A Variational Adjoint Approach on Wide-Angle Parabolic Equation for Refractivity Inversion

A comprehensive study on the anomalous propagation (AP) conditions occurring over the central and west African stations was made from 2 years (January 2005–December 2006) high-resolution data measured by GPS (Global Positioning System) radio survey observations. Through data quality control and diagnostic analysis, the probability of AP occurrence and characteristic quantities of the three typical anomalous propagation conditions were given. The sub-refraction, super-refraction and ducting cases were investigated statistically using the vertical profile of modified refractivity gradient. Strong diurnal variation in the percentage occurrence of the AP had its peak during the wet months, while the dry months had the lowest values. From 0600 to 1800 local time (LT) at day (1800–0600 LT at night), the total percentage occurrence of super-refraction, sub-refraction and ducting were 82.5% (78.5%), 11% (15.5%) and 6.5% (6%), respectively. Besides statistical results, local meteorological conditions prevailing over central and west Africa have also been discussed.

THE EFFECTS OF ATMOSPHERIC REFRACTIVITY ON MICROWAVE PROPAGATION

The authors review the technique of phase noise stability measurement using complex demodulation and the approach to an unlocked spread spectrum communications probe. This system has recently been developed to characterize a transhorizon communications channel at 11.6 GHz under both tropospheric scatter and anomalous propagation conditions. The receiver has a bandwidth of 31.25 MHz but has no carrier recovery and therefore no absolute phase reference. However, provided that the phase jitter and the frequency drift are small enough during the channel measurement period, it is possible to obtain an accurate channel transmittance estimate. The system has the capability of implementing time-domain signal averaging on the fly, thus permitting high-speed channel identification under low SNR regimes. As the averaging period is increased (to improve the SNR), low phase jitter in the oscillators becomes imperative. The SNR improvement achieved is thus controlled by the maximum averaging period, which is a direct reflection of the stability of the oscillators. >

Refractivity is an important factor in predicting the performance of radio link. The structures of the refractive index of the troposphere are responsible for many complicated mechanisms such as, multipath effects, absorption, scattering of radio signals, etc. and these mechanisms can cause a lot of propagation impairment such as, loss of signal for mobile communication, the accuracy of tracking radio source (such as stars) with radio telescopes, radar system etc. Understanding the effects of the weather on radio wave propagation will help to improve the quality of radio signals, as the Weather variables are useful ingredients in the predictions of refractivity distributions and the anomalous radio wave propagation conditions of the atmosphere. This study contends that refractivity variations significantly affects received signal strength at Ultra High Frequency (UHF) and gave a brief theoretical background on Electromagnetic wave, Atmospheric effects on radio propagation anomalous propagation condition and the mathematical expression for refractive index and refractivity of the troposphere. Also, the paper review related articles in the following area; refractive index and refractivity of the troposphere, effects of weather variables and refractivity of troposphere. The analysis of the review articles summits that, refractivity reveals seasonal variations with high value in rainy season and low values in dry season. Also observed from the review articles, is that the diurnal variations are influenced by three atmospheric parameters; water vapour has the greatest effect on refractivity variations followed by temperature and pressure. Furthermore, the Refractivity is function of local meteorological parameters. Eventually, the study suggests that, the effect of Atmospheric parameters and consequently the Radio Refractivity should be compensated for when designing wireless communication links in an environment.

The operation is described of a diagnostic system designed to measure accurately the angle-of-arrival of tropospheric microwave signals at X-band frequencies in situations typical of terrestrial communications links; relative amplitudes, angles-of-arrival and relative path delays under multipath conditions are also accessible using the system. The results presented here indicate that, under single-path conditions, significant variations in angle-of-arrival in the vertical plane occur regularly and, further, that in many instances these variations exhibit a wave-like character.

The states of the Earth surface in terms of high-latitude freeze and thaw (FT) cycles significantly impact many physical applications that include biogeochemical transitions, hydrological phenomena, and ecosystem evolution. We have shown that land surface emissivity estimates have great potential for use in the detection of FT states since that parameter primarily depends on surface characteristics instead of on direct use of brightness temperatures. This study aims to investigate the potential of merging passive microwave sensors and their land surface emissivity estimates from Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E), Special Sensor Microwave Imager (SSM/I), AMSR2, and the Global Precipitation Measurement (GPM) Microwave Imager (GMI) to provide high temporal resolution (sub-daily) FT states. This factor is of critical importance and usage, primarily during the transitions between freeze and thaw that frequently occur at sub-daily time-frames in spring seasons. Data fusion techinques were used to construct diurnal estimates in order to accurately predicting the exact time of the freeze-thaw transition for a variety of land cover types and geographical regions. The results revealed that emissivity difference values between low and high frequencies (such as 10.7 GHz and 89GHz) at horizontal polarization from multiple platforms have a strong correlation with ground-based soil temperature diurnal values at 5-cm depth. Evaluation of the proposed approach with independent ground observations from year 2015 to 2017 showed that the data fusion of land surface emissivities in high-latitudes was able to notably capture the frequent FT transitions.

