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The present study introduces a novel algorithm based on the homotopy analysis method (HAM) to efficiently solve the equation of motion of simply supported transversely and axially loaded double-beam systems. The original HAM was developed for single partial differential equations (PDEs); the current formulation applies to systems of PDEs. The system of PDEs is derived by modeling two prismatic beams interconnected by a nonlinear inner layer as Euler–Bernoulli beams. We employ the Bubnov–Galerkin technique to turn the PDEs' system into a system of ordinary differential equations that is further solved with the HAM. The flexibility and straightforwardness of the HAM in computing time-dependent components of the system's transverse deflection and natural frequencies, in conjunction with the observed fast convergence, offer a robust semi-analytical method for analyzing such systems. Finally, the transverse deflection is built through the modal superposition principle. Thanks to a judicious and high-flexibility selection of initial guesses and convergence control parameters, numerical examples confirm that at most six iterations are needed to achieve convergence, and the results are consistent with the selected benchmark cases. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical simulations based on an immersed boundary method are performed for the two-foil system with varying spacings, and a staggered arrangement with a spacing normalized by chord length (X d / D , Y d / D) = (2.0 , 2.5) is found to be the most favorable for energy harnessing with the efficiency enhancement of nearly 40% for the downstream foil. By scrutinizing the mean flow characteristics and the instantaneous wake dynamics, it is revealed that convective flow acceleration and vortex impingement in synchronized timing with foil motion are the two primary mechanisms that contribute to significant efficiency improvement. Based on the staggered arrangement for the two-foil system, a V-shape formation is further proposed for the multiple-foil system and verified with the highest efficiency of energy harnessing up to nearly twice that of a single foil. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical computations are carried out for two rigidly connected cylinders of unequal sizes undergoing vortex-induced vibrations (VIV) perpendicular to the free stream. Results are examined for Re = 250 and a fixed diameter ratio of d / D = 0.2. The VIV response of the system is investigated for various positions of the small cylinder, covering a fine grid of wide radial (r) and azimuthal (θ) ranges, relative to the origin of the main cylinder. It is shown that the structural dynamics and hydrodynamic forces are strongly dependent on the arrangements. Regions of VIV reduction and amplification are distinguished, and the highest and lowest oscillation amplitudes are, respectively, acquired at configurations of (r , θ) = (0.7 D , 90 °) and (r , θ) = (0.88 D , 130 °). A deeper analysis in terms of the wake topology and surface pressure is then provided for these two extreme cases, to figure out the underlying mechanisms that lead to such markedly distinct responses. For the former case, the shear layers from two cylinders intensely interact and amalgamate during the oscillation, setting off subsequent processes of shear layer reattachment and downflow that are responsible for the observed high-amplitude response, while for the latter case, the shear layers from the small cylinder are highly stretched and absent from direct interaction with that from the large cylinder, which is favorable for stabilizing the wake and maintaining the low-amplitude response. Proper orthogonal decomposition (POD) is further utilized to correlate the key features of the wake with the dominant coherent structures in the flow. [ABSTRACT FROM AUTHOR]

In this study, we conducted three-dimensional direct numerical simulations to investigate the impact of incoming turbulence on the flow dynamics behind a single main cylinder at Reynolds number of Re = 3900. The incoming turbulence is generated by ten side-by-side cylindrical grid bars, whose center-to-center distance T is set equal to 2D, where D is the diameter of both the grid bars and the main cylinder. It is found that in the wake of the grid bars, the non-Gaussian production region and the homogeneous decay region, which are, respectively, located at X / D < 4 and X / D > 4 , were separated by the development of turbulence intensity Ti. Compared to the uniform incoming flow, the hydrodynamic forces induced by the present incoming turbulence are markedly distinct in magnitude. Specifically, the time-averaged drag coefficient C ¯ d is increased by 29.4%, and more surprisingly, the root mean square value of the lift coefficient is increased by as much as 500%. Statistical analysis is then performed, in terms of the Reynolds stresses, mean field, and the turbulent wake visualization to show variations in the flow dynamics. Results indicate that the incoming turbulence could accelerate the turbulent transition of shear layers and lead to a contraction of the vortex formation region. In addition, the long streamwise streaky vortical structures leading to an increase in Reynolds stress are found in front of the cylinder surface, which illustrates that the effect of incoming turbulence has penetrated inside the front boundary layer. [ABSTRACT FROM AUTHOR]

