test anxiety. However, there has been limited research on the potential relationship between perceived social support, academic buoyancy, and test anxiety. The purpose of this study is to examine the effects of specific sources and types of students perceived social support (e.g., emotional support from parents, teachers, and friends) on test anxiety and examine whether academic buoyancy serves as a mediating variable in the relationship between perceived social support and test anxiety. A total of 565 final-year high school students were selected as respondents from Heilongjiang Province in China. The result of SEM analysis indicated that the three sources of student-perceived emotional support could not predict test anxiety directly, but the students perceived three sources emotional support may have an indirect effect on test anxiety through the mediating role of academic buoyancy. In particular, perceived friend emotional support is the most beneficial among these sources of support for students. The anticipated outcomes of this study are expected to provide educators, counselors, and parents with key insights into the factors that alleviate test anxiety in high school students. Full article

Be stars are rapidly rotating B-type stars surrounded by circumstellar disks formed from self-ejected material. Understanding the mechanisms driving mass ejection and disk formation, known as the Be phenomenon, requires a detailed investigation of their variability and underlying physical processes. In this study, we analyze the photometric, spectroscopic, and seismic characteristics of three Be stars—HD 212044, 28 Cyg, and HD 174237—using high-cadence data from the TESS mission and spectral data from the BeSS database. Photometric variability was analyzed through iterative prewhitening and wavelet techniques, revealing distinct frequency groups associated with non-radial pulsations (NRPs). Spectral data provided equivalent width measurements of the H$\alpha$ line, which correlated with photometric changes, reflecting dynamic interactions between the stars and their disks. Seismic analysis identified core rotation rates and buoyancy travel times for HD 212044 and 28 Cyg, offering insights into internal stellar processes and angular momentum distribution. HD 212044 exhibits a strong negative correlation between photometric brightness and H$\alpha$ equivalent width, whereas this correlation is weaker in the case of 28 Cyg. The findings for these two stars highlight the interplay between NRPs, rapid rotation, and circumstellar disk dynamics. In contrast, the case of HD 174237 presents an example of how a binary system with mass transfer and a B-type component is revealed when observed simultaneously with space-based photometry and ground-based spectroscopy, demonstrating the importance of distinguishing classical Be stars from interacting binaries. Full article

Arthrospira platensis, a filamentous cyanobacterium, exhibits morphological variability influenced by biotic and abiotic factors. We investigated the effect of sodium ion concentration on filament length, growth, and harvest efficiency. Increasing the sodium concentration from 0.2 M to 0.4 M (using NaHCO₃ or Na₂CO₃) led to a significant increase in filament length, from 0.3393 to 0.7084 mm, and longer filaments had increased auto-flotation efficiency (from 87% to 94%) within 3 h. The linear filaments, obtained via spontaneous morphological conversion, also had increased photosynthetic activity and growth rates compared to coiled filaments, and we speculate this was due to decreased self-shading and increased light penetration. However, linear filaments also had poor auto-flotation efficiency (10% after 24 h) and decreased buoyancy, and this likely limits their survival in natural ecosystems. These findings provide insights into optimizing the cultivation of A. platensis for biomass harvesting. Full article

This paper presents the design of an electric amphibious vehicle with buoyancy provided by inflatable pontoons, referred to as the Electric Inflatable Pontoon amphibious vehicle (E-IPAMV). To investigate the effect of pontoon arrangements on resistance performance, maneuverability, seakeeping, transverse stability, and longitudinal stability of E-IPAMV, STAR-CCM+ and Maxsurf are used to solve the above performance parameters. A constrained space Latin hypercube experimental design is employed, using the lengths of the inflatable pontoons at five installation positions as input variables, and total resistance, steady turning diameter, maximum pitch angle, transverse metacentric height, and longitudinal metacentric height as output variables. A neural network model is then established and validated. Based on this model, NSGA-II is employed to optimize the pontoon lengths at the five installation positions, yielding Pareto-optimal solutions. Finally, considering project and manufacturing requirements, two optimized design schemes are proposed. Compared to the original design, optimization scheme 1 shows a slight reduction in seakeeping but improvements in other hydrodynamic performances. Meanwhile, optimization scheme 2 enhances all hydrodynamic performances. Specifically, in optimization scheme 2, maneuverability increases by the smallest amount, showing 23.43% improvement compared to the original design, while transverse stability sees the greatest improvement, increasing by 290.99% compared to the original design. Full article

This study investigates mixed convection flow over a vertical thin needle with variable surface heat, mass, and microbial flux, incorporating the influence of gyrotactic microorganisms. The governing partial differential equations are transformed into ordinary differential equations using appropriate similarity transformations and then solved numerically by employing MATLAB’s Bvp4c solver. The primary focus lies in examining the influence of various dimensionless parameters, including the mixed convection parameter, power-law index, buoyancy parameters, bioconvection parameters, and needle size parameters, on the velocity, temperature, concentration, and microbe profiles. The results indicate that these parameters significantly affect the surface (wall) temperature, fluid concentration, and motile microbe concentration, as well as the corresponding velocity, temperature, concentration, and microorganism profiles. The findings provide insights into the intricate dynamics of mixed convection flow with bioconvection and have potential applications in diverse fields such as biomedicine and engineering. Full article

