Levels Of Atmospheric Turbulence Research Articles

Evolutions of a ring Airy vortex beam and a ring Pearcey vortex beam in turbulent atmosphere and a comparative analysis of their channel efficiencies and OAM spectra

An optical vortex beam propagating through turbulent atmosphere encounters distortions in the wavefront that result in modal scattering. Abruptly autofocusing (AAF) beams with orbital angular momentum have gained significant attention due to their non-diffracting and self-healing nature. These warrant understanding of the behavior of these beams through turbulent atmosphere absolutely necessary. With this intuition, in the present work we investigate the behavior of two AAF beams, namely the ring Airy vortex beam (RAVB) and ring Pearcey vortex beam (RPVB) through the turbulent atmosphere in two cases—multiplexed and non-multiplexed. We propagate multiplexed as well as non-multiplexed RAVB and RPVB in different levels of turbulent atmosphere. In the non-multiplexed case, channel efficiency declines for both beams with increase in mode numbers. In the multiplexed case, increasing the gap between the mode sets results in a decrease in channel efficiency. We also report that in weak atmospheric turbulence RAVB outperforms RPVB in terms of channel efficiency. We use the optical transformation sorting (log-polar) method to demultiplex the optical beams at the output. Furthermore, we investigate and compare the orbital angular momentum (OAM) spectra of both beams in different levels of atmospheric turbulence and at different propagation distances. The comparison reveals that the spectra of RPVB are more dispersive as compared to those of RAVB.

Hollow Gaussian beam-based experimental investigation on echo measurements under atmospheric turbulence and central obstruction.

Despite their potential, Cassegrain systems face challenges due to central obstruction, resulting in reduced emitting efficiency. Optical vortex (OV) beams, known for their unique orbital angular momentum (OAM) characteristics, show promise in enhancing transmission efficiency. However, the existence of central phase singularity in OV beams may limits their effectiveness in laser ranging. In this work, a solid 532 nm laser and a spatial light modulator (SLM) are used to produce the OV and hollow Gaussian beam (HGB). Per the requirements of our experiments, the transmission characteristics of OVs in the Fresnel region have been thoroughly investigated. Based on the healing properties of HGBs, Experimental validation is conducted using HGBs and OV beams under varying atmospheric turbulence levels, and stronger echo signals with HGBs are realized. Additionally, a simplified model simulating central obstruction challenges in Cassegrain systems is developed, HGB shows the superior performance of HGBs in improving transmission efficiency and enhancing echo signals. This research provides valuable insights for optimizing Cassegrain antenna systems and advancing laser ranging technology and highlighting the potential of HGBs as a promising solution for overcoming central obstruction challenges and improving overall system performance.

Free space optical communication system in the presence of atmospheric losses

<span>Free space optical communication is gaining importance in the field of optical communication due to its high speed and high bandwidth applications. Free space optical communication system (FSOCS) provides many benefits as compared to traditional wireless communication system and fiber optic cables. This makes this technology the reasonable extension of metropolitan area network and also provides the quick recovery during natural disaster. This system performance is limited due to the atmospheric turbulence effect and various atmospheric losses such as rain, and fog. Gamma gamma atmospheric turbulent model is used to analyze the system performance in the presence of moderate to strong atmospheric turbulence. We have designed the FSO gamma <span>gamma turbulent model with non-return to zero (NRZ) modulation format employing wavelength division multiplexing (WDM), spatial diversity multiple input multiple output (MIMO) (8×8) at various atmospheric turbulence levels and attenuation loss of 10 dB/km at the distance of 2-4 km. Using the proposed model, the link distance is enhanced up to 4km in the presence of turbulence and atmospheric losses with minimum laser transmitted power.</span></span>

