Search Results (104)

Atmospheric and oceanic turbulence can severely degrade the orbital angular momentum (OAM) mode purity of vortex beams in cross-media optical links. Here, we propose a hybrid correction framework that fuses multiscale phase-screen modeling with a lightweight U-Net predictor for phase-distortion—driven solely by measured optical intensity—and augments it with a feed-forward, Gaussian-reference subtraction scheme for iterative compensation. In our experiments, this approach boosts the l = 3 mode purity from 38.4% to 98.1%. Compared to the Gerchberg–Saxton algorithm, the Gaussian-reference feed-forward method achieves far lower computational complexity and greater robustness, making real-time phase recovery feasible for OAM-based communications over heterogeneous channels. Full article

LiDAR-assisted wind turbine control holds strong potential for reducing structural loads and improving rotor speed regulation, thereby contributing to more sustainable wind energy generation. However, key research gaps remain: (i) the practical limitations of commercially available fixed beam LiDARs for large turbines, and (ii) the performance assessment of commonly used LiDAR assisted feedforward control methods. This study addresses these gaps by (i) analysing how the coherence of LiDAR estimated rotor effective wind speed is influenced by the number of beams, measurement locations, and turbulence box resolution, and (ii) comparing two established control strategies. Numerical simulations show that applying a low cut-off frequency in the low-pass filter can impair preview time compensation. This is particularly critical for large turbines, where reduced coherence due to fewer beams undermines the effectiveness of LiDAR assisted control compared to the smaller turbines. The subsequent evaluation of control strategies shows that the Schlipf method offers greater robustness and consistent load reduction, regardless of the feedback control design. In contrast, the Bossanyi method, which uses the current blade pitch measurements, performs well when paired with carefully tuned baseline controllers. However, using the actual pitch angle in the feedforward pitch rate calculation can lead to increased excitation at certain frequencies, particularly if the feedback controller is not well tuned to avoid dynamics in those ranges. Full article

Atmospheric turbulence introduces distortions to the wavefront of propagating optical radiation. It causes image resolution degradation in astronomical telescopes and significantly reduces the power density of radiation on the target in focusing applications. The impact of turbulence fluctuations on the wavefront can be investigated under laboratory conditions using either a fan heater (roughly tuned), a phase plate, or a deformable mirror (finely tuned) as a turbulence-generation device and a wavefront sensor as a wavefront-distortion measurement device. We designed and developed a software simulator and an experimental setup for the reconstruction of atmospheric turbulence-phase fluctuations as well as an adaptive optical system for the compensation of induced aberrations. Both systems use two 60 mm, 92-channel, bimorph deformable mirrors and two tip-tilt correctors. The wavefront is measured using a high-speed Shack–Hartmann wavefront sensor based on an industrial CMOS camera. The system was able to achieve a 500 Hz correction frame rate, and the amplitude of aberrations decreased from 2.6 μm to 0.3 μm during the correction procedure. The use of the tip-tilt corrector allowed a decrease in the focal spot centroid jitter range of 2–3 times from ±26.5 μm and ±24 μm up to ±11.5 μm and ±5.5 μm. Full article

We propose an empirical motion compensation algorithm for a better turbulence intensity (TI) measurement by Floating LiDAR systems (FLSs) with a newly introduced motion parameter, the significant tilt angle ${\theta }_{\alpha ,1/3}$, using four datasets from three different FLSs in Japan. The parameter was compared to other environmental parameters; it was confirmed to well represent various types of buoy motion. A sensitivity assessment was conducted for the error of the FLS’s standard deviation of wind speed to the buoy motion. The strong correlation obtained by the assessment suggests that the error of the FLS TI is dominated by the motion and that it is possible to offset the error by applying the relationship back to the measurement. The corrected TI shows good agreement with that of a reference fixed vertical LiDAR (VL). Moreover, the similarity of the relationships for the same type of VL mounted on different buoys implies that the correction may be VL-specific rather than FLS-specific, and, therefore, universal regardless of the FLS type. The successful validation suggests that the correction based on ${\theta }_{\alpha ,1/3}$ can be applied not only to the future campaign but also to those performed in the past to revitalize numerous existing FLS datasets. Full article

The objective of this work is to study the equilibrium stability of a switched linear model with time-delayed control and additive disturbances, that in subsidiary represents the control of wing vibrations in the presence of the turbulence disturbances in an aerodynamic tunnel. The state system is modeled as a collection of subsystems, each corresponding to different levels of air velocity in the wind tunnel. The problem is closely related to the gain scheduling approach for stable control synthesis and to the design of stable, switched systems with time-delay control. A state-predictive feedback method is employed to compensate for actuator delay, resulting in closed-loop free delay switching systems both in presence and absence of disturbances. The main contribution of this study is a thorough analysis of system stability in the presence of disturbances. Finally, numerical simulation results are provided to support and complement the findings. Full article

