Rayleigh Brillouin Research Articles

A non-localized spatial–temporal constitutive relation in rarefied gas dynamics

Although the Boltzmann equation is instrumental in capturing the dynamics of rarefied gases, finding its solutions in engineering problems is challenging. Therefore, over the past century and a half, numerous partial differential equations based on a few macroscopic variables have been introduced. However, they not only have complicated forms but also cannot make satisfactory prediction when the Knudsen number is large. Here, we propose a non-localized spatial–temporal (NiST) constitutive relation for rarefied gas dynamics, where the stress/heat flux at time t and position x is determined by the velocity/temperature gradient in the nearby spatial–temporal coordinates, via convolution operators. Utilizing solutions of the Boltzmann equation for the Couette/Fourier/Poiseuille flow and the spontaneous Rayleigh–Brillouin scattering, we extract the universal parameters of non-locality as functions of the spatial and temporal Knudsen numbers. Subsequent validation through sound propagation and backward-facing step flow demonstrates that the NiST constitutive relation is capable of accurately forecasting rarefied gas flows across a broad spectrum of Knudsen numbers.

A Fast and Accurate Mapping Method for an OPGW Tower Based on Hybrid Distributed Optical Fiber Sensing.

The combination of the dark fiber in existing Optical Fiber Composite Overhead Ground Wire (OPGW) with Distributed Optical Fiber Sensing (DOFS) technology can be used to enable online monitoring and provide early warnings of anomalies in high-voltage transmission lines. Accurate mapping of the optical cable length to the geographic coordinates of actual towers is a key factor in achieving this goal. This paper discusses the principle of using a DOFS system for transmission line tower positioning and presents four available positioning features. To overcome the limitations of single physical parameter positioning, this paper presents a self-developed hybrid DOFS that simultaneously captures Rayleigh backscattering and Brillouin scattering signals. Several physical parameters, including temperature, strain, and vibration, are acquired synchronously. Through hybrid multi-parameter analysis, the rapid and accurate positioning of OPGW line towers is achieved. Experimental results have shown that the proposed method, based on the hybrid DOFS system, can locate up to 82 towers, while the traditional method could only identify 12. The hybrid system was able to complete 80% of the tension towers in 40 h. This paper presents a novel multi-parameter localization method that has the potential to significantly improve the efficiency and reliability of grid operation and maintenance.

Performance Enhancement in a Few-Mode Rayleigh-Brillouin Optical Time Domain Analysis System Using Pulse Coding and LMD Algorithm

Rayleigh Brillouin optical time domain analysis (BOTDA) uses the backscattered Rayleigh light generated in the fiber as the probe light, which has a lower detection light intensity compared to the BOTDA technique. As a result, its temperature-sensing technology suffers from a low signal-to-noise ratio (SNR) and severe sensing unreliability due to the influence of the low probe signal and high noise level. The pulse coding and LMD denoising method are applied to enhance the performance of the Brillouin frequency shift detection and temperature measurement. In this study, the mechanism of Rayleigh BOTDA based on a few-mode fiber (FMF) is investigated, the principles of the Golay code and local mean decomposition (LMD) algorithm are analyzed, and the experimental setup of the Rayleigh BOTDA system using an FMF is constructed to analyze the performance of the sensing system. Compared with a single pulse of 50 ns, the 32-bit Golay coding with a pulse width of 10 ns improves the spatial resolution to 1 m. Further enhanced by the LMD algorithm, the SNR and temperature measurement accuracy are increased by 5.5 dB and 1.05 °C, respectively. Finally, a spatial resolution of 1.12 m and a temperature measurement accuracy of 2.85 °C are achieved using a two-mode fiber with a length of 1 km.

Rayleigh and Brillouin self-heterodyne detection Brillouin optical time domain reflectometer system based on phase shift keying pulse encoding

Rayleigh and Brillouin self-heterodyne detection Brillouin optical time domain reflectometer system based on phase shift keying pulse encoding

Demonstration of a Rayleigh-Brillouin scattering spectrometer with a high spectral resolution for rapid gas temperature detection.