In light of recent interest in adjoint modelling in underwater acoustics, we present a selective review which is mainly focused on the underlying concept of backpropagation. The different implementations of that concept to date are compared and discussed in the framework of experimental acoustic inversion in shallow water with application to source localisation, ocean acoustic tomography, geoacoustic inversion and underwater communications. Well established inversion or focalisation methods based on matched field processing, model-based matched filter and time reversal mirror are related to less popular ones such as acoustic retrogation and other variants of backpropagation. In contrast to the latter, adjoint-based, variational inversion approaches make use of the adjoint of a forward model to backpropagate the model-data mismatch at the receiver toward the source. The paper describes in greater detail adjoint methods and applications in underwater acoustics. We also present results using environmental data obtained during a geoacoustic inversion experiment in the Mediterranean. © 2006 Springer.

In this paper, we propose a new approach to four-dimensional variational data assimilation (4DVar) with analysis of the initial condition (IC) at the end of assimilation window. The new approach is referred to as “backward 4DVar (B-4DVar).” The minimization of its cost function is fulfilled through an ensemble of historical prediction samples. B-4DVar is computationally efficient because it does not use tangent linear or adjoint models. To prepare it for numerical weather prediction, a B-4DVar assimilation system is developed based on an operational regional prediction model, the Advanced Regional Eta Model (AREM). Two single observation experiments (SOEs) and an observing system simulation experiment (OSSE) are conducted to evaluate the system. The SOEs reveal a characteristic of flow dependence in B-4DVar, which is consistent with statistical estimation of the background error covariance matrix (simply B-matrix) using a group of IC-reliant historical prediction samples. The OSSE suggests that the B-4DVar approach improves the analytic quality of IC by effectively incorporating conventional observations, thereby outperforming the 3DVar. The time-saving feature, flow dependence in B-matrix, and good performance in assimilating conventional observations indicate the potential and feasibility of B-4DVar in operational use.

In the context of designing a next-generation ultrasonic polar scan (UPS) measurement system for viscoelastic material characterization, a novel approach is proposed, which draws on a set of cylindrically focused emitters in conjunction with a circular phased array (C-PA) receiver in order to create a portable measurement system while improving the data quality and ease the data interpretation. To explore the potential of the new approach and determine its optimal design parameters, a 3-D analytical model is presented to numerically simulate UPS experiments with the proposed system. Furthermore, a postprocessing procedure is worked out to treat the acquired raw data with the aim to deal with the integrating effect of finite size transducers and directly reconstruct the angle-dependent plane wave reflection coefficients of the sample under study. As the accuracy of the reconstruction heavily depends on various design parameters, a parameter study focusing on the influence of three main experimental parameters is performed to guide the optimal design. For each of these parameter studies, the UPS simulation results have been inverted, and the errors on the estimated C-tensor parameters have been deduced. First, it is shown that, for a given frequency, the radius of the C-PA must be large enough to capture both the specular and nonspecular reflected field, which is crucial to assure a correct reconstruction of the plane wave characteristics and find proper estimates of the C-tensor parameters. Second, the impact of the emitter and receiver lengths on the quality of the reconstruction and the C-tensor parameters has been investigated, yielding superior results upon increasing either of them. Finally, a dedicated study of the pitch of the C-PA elements and the angular range of the cylindrically focused emitters shows that aliasing effects disturb the results if the pitch is too large. However, this effect can somewhat be mitigated by employing multiple emitters with a restricted angular range. Using the knowledge of the abovementioned parameter studies, a simulated UPS experiment using a proper set of design parameters is performed for a cross-ply carbon epoxy laminate. The postprocessed reconstruction based on these data shows an excellent agreement with the theoretical plane wave results.

Performance of Fiber Reinforced High-strength Concrete with Steel Sandwich Composite System as Blast Mitigation Panel

Artificial neural networks (ANNs) are an emerging field of research and have proven to have significant potential for use in structural engineering. In previous literature, many studies successfully utilized ANNs to analyze the structures under different loading conditions and verified the accuracy of the approach. Several studies investigated the use of ANNs to analyze the shear behavior of reinforced concrete (RC) members. However, few studies have focused on the potential use of an ANN for analysis of the torsional behavior of an RC member. Torsion is a complex problem and modeling the torsional fracture mechanism using the traditional analytical approach is problematic. Recent studies show that the nonlinear behavior of RC members under torsion can be modeled using ANNs. This paper presents a comprehensive analytical and parametric study of the torsional response of RC beams using ANNs. The ANN model was trained and validated against an experimental database of 159 RC beams reported in the literature. The results were compared with the predictions of design codes. The results show that ANNs can effectively model the torsional behavior of RC beams. The parametric study presented in this paper provides greater insight into the torsional resistance mechanism of RC beams and its characteristic parameters.

