Descent Velocity Research Articles

Comparative Analysis of the Kinematic Characteristics of Lunge-Style and Squat-Style Jerk Techniques in Elite Weightlifters.

This study aimed to discuss the differences in technical characteristics between the lunge-style and squat-style jerk techniques and to reveal the adaptability of these techniques for individual weightlifters. A total of 52 attempts by 32 male weightlifters were selected, and the video data were digitized manually using the SIMI Motion 7.50 three-dimensional analysis system. The technical characteristics of the lunge split and squat jerk were fundamentally consistent during the pre-squat, force exertion, and inertia ascent phases. The primary differences between the lunge split and squat jerk techniques were observed during the squatting and support phases, including the vertical descent velocity of the barbell at the end of the squat shoulder-locking phase and the stability angles in the sagittal and coronal axes. The vertical velocity of the barbell at the end of the squat shoulder-locking phase was significantly greater in the squat style (-0.41 ± 0.17 vs. -0.88 ± 0.14) compared to the lunge style (t = 6.393, p < 0.05). The stability angle on the sagittal axis at the end of the squat-supporting phase in the lunge style was significantly greater (46.99 ± 3.23 vs. 13.64 ± 0.51) than that of the squat style (t = 45.639, p < 0.05).

Simulation study on heat and mass transfer characteristics within tubular moving bed heat exchangers

In the field of waste heat recovery, tubular moving bed heat exchangers (MBHEs) hold great promise as equipment capable of effectively handling large-sized granular materials with high temperature. To further clarify the heat and mass transfer characteristics within tubular MBHEs, this study firstly conducted detailed investigations on the contributions of different heat transfer mechanisms. The result shows that heat radiation (∼45 %) and gas film heat conduction (∼42 %) control the heat transfer processes involving tube walls, with an inlet particle temperature of 600 °C, tube wall temperature of 50 °C, and particle descent velocity of 2 mm/s. These contributions can be influenced by the inlet particle temperature and the tube wall temperature, rather than the particle descent velocity. Additionally, for a specific particle, its heat transfer state is highly depended on where it is located. Subsequently, the time-varying and space-varying characteristics of the heat transfer processes involving tube walls are studied. The results suggest that the time-varying and space-varying characteristics are inherent in tubular MBHEs due to the difficulty to achieve complete stability. Meanwhile, temporal fluctuations intensify with an increase in the inlet particle temperature, a decrease in wall temperature, or an increase in particle descent velocity. The result of power spectral density (PSD) indicates that the heat transfer processes exhibit a degree of chaotic behavior due to the nonlinear property of sub-models, as well as the randomness in particle feeding and contacts. The space-varying characteristic is mainly determined by the inlet particle temperature and the tube wall temperature, rather than the particle descent velocity. This characteristic can be observed in both the axial and circumferential directions, with the circumferential variation being more pronounced than the axial variation. Furthermore, a double-tube MBHE, a topic that has received scant attention in previous research, is investigated in comparison to a single-tube MBHE to explore the differences in heat and mass transfer between the two configurations. It indicates that under the same boundary conditions, there are differences in heat transfer results, which are attributed to the interactions between two vertically arranged tubes. These interactions are reflected in the reconstruction of particle flow patterns and the effect altering the temperature distribution for the particles or fluid surrounding the tubes. The obtained results provide valuable insights into the mass and heat transfer behaviors of particles and fluids within tubular MBHEs, laying a foundation for advancing the development of efficient tubular MBHEs.