A novel, to the best of our knowledge, Rayleigh-Brillouin scattering (RBS) spectrometer based on a virtually imaged phased array (VIPA) with a high spectral resolution is proposed for rapid gas temperature detection. CO2 RBS spectra at gas pressure of 0.5-4 bar were acquired with a spectrum acquisition time of 10 s, and temperature inversion analysis was performed using TENTI S6 model. The root-mean-square error (RMSE) of the RBS profile fitting is less than 2.95%, and the maximum absolute error of temperature inversion is less than 2.45 K. Compared with traditional methods, this method has low RBS signal loss and short acquisition time without the frequency scanning process, which is more conducive to real-time detection applications.

On the use of consistent bias corrections to enhance the impact of AeolusLevel‐2B Rayleigh winds on National Oceanic and Atmospheric Administration global forecast skill

AbstractThe operational Aeolus Level‐2B (L2B) horizontal line‐of‐sight (HLOS) retrieved Rayleigh winds, produced by the European Space Agency (ESA), utilize European Centre for Medium‐Range Weather Forecasts (ECMWF) short‐term forecasts of temperature, pressure, and horizontal winds in the Rayleigh–Brillouin and M1 correction procedures. These model fields or backgrounds can contain ECMWF model‐specific errors, which may propagate to the retrieved Rayleigh winds. This study examines the sensitivity of the retrieved Rayleigh winds to the changes in the model backgrounds, and the potential benefit of using the same system, in this case the National Oceanic and Atmospheric Administration's Finite‐Volume Cubed Sphere Global Forecast System (FV3GFS), for both the corrections and the data assimilation and forecast procedures. It is shown that the differences in the model backgrounds (FV3GFS minus ECMWF) can propagate through the Level‐2B horizontal line‐of‐sight Rayleigh wind retrieval process, mainly the M1 correction, resulting in differences in the retrieved Rayleigh winds with mean and standard deviation of magnitude as large as 0.2 m·s−1. The differences reach up to 0.4, 0.6, and 0.7 m·s−1 for the 95th, 99th, and 99.5th percentiles of the sample distribution with maxima of ∼1.4 m·s−1. The numbers of the large differences for the combined lower and upper 5th, 1st, and 0.5th percentile pairs are ∼6,100, 1,220, and 610 between 2.5 and 25 km height globally per day respectively. The ESA‐disseminated Rayleigh wind product (based on the ECMWF corrections) already shows a significant positive impact on the FV3GFS global forecasts. In the observing system experiments performed, compared with the ESA Rayleigh winds, the use of the FV3GFS‐corrected Rayleigh winds lead to ∼0.5% more Rayleigh winds assimilated in the lower troposphere and show enhanced positive impact on FV3GFS forecasts at the day 1–10 range but limited to the Southern Hemisphere.

Torr-level, seedless, non-resonant velocity distribution function measurement with a dual-color, single-shot coherent Rayleigh–Brillouin scattering scheme

A two order of magnitude spectral acquisition improvement in velocity distribution function measurement is demonstrated with a novel single-shot, dual-color coherent Rayleigh–Brillouin scattering (CRBS) scheme. By performing this non-resonant and seedless spectral diagnostic technique, capable of obtaining a spectrum in ns, we demonstrate accurate temperature (ranging from 300 K to 500 K) and pressure (ranging from 760 Torr down to 1 Torr) measurement capabilities, for a variety of atomic, molecular and multispecies gases. This demonstrated gas thermodynamic characterization capability of the dual-color CRBS scheme over broad ranges of pressure and temperature for a variety of gases is anticipated to be of great interest to a plethora of fields, ranging from aerospace applications to low-temperature plasmas, providing with an accurate measurement of physical properties of neutral particles, for a range of gases.