Three-dimensional large-eddy simulations are carried out for flow past a cylinder beneath a deformable free surface at a fixed Reynolds number of Re = 7550. The results are examined for two Froude numbers of F r = 0.2 and 0.6 and a gap ratio of 0.4, to investigate the effects of the distortion of the free surface on the flow fields and hydrodynamics. At the low Froude number of F r = 0.2 , the deformation of the free surface is small with little influence on the wake characteristics, and an alternative vortex shedding modulated by the free surface is detected in the wake. As the Froude number increases to 0.6, intense interface distortion occurs, which can be divided into three different regions: a hydraulic jump in the region of overtopping, a well-defined long-wavelength wave generation region in the large-scale recirculation zone near the free surface, and a water level recovery slope further downstream. The sudden change in flow regime from locally supercritical to subcritical allows the occurrence of the hydraulic jump. The induced surface waves behind the cylinder are ascribed to the shedding process of three shear layers, two of which are separated from the cylinder surface and one from the free surface. In addition, a jet-like flow originated from the gap between the free surface and the top of the cylinder occurs, exerting a downward thrust on the cylinder and pushing the wake away from the free surface. The Kármán vortex shedding in the wake is suppressed due to the interruption of the jet-like flow. The fluctuations of the wake turbulence and hydrodynamic forces are also suppressed to a low level. [ABSTRACT FROM AUTHOR]

As a hydraulic lifting pipeline structure widely used in deep-sea oil, gas transportation, and sediment dredging projects, the pipeline configuration is related to the improvement of transportation efficiency and pipeline safety. Particularly, the bending section consumes the most energy and withstands severe erosion. Understanding and predicting the conveying characteristics of two-phase flow in bends is therefore crucial. In this study, CFD-DEM (computational fluid dynamics-discrete element method) simulation method is employed to calculate various cases, considering five parameters: pipeline bending radius and angle, conveying velocity, particle diameter, and concentration, to explore the influence of these parameters on pressure drop and erosion rate of pipeline and result in a data set of hundreds of cases. Based on this data set, seven machine learning models are trained to predict pressure drop and erosion rate, respectively. To enhance model accuracy, the stacking method in ensemble learning is employed to combine multiple models with good performance. Additionally, the Optuna and SHAP (SHapley Additive exPlanation) methods are utilized to optimize hyperparameters and explain the degree to which parameters impact the predictions. The result demonstrates that pressure drop is almost unaffected by bending radius, while erosion rate initially decreases and then increases with bending angle, and both increase with other parameters. Among the evaluated models, artificial neural network, XGBoost, and random forest all demonstrate high prediction accuracy. The stacking model further improves the accuracy, with mean absolute error improving by 21.7% and 32.2%, and the SHAP method demonstrated good interpretability, which is basically consistent with CFD-DEM results. [ABSTRACT FROM AUTHOR]

Quantum well (QW) diodes have the capability to function as a light-emitting diode or a photodiode and inherently feature a partial emission-detection spectral overlap. Therefore, QW diode can sense light emission from another diode sharing the same QW active region. In association with distributed Bragg reflection (DBR) technique, we here present a wavelength-division multiplexing (WDM) visible light communication (VLC) over a single channel by using vertical assembly of red, green, and blue (RGB) QW diodes. The identical QW diodes separately functioning as a transmitter and a receiver establish a wireless communications link. The DBRs enable the transmission of longer-wavelength photons or the reflection of shorter-wavelength photons, creating an optical bandpass filter in conjunction with emission-detection spectral overlap, effectively boosting the capacity of an initially single communication channel. Both the transmitter and the receiver can be switched freely by software, forming time-division multiplexing (TDM) wireless light communication system using single optical path. We unite TDM and WDM together to demonstrate real-time TDM multichannel bidirectional communication using the vertical integration of RGB QW diodes, offering the great potential to establish TDM-WDM VLC. [ABSTRACT FROM AUTHOR]