Celestial Choreography: Optimizing Mars Mission Trajectories through Gravity Assists

Mars exploration has become a focal point for medical, technological, and human endeavours. The success of these missions closely depends on optimizing spacecraft trajectories to ensure performance and cost-effectiveness. This paper delves into the significance of trajectory optimization and the pivotal role gravity assists play in interplanetary missions, particularly those aimed at Mars. In our study, we applied advanced simulation tools, including MATLAB, to model and optimize mission trajectories. Our research also includes Computational Fluid Dynamics (CFD) simulations of the Mars probe, focusing on incompressible flow tests around the probe's frame. A novel aspect of our design is the addition of a turbo-dynamic fan mounted on the outer boundary of the rover. This innovation is intended to reduce the descent velocity of the probe through the Martian atmosphere, improving landing precision and safety. Furthermore, our study explores the Olympus Mons area, a towering 27 km high Martian volcano, as a prime location for sample collection. By optimizing the trajectory and fuel usage, our simulations show how samples can be efficiently collected and returned to Mars orbit, ultimately facilitating their return to Earth. The trajectory calculations and visualizations provided offer valuable insights for future missions, aiming to balance fuel efficiency and mission success. This research not only highlights the quantitative benefits of trajectory optimization through gravity assists but also showcases the practical implications of innovative design improvements in space exploration. The lessons learned from our simulations and analyses are poised to significantly impact future Mars missions, paving the way for more efficient and effective exploration strategies.

Numerical computation and experimental assessment of a pressure-retaining gas-tight sediment sampler

The pressure of the recovered sample is intricately connected to seawater temperature, the recovery velocity, and the pressure of the pre-charged gas. To better understand the sample pressure dynamics during the sampling recovery process, we focus on a gas-tight sediment sampler, delving into its pressure-compensation and pressure-retaining mechanisms. A comprehensive thermal and thermodynamic analysis is conducted throughout the entire pressure-retaining sampling process, examining the temporal variations in the temperatures of seawater and nitrogen within the sampler at various descending velocities. The heat transfer and thermodynamics are examined throughout the entire pressure-retaining sampling process to determine how changes in the temperatures of seawater and nitrogen, as well as the descent velocity, affect the pressure-retaining performance. The influence of pre-charging pressure and recovery velocities on the pressure-retaining performance of the sampler is examined. Then the proposed numerical model was well verified by the ultra-high-pressure vessel experiments of the sampler under 115 MPa. Finally, the sea trial results further verified the accuracy of the numerical model.

An Improved Reacceleration Optimization Algorithm Based on the Momentum Method for Image Recognition

The optimization algorithm plays a crucial role in image recognition by neural networks. However, it is challenging to accelerate the model’s convergence and maintain high precision. As a commonly used stochastic gradient descent optimization algorithm, the momentum method requires many epochs to find the optimal parameters during model training. The velocity of its gradient descent depends solely on the historical gradients and is not subject to random fluctuations. To address this issue, an optimization algorithm to enhance the gradient descent velocity, i.e., the momentum reacceleration gradient descent (MRGD), is proposed. The algorithm utilizes the point division of the current momentum and the gradient relationship, multiplying it with the gradient. It can adjust the update rate and step size of the parameters based on the gradient descent state, so as to achieve faster convergence and higher precision in training the deep learning model. The effectiveness of this method is further proven by applying the reacceleration mechanism to the Adam optimizer, resulting in the MRGDAdam algorithm. We verify both algorithms using multiple image classification datasets, and the experimental results show that the proposed optimization algorithm enables the model to achieve higher recognition accuracy over a small number of training epochs, as well as speeding up model implementation. This study provides new ideas and expansions for future optimizer research.

Mechanism analysis of the vortex ring and its effects on an axial descending rotor