Single-end hybrid Rayleigh Brillouin and Raman distributed fibre-optic sensing system

Single-end hybrid Rayleigh Brillouin and Raman distributed fibre-optic sensing system

Measurements of Rayleigh–Brillouin scattering spectra of Ar, N[formula omitted], and CH[formula omitted] at pressures up to 20 atm

This paper describes the spectral measurement of Rayleigh–Brillouin scattering (RBS) in Ar, N2, and CH4 at 300 K and pressures up to 20 atm, which is relevant for the application of filtered Rayleigh scattering (FRS) diagnostics at elevated pressure. The measurement scheme for RBS spectra was relatively novel, based on an air-spaced virtually imaged phased array (VIPA), capable of rapid acquisition at high resolution. However, the VIPA’s nonuniform dispersion, irregular instrument response function, and sensitivity to local ambient conditions challenged the reconstruction of high-fidelity lineshapes in frequency space. Nonuniform dispersion was addressed using the VIPA dispersion function, and sensitivity of the air-spaced optic to the refractive index of air was minimized by tracking local temperature and pressure during measurements. Irregular instrument response was remedied with a computational model of the VIPA’s optical field, and by deconvolution. A comparison of experimental lineshapes with model predictions by Tenti S6 yielded good agreement. In the kinetic and nominal transition regimes, local residuals were within 3% for Ar and N2, and 5–10% for CH4. Beyond the transition regime, experimental lineshapes continued to display good qualitative agreement, with frequency shifts of the Brillouin peaks representing the main deviation from model prediction. Artifacts in the experimental data from uncertainty and the deconvolution algorithm were identified and discussed. This work establishes an experimental foundation for future measurements of RBS spectra, which will assist in the development of FRS diagnostics of reacting flows at high pressure.

Airborne temperature profiling in the troposphere during daytime by lidar utilizing Rayleigh-Brillouin scattering.

The airborne measurement of a temperature profile from 10.5 km down towards ground (≈1.4km above sea level) during daytime by means of a lidar utilizing Rayleigh-Brillouin (RB) scattering is demonstrated for the first time, to our knowledge. The spectra of the scattered light were measured by tuning the laser (λ=354.9nm) over a 11 GHz frequency range with a step size of 250 MHz while using a Fabry-Perot interferometer as a spectral filter. The measurement took 14 min and was conducted over a remote area in Iceland with the ALADIN Airborne Demonstrator on-board the DLR Falcon aircraft. The temperature profile was derived by applying an analytical RB line shape model to the backscatter spectra, which were measured at different altitudes with a vertical resolution of 630 m. A comparison with temperature profiles from radiosonde observations and model temperatures shows reasonable agreement with biases of less than ±2K. Based on Poisson statistics, the random error of the derived temperatures is estimated to vary between 0.1 K and 0.4 K. The work provides insight into the possible realization of airborne lidar temperature profilers based on RB scattering.

High-spectral-resolution lidar for measuring tropospheric temperature profiles by means of Rayleigh-Brillouin scattering.

A novel high-spectral-resolution lidar receiver based on a Fizeau interferometer and a photomultiplier tube array for tropospheric temperature profiling is introduced. Compared to other temperature lidars, an imaging approach is used to resolve the entire Rayleigh-Brillouin (RB) spectrum without applying frequency scanning techniques. The functionality of the system is demonstrated by means of a nighttime measurement. Atmospheric temperature is retrieved from 4.0 km to 9.2 km by analyzing the measured RB spectra with the Tenti S6 line shape model. The systematic error of the retrieved temperatures is determined to be smaller than 3 K, and the corresponding random error varies between 1.7 K (4.0 km) and 2.3 K (9.2 km) for an observation time of 5 min and a vertical resolution of 0.3 km. Considering the short averaging time and the stable arrangement of the system, the suggested approach is also attractive for future airborne applications.