Performance analysis of the RoFSO system over a log-normal turbulent link

Abstract This study provides a comprehensive analysis of the design and performance evaluation of a radio over free space transmission link using Optisystem v.20 software. The research focuses on the integration of four transmission channels through the use of a multiplexer to achieve a transfer rate of 40 Gbps over a distance of 2 km, with each channel carrying 10 Gbps of data. This is achieved by modulating data on 20 GHz radio frequency signals, which are subsequently transmitted via laser waves at different frequencies. The performance of the proposed communication is assessed by evaluating its behavior in the presence of different levels of atmospheric turbulence, classified as light, medium, and strong. This analysis involves calculating based on metrics such as the Q-factor, bit error rate (BER), and log values (BER). The results showed that the Q factor decreased by (4.7) under the influence of a strong disturbance, while in contrast, it decreased by (0.2) under the influence of a weak disturbance. The results also indicated a minimum log BER (at −7.96) for the wavelength (193.3 THz) at 1.5 km. Besides, the best frequency for data transfer was (193.1 THz), followed by (193.4 THz).

Impact of probe volume and peak detection methods on lidar rotor effective wind speed and turbulence intensity estimations

Lidar simulation techniques are a suitable and increasingly reliable alternative for testing lidar measuring strategies and illustrating their response when combined with modelled wind fields. In this work, two simulation tools are combined to assess the uncertainty in the derivation of the rotor effective wind speed and the wind speed variance from a forward-looking nacelle-mounted continuous wave lidar wind speed estimations. These uncertainties are analysed for a variety of atmospheric turbulence levels and lidar measuring strategies. A synthetic turbulence generator is used to create the reference wind fields. Subsequently, a lidar simulator operated in a continuous-wave mode is used to scan the synthetic wind fields and perform a sensitivity analysis by comparing first- and second-order statistics against reference values. The lidar simulator is enhanced with three Doppler peak detection methods, namely the maximum, the median and the centroid, to extract radial wind speeds from the velocities found within the probe volume. The results show that probe volume and peak detection methods influence the uncertainty of the wind speed variance. The uncertainty in time-averaged and instantaneous rotor effective wind speed estimations is not sensitive to the lidar spatial averaging or peak detection methods investigated. Finally, we saw that the turbulence intensity influences the derived lidar quantities and is the main driver of the variations in rotor effective wind speed uncertainty estimations.

Modeling of optical wave propagation through turbulent atmosphere using fractional approach for FSO wireless communication

Free space optical (FSO) wireless communication has emerged as a viable alternative to the existing fiber optics and radio frequency (RF) communications due to its ability to operate in unlicensed spectrum, offering huge data handling capacities and being cost effective with easy deployment. The underlying communication mechanism in the FSO link is based upon the laser beam propagation model. The propagating free space optical beam undergoes signal degradation due to refraction and diffraction caused by atmospheric turbulence measured by refractive index structure parameter (C n 2). In this work, a novel, robust analytical model for propagating Gaussian-beam through atmospheric turbulence is presented, by solving the space-fractional paraxial wave equation. We report analytical expressions for the intensity and long-term beam spreading of a Gaussian beam in terms of space-fractional parameter D, for the range 2 < D ≤ 3. This range of parameter D, defines the effective number of euclidean space corresponding to atmospheric turbulence levels faced by the propagating Gaussian beam in the FSO link. The results of the proposed fractional model and the existing models agreed well, therefore the D-dimensional parameter can be used to effectively express the value of refractive index structure parameter (C n 2). The classical values of C n 2 ranges form 10−13 to 10−16 for strong to weak fluctuations respectively. However, in fractional-dimension scale, these fluctuations can be express as D = 2.668 (strong fluctuations) to D = 2.999 (weak fluctuations). The ideal case of free space beam propagation can be expressed as D = 3, with no turbulence. Moreover, we have studied the fractional-dimension model performance for varying wavelengths. Further, based upon a practical example, we proposed a self-sustainable FSO link architecture to efficiently minimize the effect of weak and strong fluctuations on the propagating optical beam, based upon the D-dimension fractional parameter, hence ensuring reliability of FSO link.