A vortex ring is a flow phenomenon involving a complex toroidal vortex system. When a rotor is in descending flight, it may enter the vortex ring state (VRS), which will affect the rotor’s aerodynamic characteristics and even endanger it. In this paper, to clarify the aerodynamic mechanism by which the vortex ring exerts its effects on an axial descending rotor, an unsteady numerical simulation method for the rotor integrated with structured moving overset grids is proposed and validated using experimental data. This numerical simulation method is then applied to analyze the aerodynamic characteristics of the rotor in descending flight. The variations of the aerodynamic forces and the flow characteristics in the slipstream are analyzed to elucidate the physical mechanisms responsible for the relationships between the aerodynamic loads, flow field, and vortices when the rotor is in the VRS. The effects of the VRS cause a sharp drop in the average aerodynamic forces, which directly affects the safety and reliability of a rotor aircraft. In the slipstream of the descending rotor, a distinct vortex ring forms and moves upward as the velocity of descent increases. The most severe VRS occurs at a nondimensional velocity of descent of 1.13 when the center of the highly unsteady vortex ring is right at the blade tip, and this can be used as an indicator of the VRS. The physical mechanism by which the VRS exerts its effects can be attributed to the low pressure induced by strong vortices. The VRS decreases the pressure on the lower surface of the blade and increases the pressure on the upper surface, resulting in a reduction in the aerodynamic loads. In comparison with the hovering state, the VRS results in a much larger vortex strength and possesses a different vortex structure.

Computational investigations of Ka-226T helicopter coaxial rotor aerodynamics in the vortex ring state

The vortex ring states are observed when the rotor is flowed at positive angles of attack. For the main rotor, these conditions are realized during a steep descent of the helicopter at low speeds. The rotor vortex ring states are affected by significant phenomena related to the behavior of its aerodynamics, including negative phenomena. The latter is, firstly, referred to decrease in rotor thrust, increase in the required power, pulsations of thrust and torque, unsteady flapping blade motion, etc. In terms of helicopter piloting, it means a sharp loss of altitude, increase in the control force, a high level of vibration, defocusing attenuation of rotor spinning cone, as well as controllability deterioration. All these factors determine the relevance of research on these modes and the importance of solving a problem of defining their boundaries. Recently, due to the rapid development of computer technologies and computational models, it has become practical to perform numerical research of the rotor aerodynamics in vortex ring states. The paper presents the study results of Ka-226T helicopter coaxial rotor aerodynamics in steep descent modes in the field of vortex ring states. The angles of rotor attack αВ = 90...30° and the range of vertical descent velocities Vу = 0...26 m/s are considered. The original nonlinear bladed vortex model of the rotor developed at the Moscow Aviation Institute (MAI) was used. The total and distributed aerodynamic rotor characteristics were calculated. The shapes of the vortex wake and the rotor flow patterns were analyzed. The boundaries of the vortex ring states in velocity coordinates “Vx − Vу” were constructed according to various criteria reflecting the known features of these states. The results obtained significantly complement the existing experience of experimental and numerical research in this field.

Investigation of the Effect of Physical Ability on the Fall Mitigation Motion Using the Combination of Experiment and Simulation

The simulation of fall plays a critical role in estimating injuries caused by fall. However, implementing human fall mitigation motions on a simulator proves challenging due to the complexity and variability of fall movement. Our simulator estimates fall motion by extrapolating the motion observed in fall experiments. By incorporating actual fall motion data for the upper limbs, we enhanced the realism of the fall simulation. The application of forward dynamics control to the lower limbs allowed for the adjustment of mitigation motions, taking into account individual physical capabilities. In this study, fall simulations were conducted under the constraints of maximum joint torque and maximum torque change rate, emulating the physical capabilities of both the elderly and young adults. Our results successfully demonstrated the mitigation motion facilitated by the stance leg reduced the descent velocity of the center of mass by 0.75 m/s for elderly individuals and by 1.25 m/s for young adults, compared to a zero torque condition. This indicates that our study introduced a novel method for quantifying the impact of the lower limbs’ physical capabilities on fall velocity. Such a method represents a significant advancement in understanding how mitigation motions can influence fall injury simulations.