Sensitivity of Aeolus HLOS winds to temperature and pressure specification in the L2B processor

Abstract. The retrieval of wind from the first Doppler wind lidar of European Space Agency (ESA) launched in space in August 2018 is based on a series of corrections necessary to provide observations of a quality useful for numerical weather prediction (NWP). In this paper we examine the properties of the Rayleigh–Brillouin correction necessary for the retrieval of horizontal line-of-sight wind (HLOS) from a Fabry–Pérot interferometer. This correction is taking into account the atmospheric stratification, namely temperature and pressure information that are provided by a NWP model as suggested prior to launch. The main goal of the study is to evaluate the impact of errors in simulated atmospheric temperature and pressure information on the HLOS sensitivity by comparing the Integrated Forecast System (IFS) and Action de Recherche Petite Echelle Grande Echelle (ARPEGE) global model temperature and pressure short-term forecasts collocated with the Aeolus orbit. These errors are currently not taken into account in the computation of the HLOS error estimate since its contribution is believed to be small. This study largely confirms this statement to be a valid assumption, although it also shows that model errors could locally (i.e. jet-stream regions, below 700 hPa over both earth poles and in stratosphere) be significant. For future Aeolus follow-on missions this study suggests considering realistic estimations of errors in the HLOS retrieval algorithms, since this will lead to an improved estimation of the Rayleigh–Brillouin sensitivity uncertainty contributing to the HLOS error estimate and better exploitation of space lidar winds in NWP systems.

A multiple-relaxation-time collision model for nonequilibrium flows

Despite yielding correct hydrodynamics in the continuum limit, the Bhatnagar–Gross–Krook collision model is too simplistic to model the full details of the collision, which becomes increasingly important as the quasi-equilibrium assumption breaks down. In a recent phenomenological collision model, independent relaxation rates are assigned to the components of the tensorial Hermite expansion of the distribution corresponding to the irreducible representations of SO(3), yielding arguably the most general form of multirelaxation without violating rotation symmetry. Here we show that by using the relaxation rates obtained analytically from Boltzmann collision term with Maxwell molecular model, lattice Boltzmann method yields results in good agreement with the accurate fast spectral method in simulation of the spontaneous Rayleigh–Brillouin scattering problem. The hydrodynamically insignificant relaxation rates of the higher moments are found to be significant as the Knudsen number increases. These results suggest that with properly tuned relaxation rates, the collision model could potentially mimic the behavior of arbitrary collision kernels.

Improved method for gas temperature and pressure retrieval in Brillouin lidar remote sensing

The Rayleigh-Brillouin scattered spectrum is an important tool for analyzing the temperature and pressure of gas in Brillouin lidar remote sensing. The Tenti-S6 model has been widely used to retrieve atmospheric temperatures. However, the retrieval accuracy of this method is unsatisfactory. We analyzed the influence of several factors on the retrieval accuracy of this method and developed an improved method for temperature and pressure retrieval. First, the Rayleigh-Brillouin spectral baseline was corrected using a new fitting procedure, and an experimental spectrum that is of high coincidence with the line shape of the S6 model could subsequently be obtained. Second, the influence of the Airy function on the retrieval accuracy was analyzed, and the retrieval error could be decreased using the Tenti-S6 model without the Airy function. We found that the gas parameters could be precisely detected under low-pressure conditions. Compared with the traditional method, our improved method could effectively reduce the temperature and pressure retrieval errors. The experimental results of nitrogen scattering in the laboratory and air scattering demonstrate the effectiveness, universality, and viability of the proposed improved method.

Review of Filtered Rayleigh Scattering technique for mixing studies in supersonic air flow