Emerging deep-ultraviolet (DUV) light communication with attractive features fulfills the increasing demand for novel wireless communication without electromagnetic interference or solar noise influence. Here, we propose, manufacture, and characterize a solar-blind full-duplex light communication system using 275-nm DUV light-emitting diodes (LEDs). Four DUV LEDs connected in series form a unit, and the transmitter consists of an array of three independent units, which can be synchronously controlled using a main processing unit to modulate the emitted light. A solar-blind filter is inserted in front of the receiver due to the lack of an ultraviolet photodetector. Both the transmitter and receiver are encapsulated together to manufacture a full-duplex DUV light communication system, which can independently transmit and receive data with a maximal communication distance of 7 m at the same time. All users within a 46 m2 area of the system have access to the DUV communication network through an integrated wireless module. Real-time video is demonstrated and different types of files could be shared. Based on Transmission Control Protocol/Internet Protocol (TCP/IP), the system is finally performed in sunlight with a transmission rate of 10 Mbps and a packet loss rate of 1.28%. [ABSTRACT FROM AUTHOR]

While it is challenging for a traditional propulsor to achieve a wide range of force profile manipulation and propulsion efficiency, nature provides a solution for a flapping foil such as that found in birds and turtles. In this paper, we introduce a deep reinforcement learning (DRL) algorithm with great potential for solving nonlinear systems during the simulation to achieve a self-learning posture adjustment for a flapping foil to effectively improve its thrust performance. With DRL, a brute-force search is first carried out to provide intuition about the optimal trajectories of the foil and also a database for the following case studies. We implement an episodic training strategy for intelligent agent learning using the DRL algorithm. To address a slow data generation issue in the computational fluid dynamics simulation, we introduce a multi-environment technique to accelerate data exchange between the environment and the agent. This method is capable of adaptively and automatically performing an optimal foil path planning to generate the maximum thrust under various scenarios and can even outperform the optimal cases designed by users. Numerical results demonstrate how the proposed DRL is powerful to achieve optimization and has great potential to solve a more complex problem in the field of fluid mechanics beyond human predictability. [ABSTRACT FROM AUTHOR]

Flow pattern and hydrodynamic characteristics of coarse particles in deep-sea hydraulic lifting pipes are simulated using a numerical approach developed by combining the computational fluid dynamics method with a discrete element method in the Euler–Lagrange framework. This paper examines the effects of feed concentration, two-phase flow initial mixture velocity, and particle gradation on the dynamic characteristics of particles and flow pattern in the pipe by validating the rationality of numerical simulation. The results demonstrate that particles are distributed from the pipe center to the pipe wall, while the lift force causes more particles to be distributed in the pipe center. Moreover, greater inertia makes large particles more concentrated in the center. Particle-induced turbulence modifies the lift force and causes particles to move from the pipe center to its wall. Due to the increasing trend of lift force, particles gather toward the center of the pipe at an increasing pace as the initial velocity increases. The greater the feed concentration, the more particles disperse toward the pipe wall due to the violent momentum exchange caused by the high concentration and the significantly altered lift force caused by the high turbulent kinetic energy resulting from a high concentration. From the particle gradation 1:1:1 to 1:6:1, the pressure drop decreases gradually as the reduction in small particles decreases the number of particles near the wall, and the frictional energy loss between the particles and the pipe wall decreases. [ABSTRACT FROM AUTHOR]

GaN-based devices have grown rapidly in recent decades, due to their important research value and application prospects. There is a desire to monolithically integrate different GaN devices into a single chip for the development of future optoelectronic systems with low power consumption. In addition to improved multifunctional performance, a miniature integrated system can result in a significant reduction in material costs, processing costs, and packaging costs. In view of such prospects, we propose monolithic, top-down approaches to build III-nitride transmitter, modulator, waveguide, beam splitter, receiver, and monitor as a single unit onto a conventional GaN-on-silicon wafer without involving regrowth or postgrowth doping. Data communication among these components is realized through light propagation, opening up horizons for GaN optoelectronic systems on a chip. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) is a key technology for 6G. Here, we propose, fabricate, and characterize a vertical-structure light-emitting diode (VSLED) to manage the dilemma of both power and speed for VLCs. Ultrathin VSLED architecture offers the unique features of decreasing the RC time constant for increasing modulation bandwidth and reducing confined optical modes inside the diode for enhancing light extraction. A 580-nm-thick VSLED with a dominant emission wavelength of 427.8 nm is implemented on a 2-in. metal-based bonded III-nitride-on-silicon wafer. Based on a bit-loading discrete multitone modulation scheme, we establish a visible light communication system using 1 × 1 mm2 VSLED, which can achieve a data transmission rate of 608 Mbps at a baud rate of 200 MBaud. [ABSTRACT FROM AUTHOR]