Safety Analysis of Initial Separation Phase for AUV Deployment of Mission Payloads

This study verifies the effects of deployment parameters on the safe separation of Autonomous Underwater Vehicles (AUVs) and mission payloads. The initial separation phase is meticulously modeled based on computational fluid dynamics (CFD) simulations employing the cubic constitutive Shear Stress Transport (SST) k-ω turbulence model and overset grid technologies. This phase is characterized by a 6-degree-of-freedom (6-DOF) framework incorporating Dynamic Fluid-Body Interaction (DFBI), supported by empirical validation. The SST k-ω turbulence model demonstrates superior performance in managing flows characterized by adverse pressure gradients and separation. DFBI entails computationally modeling fluid–solid interactions during motion or deformation. The utilization of overset grids presents several advantages, including enhanced computational efficiency by concentrating computational resources solely on regions of interest, simplified handling of intricate geometries and moving bodies, and adaptability in adjusting grids to accommodate changing simulation conditions. This research analyzes mission payloads’ trajectories and attitude adjustments after release from AUVs under various cruising speeds and initial release dynamics, such as descent and angular velocities. Additionally, this study evaluates the effects of varying ocean currents at different depths on separation safety. Results indicate that the interaction between AUVs and mission payloads during separation increases under higher navigational speeds, reducing the separation speed and degrading the stability. As the initial drop velocities increase, fast transition through the AUV’s immediate flow field promotes separation. The core of this process is the initial pitch angle management upon deployment. Optimizing initial pitching angular velocity prolongs the time for mission payloads to reach their maximum pitch angle, thus decreasing horizontal displacement and improving separation safety. Deploying AUVs at greater depths alleviates the influence of ocean currents, thereby reducing disturbances during payload separation.

A comprehensive heat transfer prediction model for tubular moving bed heat exchangers using CFD-DEM: Validation and sensitivity analysis

This study established a complete heat transfer prediction model that integrates four heat transfer sub-models, including contact heat conduction, gas film heat conduction, heat radiation, and heat convection, along with an artificial softening correction based on combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The model’s precision was confirmed by experimental results with relative errors below 5 %. The discussion on the heat flux contributions of the above four heat transfer mechanisms shows that with an initial particle temperature of 200℃, tube wall temperature of 30℃, and particle descent velocity of 1.3 mm/s, gas film conduction heat flux is the primary contributor to the total tube wall heat flux at 60.5 %, followed by radiation heat flux (19.5 %), contact conduction heat flux (12.1 %), and convection heat flux (7.9 %). Besides, the presentation of physical fields related to particles and the fluid showcases the model’s capability in elucidating the heat and mass transfer characteristics of the particle–fluid-wall system within tubular MBHEs. Furthermore, a local nominal range sensitivity analysis, along with a study of factors’ influence rules, was performed to quantify the effects of the 8 key model parameters on heat transfer results. The analysis indicates that fluid thermal conductivity is the predominant factor with its nondimensionalized first-order sensitivity coefficient of 54.9 %, succeeded by particle-to-wall angle factor (15.4 %), wall emissivity (14.4 %), gas film thickness (9.6 %), wall thermal conductivity (2.3 %), wall roughness (-1.6 %), particle Young’s modulus (-1.4 %), and particle specific heat capacity (0.4 %). Finally, their impacts on the heat prediction results were presented in detail.

Effects of Wave Parameters on Load Reduction Performance for Amphibious Aircraft with V-Hydrofoil

An investigation of the influence of the hydrofoil on load reduction performance during an amphibious aircraft landing on still and wavy water is conducted by solving the unsteady Reynolds-averaged Navier–Stokes equations coupled with the standard k−ω turbulence model in this paper. During the simulations, the numerical wave tank is realized by using the velocity-inlet boundary wave maker coupled with damping wave elimination techniques on the outlet, while the volume of fluid model is employed to track the water–air interface. Subsequently, the effects of geometric parameters of hydrofoil have been first discussed on still water, which indicates the primary factor influencing the load reduction is the static load coefficient of hydrofoil. Furthermore, the effects of descent velocity, wave length, and wave height on load reduction are comprehensively investigated. The results show that the vertical load reduces by more than 55% at the early stage of landing on the still water by assembling the hydrofoil for different descent velocity cases. Meanwhile, for amphibious aircraft with high forward velocity, the bottom of the fuselage will come into close contact with the first wave when landing in crest position, and then the forebody will impact the next wave surface with extreme force. In this circumstance, the load reduction rate decreases to around 30%, which will entail a further decline with the increase in wave length or wave height.