Filtered Rayleigh Scattering or FRS is a non-intrusive laser diagnostic technique that is utilized to observe and quantify various flow properties in high-speed gas flows. In this technique, a laser beam is used to irradiate a portion of the gas under investigation. The receiving gas molecules scatter the incident laser beam in all the directions. The scattered radiation from a single molecule stems from the induced oscillations of the electron cloud that acts as a classical dipole radiator. The intensity of the net scattered radiation from all the molecules that are in random motion depend on the spatio-temporal fluctuations of the dielectric permittivity of the gas. The thermodynamic properties of the gas govern the nature of these variations and the resulting spectra of the scattered radiation is referred to as Rayleigh Brillouin spectrum. In high-speed gas flows applications, the bulk velocity of the gas molecules causes the entire scattered spectrum to be Doppler shifted from the lasing frequency. The extent of this shift is very significant in supersonic flows and this enables the isolation of the scattered spectrum from the laser reflections using a molecular notch filter. Measurement of flow properties can be made based on the intensity of the RBS signal that is collected by a charged coupled device. The FRS technique is also utilized to perform mixing measurements in binary mixture of gases in high-speed flows. In non-reacting mixing studies, helium is often used as a passive scalar and simulant gas for hydrogen fuel. Helium concentration measurements in supersonic flows based on the FRS technique require two independent experiments. The first experiment involves injection of helium gas and the second separate experiment requires air injection. The helium mole-fraction is retrieved from the FRS imaging data under the key assumptions that the total number density profiles and the extent of the Doppler shift that is associated with each one of the two independent experiments at the measuring plane are identically the same. In this review, the theory governing Rayleigh scattering and the FRS technique to retrieve helium mole-fraction in supersonic flow will be discussed in detail. Measurement results from recent FRS studies on complex vortex interactions in supersonic flow will be also presented and discussed as well as the impact on the results of a departure from the key assumptions in FRS mixing studies will be analyzed for a canonical parallel injection using strut injector in supersonic flow.

Simultaneous measurement of gas temperature and pressure based on Rayleigh–Brillouin​ scattering in kinetic region of less than one standard atmospheric pressure

In order to detect high-precision atmospheric temperature data and realize simultaneous detection of atmospheric temperature and pressure, a decoupling method for the envelope spectrum of Rayleigh–Brillouin scattering in kinetic region of less than one atmosphere is presented. The parameters of envelope spectrum are characterized by using an experimental setup with the frequency locking, continuous tunable cavity Fabry–Perot interferometer, temperature and pressure control technologies. The nitrogen with the purity of 99.99% and compressed air acted as the examples are putted into the gas cell. Combined with the characteristics of actually detecting atmospheric parameters, the correction method is adopted to eliminate the influence of residual Mie scattering on the envelope spectrum. By choosing four discrete points on the envelope spectrum of Rayleigh–Brillouin scattering, the gas temperature and pressure can be inversed. There is a nice linearity between the measured results and theoretical values.

Accuracy of high-order lattice Boltzmann method for non-equilibrium gas flow

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Research on Rayleigh Scattering and Brillouin Scattering of Bidirectional Wavelength Division Multiplexing System

In order to study the influence of Rayleigh and Brillouin scattering on the performance of bidirectional Wavelength Division Multiplexing (WDM) system. On the VPI simulation platform, a two-way WDM simulation system was built. Rayleigh scattering and Brillouin scattering were generated by setting the parameters of the optical fiber module and system parameters such as Brillouin Scattering, Rayleigh scattering, Rayleigh Distortion, and Rayleigh Distortions. Then, the frequency spectrum of the received signal and the bit error rate were observed and analysed. Experimental results show that Rayleigh and Brillouin scattering will cause the performance of the two-way WDM system to degrade. By separating the back propagation channel, the impact of these two effects on the performance of the two-way WDM system can be reduced.

Evaluation of the Tenti S6 model for hydrocarbon fuels at elevated temperatures using filtered Rayleigh scattering measurements.

This Letter targets the assessment of the well-known Tenti S6 model for predicting the Rayleigh-Brillouin scattering (RBS) spectra of select gas-phase hydrocarbon fuels (CH4, C2H2, C2H4, C3H8, and C4H10) over a temperature range of 300 to 700 K. The Tenti S6 model is evaluated by comparing filtered Rayleigh scattering (FRS) measurements to synthetic FRS signals generated from the combination of the Tenti S6 output and an accurate iodine absorption filter model. The experimental and synthetic FRS results agree very well (<3% difference) over the full temperature range for CH4, C2H2, and C2H4, indicating accurate calculation of the RBS spectra. For C3H8 and C4H10, there are some large differences between the experimental and synthetic FRS results which cannot be resolved through tuning of bulk viscosity, internal heat capacity, or inclusion of vibrational degrees of freedom, suggesting the need for detailed measurements of the Rayleigh-Brillouin spectra.

Extraction of the translational Eucken factor from light scattering by molecular gas

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