Settling and clustering of snow particles in atmospheric turbulence

The effect of turbulence on snow precipitation is not incorporated into present weather forecasting models. Here we show evidence that turbulence is in fact a key influence on both fall speed and spatial distribution of settling snow. We consider three snowfall events under vastly different levels of atmospheric turbulence. We characterize the size and morphology of the snow particles, and we simultaneously image their velocity, acceleration and relative concentration over vertical planes approximately$30\ \textrm {m}^2$in area. We find that turbulence-driven settling enhancement explains otherwise contradictory trends between the particle size and velocity. The estimates of the Stokes number and the correlation between vertical velocity and local concentration are consistent with the view that the enhanced settling is rooted in the preferential sweeping mechanism. When the snow vertical velocity is large compared to the characteristic turbulence velocity, the crossing trajectories effect results in strong accelerations. When the conditions of preferential sweeping are met, the concentration field is highly non-uniform and clustering appears over a wide range of scales. These clusters, identified for the first time in a naturally occurring flow, display the signature features seen in canonical settings: power-law size distribution, fractal-like shape, vertical elongation and large fall speed that increases with the cluster size. These findings demonstrate that the fundamental phenomenology of particle-laden turbulence can be leveraged towards a better predictive understanding of snow precipitation and ground snow accumulation. They also demonstrate how environmental flows can be used to investigate dispersed multiphase flows at Reynolds numbers not accessible in laboratory experiments or numerical simulations.

BER performance analysis of FSO using hybrid-SIM technique with APD receiver over weak and strong turbulence channels

Abstract In this paper, a hybrid-subcarrier-intensity-modulation (hybrid-SIM) technique for the performance improvement of free-space-optical (FSO) communication system has been proposed. Subsequently, for further error performance improvement, avalanche photodiode (APD) based receiver is used in the proposed system. The system performance is analyzed at various atmospheric turbulence levels over weak and strong turbulence channels. The bit error rate (BER) is theoretically derived using Gauss–Hermite approximation and Meijer-G function and it is simulated in the MATLAB environment. The simulation result shows that the BER performance of hybrid-SIM is better than BPSK-SIM technique irrespective of the channel types and also the significant BER performance improvement is observed by APD receiver.

EXPERIMENTAL STUDY OF THE INFLUENCE OF ATMOSPHERIC TURBULENCE ON THE BOUNDARY LAYER FLOW ON THE GLIDER WING

Flight experiments aimed at studying the transition from the laminar to turbulent flow in the boundary layer on the glider wing at different levels of atmospheric turbulence performed in the 1980s are reviewed. The results are obtained by means of hot-wire measurements and visualization of the flow on the wing surface by the method of sublimating coatings. The transition is found to proceed in several stages: emergence, evolution, and failure of a discrete packet of instability waves in the region of the adverse pressure gradient. The data obtained in the study are compared with the results of similar investigations carried out in a large wind tunnel on the same real glider wing at realistic Reynolds numbers.

Adaptive low density parity check encoder for a complete free space optics/continuous variable-quantum key distribution system using commercially available off-the-shelf devices for variable throughput network considering dynamical atmospheric turbulence levels

In this paper, an adaptive LDPC encoder for complete FSO/CV-QKD system using a COTS device for emulated dynamical atmospheric turbulence levels is presented. The experimental and emulation results show the maximum and minimal final secret key rates of 105 Kbps and 10 Kbps, respectively, for minimal and maximal throughput in a commercial network, 30 Mbps and 90 Mbps, respectively. Our proposal presents an adequate performance for weak and moderate atmospheric turbulence levels and a suitable option for improve the using of QKD systems.

BER analysis and capacity evaluation of FSO system using hybrid subcarrier intensity modulation with receiver spatial diversity over log-normal and gamma–gamma channel model

In this work, the performance of free space optical system is evaluated in terms of bit error rate (BER) and capacity using PPM–BPSK–SIM (hybrid-SIM) modulation scheme. Addition to that, we have applied spatial diversity technique for the proposed system. We have calculated the BER and the capacity at various atmospheric turbulence levels over the log-normal and gamma–gamma channel models. The analysis has been carried out at different signal-to-noise-ratio (SNR) values and link distances. The simulation results show that hybrid-SIM modulation scheme achieves better BER performance than BPSK–SIM scheme in both the channel models; furthermore, the BER performance gets improved significantly by spatial diversity technique.