Maple samara flight is robust to morphological perturbation and united by a classic drag model

Winged, autorotating seeds from the genus Acer, have been the subject of study for botanists and aerodynamicists for decades. Despite this attention and the relative simplicity of these winged seeds, there are still considerable gaps in our understanding of how samara dynamics are informed by morphological features. Additionally, questions remain regarding the robustness of their dynamics to morphological alterations such as mass change by moisture or area change by damage. We here challenge the conventional approach of using wing-loading correlations and instead demonstrate the superiority of a classical aerodynamic model. Using allometry, we determine why some species deviate from interspecific aerodynamic behavior. We alter samara mass and wing area and measure corresponding changes to descent velocity, rotation rate, and coning angle, thereby demonstrating their remarkable ability to autorotate despite significant morphological alteration. Samaras endure mass changes greater than 100% while maintaining descent velocity changes of less than 15%, and are thus robust to changes in mass by moisture or damage. Additionally, samaras withstand up to a 40% reduction in wing area before losing their ability to autorotate, with the largest wings more robust to ablation. Thus, samaras are also robust to wing damage in their environment, a fact children joyfully exploit.

Propagation Velocity of Excitation Waves Caused by Turbidity Currents

Turbidity currents are important carriers for transporting terrestrial sediment into the deep sea, facilitating the transfer of matter and energy between land and the deep sea. Previous studies have suggested that turbidity currents can exhibit high velocities during their movement in submarine canyons. However, the maximum vertical descent velocity of high-concentration turbid water simulating turbidity currents does not exceed 1 m/s, which does not support the understanding that turbidity currents can reach speeds of over twenty meters per second in submarine canyons. During their movement, turbidity currents can compress and push the water ahead, generating propagating waves. These waves, known as excitation waves, exert a force on the seafloor, resuspending bottom sediments and potentially leading to the generation of secondary turbidity currents downstream. Therefore, the propagation distance of excitation waves is not the same as the initial journey of the turbidity currents, and the velocity of excitation waves within this journey has been mistakenly regarded as the velocity of the turbidity currents. Research on the propagation velocity of excitation waves is of great significance for understanding the sediment supply patterns of turbidity currents and the transport patterns of deep-sea sediments. In this study, numerical simulations were conducted to investigate the velocity of excitation waves induced by turbidity currents and to explore the factors that can affect their propagation velocity and amplitude. The relationship between the velocity and amplitude of excitation waves and different influencing factors was determined. The results indicate that the propagation velocity of excitation waves induced by turbidity currents is primarily determined by the water depth, and an expression (v2 = 0.63gh) for the propagation velocity of excitation waves is provided.

Effect of Archimedes number on the dynamics of free-falling perforated disks

The dynamics of perforated disks falling freely in a large expanse of viscous fluid at rest is investigated numerically. This complex fluid–structure interaction is solved via large eddy simulation. This numerical algorithm is verified and validated with available experimental results. The influence of Archimedes number expressing the ratio between the gravity-buoyancy and viscosity effects is discussed thoroughly, including kinematics and dynamics. Two critical Archimedes numbers are identified, Arcr1≈450 and Arcr2≈950, respectively. At these two critical Archimedes numbers, both kinematic and dynamic variables change trends. In this paper, we focus on the statistics of free-falling perforated disks. With the Archimedes number Ar increasing, the average angle of attack ⟨AoA⟩ and descent velocity ⟨Uz⟩ decrease gradually, and they arrive at a fixed value finally (here, ⟨·⟩ represents a time-average result); On the contrary, the other kinetic variables change violently when Ar is around 900, for example, terminal velocity ⟨Ut⟩. Additionally, phase differences of kinematic and dynamic variables are analyzed. A constant phase difference between the nutation angle θ and normal force FN is identified, about 66°, which is independent of Ar. Vortex structures are visualized using Q-criterion, and triangular vortex is omnipresent around holes. During the descent, a helical vortex always attaches to the perforated disk outer edge. With Ar increasing, complex vortex interaction appears, for example, merging and stretching. Some unusual behaviors in the numerical results are analyzed from the perspective of wake dynamics.