The sea ice extent in the Weddell Sea exhibited a positive trend from the start of satellite observations in 1978 until 2016 but has shown a decreasing trend since then. This study analyzes seasonal and interannual variations in sea ice thickness using ICESat-2 laser altimetry data over the Weddell Sea from 2019 to 2022. Sea ice thickness was calculated from ICESat-2’s ATL10 freeboard product using the Improved Buoyancy Equation. Seasonal variability in ice thickness, characterized by an increase from February to September, is more pronounced in the eastern Weddell sector, while interannual variability is more evident in the western Weddell sector. The results were compared with field data obtained between 2019 and 2022, showing a general agreement in ice thickness distributions around predominantly level ice. A decreasing trend in sea ice thickness was observed when compared to measurements from 2003 to 2017. Notably, the spring of 2021 and summer of 2022 saw significant decreases in Sea Ice Extent (SIE). Although the overall mean sea ice thickness remained unchanged, the northwestern Weddell region experienced a noticeable decrease in ice thickness. Full article

In Cartesian coordinates $\left(x,y,z\right),$ the gradient Richardson number $Ri$ is the ratio between the square of the buoyancy frequency N and the square of the vertical shear S, $Ri=N^2/S^2$, where $N^2=-\left(g/\rho \right)\partial \rho /\partial z$ and $S^2={\left(\partial u/\partial z\right)}^2+{\left(\partial v/\partial z\right)}^2$, with ρ potential density, $\left(u,v\right)$ the horizontal velocity components and g gravity acceleration. In isopycnic coordinates $\left(x,y,\rho \right)$, $Ri$ is expressed as the ratio between ${M^2\equiv N}^{-2}$ and the squared diapycnal shear ${S_{\rho }}^2={\left(\rho /g\right)}^2\left[{\left(\partial u/\partial \rho \right)}^2+{\left(\partial v/\partial \rho \right)}^2\right]$, $Ri=M^2/{S_{\rho }}^2$. This could suggest that a decrease (increase) in stratification brings a decrease (increase) in dynamic stability in Cartesian coordinates, but a stability increase (decrease) in isopycnic coordinates. The apparently different role of stratification arises because S and $S_{\rho }$ are related through the stratification itself, $S_{\rho }=S/N^2$. In terms of characteristic times, this is equivalent to $\tau \equiv S_{\rho }={t_o}^2/t_d$, which is interpreted as a critical dynamic time $\tau$ that equals the buoyancy period $t_o\equiv N^{-1}$ normalized by the ratio $t_d/t_o$, where $t_d=S^{-1}$ is the deformation time. Here we follow simple arguments and use field data from three different regions (island shelf break, Gulf Stream and Mediterranean outflow) to endorse the usefulness of the isopycnal approach. In particular, we define the reduced squared diapycnal shear ${{\sigma }_{\rho }}^2={S_{\rho }}^2-M^2$ and compare it with the reduced squared vertical ${\sigma }^2=S^2-N^2$, both being positive (negative) for unstable (stable) conditions. While both Ri and ${\sigma }^2$ remain highly variable for all stratification conditions, the mean ${{\sigma }_{\rho }}^2$ values approach ${S_{\rho }}^2$ with increasing stratification. Further, the field data follow the relation ${{\sigma }_{\rho }}^2=\left(1-Ri\right)/\left(N^{2}Ri\right)$, with a subcritical $Ri=0.22$ for both the island shelf break and the Mediterranean outflow. We propose ${{\sigma }_{\rho }}^2$ and ${S_{\rho }}^2$ to be good indexes for the occurrence of effective mixing under highly stratified conditions. Full article

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. Full article

The design of patrol boats, especially in Indonesian waters with extreme sea conditions, requires good stability capabilities and self-righting moments. These conditions require patrol boats to have anti-capsized capabilities where, with these capabilities, the patrol boat can return to an upright position at extreme heeling angles. This study investigates how changing the center of gravity (CoG) due to the deckhouse height factor improves self-righting moment capabilities. Four different deckhouse heights are examined to find the optimal self-righting roll moment, with a deckhouse height in the 2.01–2.31 m range. In addition, the presence of the self-righting roll moment is also validated by the computational fluid dynamics (CFD) method using three different mesh sizes. The height of the deckhouse can significantly influence the ship’s stability. The initial investigation shows ships with minimum deckhouse heights of 2.06 m have positive righting lever arms at 170° and are classified as anti-capsized ships. It has been discovered that buoyancy and the center of gravity are crucial variables in obtaining the self-righting moment. The deckhouse’s height increases the stability of the ship’s righting arm by enhancing the metacenter point. The findings demonstrate that more excellent stability is achieved with a larger deckhouse height. Full article