Nacelle-based Lidar Measurements for the Calibration of a Wake Model at Different Offshore Operating Conditions

Commonly, wake models are calibrated in wind tunnels or using flow simulations with a wide degree of physical details. In general, it is assumed that these methods cannot fully reproduce the real operating conditions of wind turbines. This research aims at investigating the calibration of an analytical single wake model in relation to full-scale measurements. Within this scope, we fitted the wake model to wake measurements realised with a lidar installed on the nacelle of an offshore wind turbine. We studied the parameters returned by the fit separating cases at different levels of atmospheric turbulence and thrust on the wind turbine rotor. Comparing the results with a published calibration based on few LES wind fields representative for partial load conditions, we achieved good agreement when the considered wind turbines operated in similar conditions. For other situations, i.e. at full load, we found different calibrations of the model parameters. Our results show that and how nacelle-based lidar measurements can be complementary in the development of wake models.

Modeling of gas adsorption by aerosol plumes emitted from industrial sources

Abstract Adsorption of trace atmospheric gases by aerosol particles contributes to the evolution of concentration distribution of the trace constituents and can affect subsequent chemical reactions in the atmosphere. We suggest a two dimensional model of adsorption of trace atmospheric constituents by aerosol particles in air pollution plume emitted from an industrial source. The model is based on an application of theory of turbulent diffusion in the atmospheric boundary layer (ABL) in conjunction with plume dispersion model and model of gas adsorption by porous solid particles. The wind velocity profiles used in the simulations were fitted from our data previously obtained in field measurements conducted in the Northern Negev (Israel) using the experimental wind mast. The developed model allows analyzing spatial and temporal evolution of adsorbate concentration in the gaseous phase as well as in the particulate matter. The adsorbate concentration distributions are calculated for the particulate matter PM2.5–10, which is typical for industrial emissions. Analysis is performed for different meteorological conditions and atmospheric stability classes. It is shown that the concentration of gases adsorbed by aerosol plume strongly depends on the level of atmospheric turbulence. The obtained results are compared with the available experimental data.

Wind-Tunnel Experiments on Vortex-Induced Vibration of Rough Bridge Cables

Surface roughness, ice accretion, and atmospheric turbulence are important issues because they can considerably alter the aerodynamic characteristics of bridge cables. The present study describes wind-tunnel experiments on vortex-induced vibrations of rough bridge cables characterized with various surface roughness and performed at various levels of atmospheric turbulence. Three different cable models were used: a smooth cylinder to represent dry cables, a helical strand cable, and a cable with modeled ice accretion. The studied parameters include the critical velocity and amplitude of vortex-induced vibration and parameter sensitivity to changes in flow turbulence and cable surface roughness. The results obtained indicate that cable roughness and turbulence of the flow have significant influences on the dynamic response of bridge cables. The experimental results indicate that the vortex-induced vibration was largest for the ice-accreted bridge cable.

High-power lasers for directed-energy applications: comment.

Sprangle etal. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] recently concluded that our experiments on coherent combining of laser beams over an atmospheric path [Opt. Lett.36, 4455 (2011)OPLEDP0146-959210.1364/OL.36.004455] were "effective only because at these low-power levels the linewidth of the lasers was very narrow… and the level of atmospheric turbulence was low…." These conclusions are inaccurate, not relevant to practical high-power coherently combined laser systems, and contradict our most recent experiments with coherent combining of 21 laser beams with a linewidth of about 1GHz over 7km distance. In this comment we also challenge the major conclusion of Sprangle etal. [Appl. Opt.54, F201 (2015)APOPAI0003-693510.1364/AO.54.00F201] and the more recently published paper by Nelson etal. [Appl. Opt.55, 1757 (2016)APOPAI0003-693510.1364/AO.55.001757] regarding inefficiency of coherent beam combining under typical atmospheric conditions.