With the growing demand for high-efficiency and secure information transmission, ultra-long-range optical communication has demonstrated significant potential. This paper proposes a method for establishing ground-based fixed-point ultra-long-range atmospheric optical communication links, aiming to overcome challenges such as atmospheric turbulence, transmission loss, and environmental interference. Through theoretical analysis and experimental validation, we developed a high-precision optical communication terminal installation model, determined the terminal’s optical axis direction via stellar calibration, and established a coordinate transformation model from geodetic coordinates to initial pointing angles. By analyzing initial pointing errors, we designed a laser link scanning strategy to compensate for uncertainties in the initial pointing region. The feasibility of this approach was verified through near-field validation and a long-distance link acquisition experiment exceeding 100 km. Experimental results demonstrated successful 100 km/100 Gbps ultra-high-speed optical communication. This breakthrough study is expected to play a critical role in future space-localized optical communication networks. Full article

Simulating atmospheric turbulence phase screens is essential for optical system research and turbulence compensation. Traditional methods, such as multi-harmonic power spectrum inversion and Zernike polynomial fitting, often suffer from sampling errors and limited diversity. To overcome these challenges, this paper proposes an improved denoising diffusion probabilistic model (DDPM) for generating high-fidelity atmospheric turbulence phase screens. The model effectively captures the statistical distribution of turbulence phase screens using small training datasets. A refined loss function incorporating the structure function enhances accuracy. Additionally, a self-attention module strengthens the model’s ability to learn phase screen features. The experimental results demonstrate that the proposed approach significantly reduces the Fréchet Inception Distance (FID) from 154.45 to 59.80, with the mean loss stabilizing around 0.1 after 50,000 iterations. The generated phase screens exhibit high precision and diversity, providing an efficient and adaptable solution for atmospheric turbulence simulation. Full article

Atmospheric refraction is a significant challenge to accurate distance and angle measurements in open-air environments, often limiting the precision of measurements obtained using electro-optic geodetic instruments despite their nominal accuracies. This study introduces a novel model, 3D-RM, designed to mitigate atmospheric effects on both distance and vertical angle measurements. The 3D-RM integrates in situ meteorological data from a network of automatic data-loggers, terrain information from a digital terrain model (DTM), and sensible heat flux from the fifth generation of European Centre for Medium-Range Weather Forecast reanalysis (ERA5), which is used in the application of the Turbulence Transfer Model (TTM) for estimating vertical refractivity gradients at various height levels. The model was tested with total station observations to 10 target points during two field campaigns. The results show that applying the model for distance correction leads to improvements in terms of closeness to reference values when compared to the standard method, which relies only on meteorological data collected at the station. Furthermore, the model has been additionally tested by removing the station meteorological data (3D-RM2). The results demonstrate that accurate corrections can be obtained even without the need of meteorological sensors specifically installed at the station point, which makes it more flexible. The 3D-RM is a cost-effective and relatively easy-to-implement solution, offering a promising alternative to existing methodologies, such as measuring meteorological values at both station and target points or the development of new instruments that can compensate the refractivity (such as a multiple-color electronic distance meter). Full article

A novel approach to underwater optical wireless coherent communications using liquid crystal spatial light modulators (LC-SLMs) and an aperture averaging lens, in combination with optical phased-array (OPA) antennas, is presented. A comprehensive channel model that includes a wide range of underwater properties, including absorption, scattering, and turbulence effects, is employed to simulate the underwater optical wireless communication (UOWC) system in a realistic manner. The proposed system concept utilizes aperture averaging and adaptive optics techniques to mitigate the degrading effects of turbulence. Additionally, OPA antennas are integrated into the system to provide electronic beam steering capabilities, facilitating precise pointing, acquisition, and tracking (PAT) between mobile underwater vehicles. This integration enables high-speed and reliable communication links by maintaining optimal alignment. The numerical results show that under strong turbulence, our combined turbulence-compensation approach (LC-SLM plus aperture averaging) can extend the communication range by approximately threefold compared to a baseline system without compensation. For instance, at a soft-decision FEC threshold of $1.25\times {10}^{-2}$, the maximum achievable link distance increases from around $10\mathrm{m}$ to over $30\mathrm{m}$. Moreover, the scintillation index is reduced by more than $90\%$, and the bit error rate (BER) improves. Full article

Hydroelectric power plants are widely used around the world, particularly in the countries of Central and South America. In Russia, there are more than 15 large hydroelectric power plants, which form the backbone of the country’s energy sector, providing about 20% of its energy needs. The construction and operation of these plants take a long time, and it is important to plan carefully and minimize environmental damage during their use. The most significant factors affecting the environmental condition of reservoirs is the low oxygen content and the impossibility of water self-purification due to low water turbulence in deep layers. Coastal erosion caused by large hydroelectric dams can lead to increased land and population destruction, as well as sedimentation in reservoirs. The objective of this review was to select a method that would enhance the quality of water in the reservoirs of hydroelectric power stations. The technical solution that has been proposed is the implementation of the aeration of the reservoir and the cleaning of the aquatorium from sediments, with the aim of compensating for the damage caused by the construction of the dam. Full article