The Measurement Method of a Piston Fall Rate

The accurate determination of the piston fall rate is one of critical parameters in dead weight testers and piston gauges, as it confirms a quality of machining, instrument accuracy class, and long-term stability. This study introduces a method for determining piston fall rate using two triangulating laser distance sensors. This approach offers versatile applicability, in high-precision standards as well as in regular-class instruments, and is robust in accommodating variations in measurement ranges, pressure transmitting mediums, materials of weights, and weight diameters. The method utilizes two laser sensors symmetrically positioned under the primary weight of the piston pressure gauge, allowing for seamless measurements without the need for sensor adjustments. The data collected from these sensors are processed to calculate the average displacement of the primary weight. The method’s effectiveness is demonstrated with high linearity and precision in determining the piston weight fall rate. This approach can lead to improvements in fluid dynamics analysis and metrology of precise pressure balances. The method’s advantages include error mitigation and reduced operator intervention, making it highly suitable for applications requiring accurate piston descent velocity measurements, particularly in older measuring instruments. The establishment of a reference dataset can enhance the accuracy of periodic examinations of piston-cylinder assemblies, thereby reducing costs and improving measurement quality.

Insights into the flame transitions and flame stabilization mechanisms in a freely falling burning droplet encountering a co-flow

The present study investigates the flame dynamics of a contactless burning fuel droplet under free fall subjected to a co-flow. The dynamic external relative flow established due to co-flow and droplet acceleration results in a series of droplet flame transitions. Different flame structures were observed, including a wake flame, reversed wake flame and enveloped flame. Following ignition, the droplet is allowed to fall through the central tube of a co-flow arrangement, and, at its exit, the droplet flame encounters the co-flow. The wake flame, which was established based on the droplet's instantaneous velocity of descent, encounters the abrupt relative velocity jump due to the co-flow. This causes the droplet flame to go through various transitions as it approaches equilibrium with the surrounding flow. Once it equilibrates, the droplet flame evolves in response to the instantaneous relative flow velocity. The droplet flame evolves by altering both its shape and the stabilization mechanism. Two stabilization mechanisms were identified for the droplet wake flame: edge-flame stabilization and bluff-body stabilization. The stabilization mechanism for different flame structures and the transition events have been theoretically analysed, and the relation between flame shape evolution and flow velocity has been determined based on the flow-field characteristics at the corresponding Re (Reynolds number) range. Furthermore, these correlations are employed in a mathematical formulation based on the spring–mass analogy, which predicts the droplet flame evolution after encountering the co-flow, including all the transition events.

Parallel water entry: Experimental investigations of hydrophobic/hydrophilic spheres

This study aims to experimentally investigate the vertical parallel water entry of two identical spheres (in geometry and material) with different surface wettability (hydrophilic or hydrophobic) pairings. The spheres simultaneously impact the water surface with velocities ranging from 1.71 to 4.32 m s−1. The corresponding ranges of the impact Froude, Weber, and Reynolds numbers are 3.87–9.75, 816–5167, and 38.5×103 to 96.8×103, respectively. The spheres' lateral distances vary from 1.0 to 5.0 times the diameter. A high-speed photography system and image processing technique analyze the event dynamics, focusing on air-entrainment cavity behavior (shapes, closure, shedding), water flow features (Worthington jets, splashes), and sphere kinetics. Results for hydrophobic/hydrophobic cases show that even at the maximum lateral distance, a slightly asymmetric cavity forms, but deep-seal pinching occurs at a single point, similar to a single water entry scenario. As the lateral distance decreases, the spheres significantly influence each other's behavior, leading to the formation of a highly asymmetric air cavity and an oblique Worthington jet. In the case of a hydrophobic/hydrophilic pairing, vortices generated behind the hydrophilic sphere influence the air cavity development of the hydrophobic sphere. This can cause a secondary pinch-off, especially at low lateral distances. This effect becomes more pronounced at higher impact velocities. Additionally, at higher impact velocities and minimum lateral distance (direct contact between the spheres), a smaller cavity detaches from the hydrophobic sphere's cavity, attaches to the hydrophilic sphere, and moves with it. These different regimes result in varying descent velocities for the spheres.