The cost of energy associated with micro wind generation: International case studies of rural and urban installations

National targets for increased renewable energy are common-place internationally and small/micro-generation may help achieve such goals. Energy yields from such technologies however, are very location and site specific. In rural environments, the average wind speed is relatively high and the homogeneous landscape promotes laminar air flow and stable (relatively) wind direction. In urban environments however, the wind resource has lower mean wind speeds and increased levels of atmospheric turbulence due to heterogeneous surface forms. This paper discusses the associated costs per unit of electricity generated by micro wind energy conversion systems from the perspective of both urban and rural locations, with three case studies that consider the potential and financial viability for such systems. The case studies ascertain the cost of energy associated with a standard HAWT (horizontal axis wind turbine), in terms of exemplar rural and urban locations. Sri Lanka, Ireland and the UK, are prioritised as countries that have progressive, conservative and ambitious goals respectively towards the integration of micro-generation. LCOE (Levelized cost of energy) analyses in this regard, offers a contextualised viability assessment that is applicable in decision making relating to economic incentive application or in the determination of suitable feed-in tariff rates.

Passive optical methods in measurement of the structure parameter of the air refractive index

A passive optical method for estimating the level of atmospheric turbulence (structure parameter of the air refractive index Cn2) from the image jitter is considered. It is proposed to use a high-speed digital video camera and effective two-dimensional parallel algorithms of image processing for measuring Cn2 in real time. Results obtained by the active and passive optical methods, as well as by an acoustic method, are compared.

An example of enhanced tephra deposition driven by topographically induced atmospheric turbulence

Spatial variations in the thickness and grain-size characteristics of tephra fall deposits imply that tephra depositional processes cannot be fully captured by models of single-particle sedimentation from the base of the eruption plume. Here, we document a secondary thickness maximum in a ∼9.75 ka tephra fall deposit from Chaitén volcano, Chile (Cha1 eruption). This secondary thickness maximum is notably coarser-grained than documented historical examples, being dominated by medium-grained ash, and an origin via particle aggregation is therefore unlikely. In the region of secondary thickening, we propose that high levels of atmospheric turbulence accelerated particles held within the mid- to lower-troposphere (0 to ∼6 km) towards the ground surface. We suggest that this enhancement in vertical atmospheric mixing was driven by the breaking of lee waves, generated by winds passing over elevated topography beneath the eruption plume. Lower atmospheric circulation patterns may exert a significant control on the dispersal and deposition of tephra from eruption plumes across all spatial scales, particularly in areas of complex topography.

Optical key distribution system using atmospheric turbulence as the randomness generating function: classical optical protocol for information assurance

We describe an experimental laboratory system that generates and distributes random binary sequence bit streams between two optical terminals (labeled Alice and Bob). The random binary sequence is generated through probing the optical channel of a turbulent atmosphere between the two terminals with coincident laser beams. The two laser beams experience differential phase delays while propagating through the atmospheric optical channel. The differential phase delays are detected and sampled at each terminal to yield raw random bit streams. The random bit streams are processed to remove bit errors and, through privacy amplification, to yield a bit stream known only to Alice and Bob. The same chaotic physical mechanism that provides randomness also provides confidentiality. The laboratory system yielded secret key bit rates of a few bits/second . For external optical channels over longer channel lengths with atmospheric turbulence levels, secret bit rates of 10 s of bits/second are predicted.

Coherence of a Bessel beam in a turbulent atmosphere

Features of the formation of an optical Bessel beam in a turbulent atmosphere are studied on the basis of a solution of an equation for the second-order mutual coherence function of the field of a Bessel optical beam. A condition has been derived for the mean intensity of a Bessel beam, which is a quantitative criterion for the formation of a beam along a long path in a turbulent atmosphere. The behavior of the degree of coherence of an optical Bessel beam versus beam and turbulent atmosphere parameters has been studied. It was found that the degree of coherence of a Bessel beam typically oscillates at low levels of fluctuations in a turbulent atmosphere, and the number of maxima and minima unambiguously depends on the Bessel beam parameter (the wave vector component normal to the radiation propagation direction) at the same level of atmospheric turbulence. At high levels of fluctuations in a turbulent atmosphere, the degree of coherence of a Bessel beam is described by a single-scale dropping curve that approaches the similar characteristic curve of a spherical wave, with an increase in the level of fluctuations along the path of laser beam formation.