This paper presents the results of processing and systematizing data from laboratory and numerical experiments to correct the distortions of laser radiation propagating through a randomly inhomogeneous medium. The information obtained allows for an accurate estimation of the effectiveness of compensating for turbulent fluctuations in the refractive index along the beam propagation path. Experimentally derived radiation parameters were compared with corresponding values calculated using an optical system model. Limitations of the numerical simulation, stemming from inherent model constraints, were identified, as were discrepancies between experimental and theoretical results, and a reason for these discrepancies was provided. Full article

Smoke simulation is a crucial yet challenging aspect of constructing ship fire scenarios. For the Eulerian smoke simulation method, the low-resolution grid results in a loss of smoke detail, while the high-resolution grid faces significant computational costs. To address this issue, a detail enhancement approach is proposed for smoke simulation in ship fire scenarios based on vortex particles, aiming at high-realism smoke simulation on a low-resolution grid. The simulation domain is first discretized using a low-resolution grid to compute the basic flow. Next, the vortex particles are sampled within the grid, and the loss of vorticity is measured before and after vortex stretching to compensate for the missing smoke details. In our approach, a geometric method is employed to efficiently capture the stretching of vortex structures. The computational results demonstrate that turbulence details can be effectively captured in a low-resolution grid while maintaining the real-time performance of the simulation. The practical application value of our approach is demonstrated in improving the realism of ship fire scenarios. Full article

This paper focuses on a typical pressure-controlled slide valve, utilizing momentum analysis and computational fluid dynamics to simulate and analyze the asymmetry of steady flow force curves under bidirectional throttling patterns. The entropy production theory is employed to reveal the causes of nonlinearity in the steady flow force of an inlet throttling slide valve. Based on flow field analysis, a flow force compensation scheme is proposed by adding a guiding shoulder and matching it with a suitably sized inner annular cavity. The study reveals that fluid momentum at the non-throttling valve port is the primary cause of the bidirectional throttling flow force difference, and under large-opening inlet throttling conditions, it may reverse the direction of the flow force. Vortex separation caused by turbulent pulsations is one of the intrinsic reasons for the nonlinearity of steady flow force. Full article

Free Space Optical (FSO) communication is extensively utilized in the telecommunication industry for both ground and space wireless links, as well as last-mile applications, as a result of its lesser Bit Error Rate (BER), free spectrum, and easy relocation. However, atmospheric turbulence, also known as Wavefront Aberration (WA), is considered a serious issue because it causes higher BER and affects coupling efficiency. In order to address this issue, a Sensor-Less Adaptive Optics (SLAO) system is developed for FSO to enhance performance. In this research, the compensation of WA in SLAO is obtained by proposing the Brownian motion and Directional mutation scheme-based Coati Optimization Algorithm, BDCOA. Here, the BDCOA is developed to search for an optimum control signal value of actuators in Deformable Mirror (DM). The incorporated Brownian motion and directional mutation are used to avoid the local optimum issue and enhance search space efficiency while searching for the control signal. Therefore, the dynamic control signal optimization for DM using BDCOA helps to enhance the coupling efficiency. Thus, the WAs are compensated for and optical signal concentration is enhanced in FSO. The metrics used for analyzing the BDCOA are Root Mean Square (RMS), BER, coupling efficiency, and Strehl Ratio (SR). The existing methods, such as Simulated Annealing (SA) and Stochastic Parallel Gradient Descent (SPGD), Advanced Multi-Feedback SPGD (AMFSPGD), and Oppositional-Breeding Artificial Fish Swarm (OBAFS), are used for evaluating the performance of BDCOA. The RMS of BDCOA for iterations 500 is 0.12, which is less than that of the SA-SPGD and OBAFS. Full article

Offshore wind turbine (WT) wake interference will reduce power generation and increase the fatigue loads of downstream WTs. Wake interference detection based on aeroacoustic noise is believed to solve these challenges in offshore wind farms. However, aeroacoustic noise is closely related to the aerodynamics around WT blades, and the acoustic detection method requires the mastery of noise emission characteristics. In this paper, FAST.Farm, combined with the acoustic model in OpenFAST, is utilized to investigate the acoustic noise emission characteristics from two 3.4 MW-130 WTs with wake interference. Multi-microphone positions were investigated for the optimal reception selection under 8 m/s and 12 m/s wind speeds with a typical offshore atmospheric turbulence intensity of 6%. The numerical simulation results indicate that wake deficit reduces the total noise emission by about 6 dBA in the overall sound pressure level (OASPL) at 8 m/s, while wake turbulence marginally increases it and its fluctuation. There is a mutual influence between these effects, and the wake deficit effect can be 100% compensated for in the OASPL at 12 m/s. Additionally, downstream observer locations are suggested based on comparisons. These investigations provide new insights into wake interference in offshore wind farms. Full article