Fluid Flow Regimes Analysis on Drilling Fluid Circulation for Cuttings Lifting in Vertical Drilling Oil Wells

Fluid Flow during the circulation of the drilling fluid is an important parameter that affects the process of successful lifting of cuttings during drilling activities. Researcher analyzed the regimes of the drilling fluid flow in vertical drilling wells through the analysis of the velocity calculation of the drilling fluid in the pipeline at conditions in the Drillpipe (v DP) of 21.11791 fps and in drill collar of (v DC) is 42.98293 fps, the velocity of drilling fluid in the annulus between the Casing and the Drill pipe is 1.14993 fps, 1.373781 fps between Open Hole and Drill pipe, and 1.594947 fps between Open Hole and Drill Collar, the critical velocity of drilling fluid in the pipeline and in the annulus, Reynolds number in the pipeline is 15986.36, inside Drill Collar of 22807.21, and in the annular between the Casing and the Drill pipe is 2847.435, between Open Hole and Drill pipe 3037.264, and 2679.938 between Open Hole and Drill Collar, and the velocity at which the drill cutting drops when in the annulus in all three zones is 0.14914 fps. The results of the analysis of fluid flow that occurs are found that in the pipe there is a turbulent flow and in the annulus a laminar flow occurs. In addition, the results of the analysis of the comparison between the velocity of the drilling fluid in the annulus and greater than the velocity of descent of the cuttings show that the cuttings can be lifted by the drilling fluid successfully. This indicates that the fluid lifting process is successful.

Vortex ring state of a shrouded rotor: an experimental survey

The vortex ring state (VRS) is a flow condition typical of rotors operating in axial descent flight which may lead to large unpredictable thrust oscillations and possibly the loss of control of the rotorcraft. Despite the dangers associated with this flight condition, there is a distinct lack of detailed experimental data related to shrouded rotors operating in axial descent, which is of relevance owing to the number of novel unmanned aerial vehicles that incorporate this technology. This manuscript presents an experimental investigation designed to assess if and how the presence of the shroud affects the development of the vortex ring state. To this end, laser Doppler anemometry and particle image velocimetry were used to investigate the flow for a range of descent velocities and results were compared with those obtained without the shroud. Time-averaged data were used to assess the general structures of the flow fields, whilst statistical analysis of the velocity fluctuations and modal analysis of the velocity field using proper orthogonal decomposition highlighted the unsteady features of the flow. The investigation showed that shrouded rotors enter the VRS similarly to their isolated counterparts, and the presence of the shroud may be responsible for a slight delay of its onset.Graphical abstract

Analysis of the Descent Process and Multi-Objective Optimization Design of a Benthic Lander

The growing need for deep-sea biological research and environmental monitoring has expanded the demand for benthic landers. Compared with remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), benthic landers can reduce overall operation cost and also possess longer endurance. Configuring a suitable descent velocity is important for benthic lander designs, helping them avoid retrieval failure and improve sea trial efficiencies. In this study, an effective scheme for the configuration and optimization of a self-developed benthic lander was outlined. First, the structural characteristics of the benthic lander were analyzed, and then a dynamic model was established. Second, the hydrodynamic coefficients of the benthic lander during its descent process were calculated using computational fluid dynamics (CFD) methods. Third, the MATLAB Simulink simulation environment was used to solve the dynamic model, and then the multi-objective optimization algorithm was introduced for the optimization design. Finally, the model was validated based on sea trial data, which demonstrated that the designed configuration and optimization scheme were correct and efficient. Collectively, this work provides a useful reference for the rational configuration and practical application of benthic landers.