Nanoswimmers are important because of their potential use for the purpose of drug delivery, monitoring, and diagnostics for in vivo biomedical application. They mimic microorganisms and mostly modeled as propelled by beating or rotating flagella. In the vast literature available on modeling of flagellar propulsion, the flagellum is considered constant diameter where as the actual microorganism have tapered flagella. The present study deals with the modeling and simulation of planar wave propagation through a tapered flagellum of a nanoswimmer for a given taper ratio. The performance parameters of velocity and efficiency are compared with the uniform diameter case. Taper diameter modeling of flagellum gives a superior performance by indicating higher velocity and efficiency. The parametric study with respect to the elasticity of the material of the flagellum and its beat frequency is also analyzed. The performance increases with increasing frequency, and the maxima for both velocity and efficiency of the tapered case is reached at a lower elasticity than the uniform case. The maximum efficiency of the taper case is almost double the maxima of the uniform diameter flagellum. The observations in the present study can be utilized while designing artificial flagellum for nanoswimmers and suggests that the tapered flagellum would be a more optimal choice of design as compared to the uniform one of the same volume.

Abstract. Reconstructing the thermo-chemical evolution of Earth's mantle and its diverse surface manifestations is a widely recognised grand challenge for the geosciences. It requires the creation of a digital twin: a digital representation of Earth's mantle across space and time that is compatible with available observational constraints on the mantle's structure, dynamics and evolution. This has led geodynamicists to explore adjoint-based approaches that reformulate mantle convection modelling as an inverse problem, in which unknown model parameters can be optimised to fit available observational data. Whilst there has been a notable increase in the use of adjoint-based methods in geodynamics, the theoretical and practical challenges of deriving, implementing and validating adjoint systems for large-scale, non-linear, time-dependent problems, such as global mantle flow, has hindered their broader use. Here, we present the Geoscientific ADjoint Optimisation PlaTform (G-ADOPT), an advanced computational modelling framework that overcomes these challenges for coupled, non-linear, time-dependent systems by integrating three main components: (i) Firedrake, an automated system for the solution of partial differential equations using the finite-element method; (ii) Dolfin-Adjoint, which automatically generates discrete adjoint models in a form compatible with Firedrake; and (iii) the Rapid Optimisation Library, ROL, an efficient large-scale optimisation toolkit; G-ADOPT enables the application of adjoint methods across geophysical continua, showcased herein for geodynamics. Through two sets of synthetic experiments, we demonstrate the application of this framework to the initial condition problem of mantle convection, in both square and annular geometries, for both isoviscous and non-linear rheologies. We confirm the validity of the gradient computations underpinning the adjoint approach, for all cases, through second-order Taylor remainder convergence tests and subsequently demonstrate excellent recovery of the unknown initial conditions. Moreover, we show that the framework achieves theoretical computational efficiency. Taken together, this confirms the suitability of G-ADOPT for reconstructing the evolution of Earth's mantle in space and time. The framework overcomes the significant theoretical and practical challenges of generating adjoint models and will allow the community to move from idealised forward models to data-driven simulations that rigorously account for observational constraints and their uncertainties using an inverse approach.

Rice stalk fiber and rice husk ash are agricultural by-products, which are available worldwide in large quantities. There is, however, no information about suitability of the rice stalk fiber and rice husk ash combination for the production of fiber-cement composites in the open literature. This paper presents a parametric experimental study, which investigates the potential use of rice stalk fiber (as reinforcement) and rice husk ash (as cement replacement) admixtures for producing a lightweight fiber-cement composite as a building material. Three levels of fibrous materials, namely 10, 25 and 40 wt% were mixed with 0, 10 and 20 wt% of rice husk ash. Effects of these variable parameters on the mechanical (modulus of rupture, modulus of elasticity and internal bond) and physical properties (water absorption and thickness swelling) of the samples were studied. The results showed the effect of high-level replacement of rice stalk fiber with rice husk ash does not exhibit a sudden brittle fracture even beyond the failure loads, indicating high energy absorption capacity of the composites. Based on the findings of this work, the water absorption and thickness swelling of the composites increased with the increasing rice stalk fiber content in the samples from 10 to 40 wt%. On the other hand, modulus of rupture and modulus of elasticity of the boards were enhanced with increase in the percentage of rice stalk fiber. Boards having 25 wt% rice stalk fiber exhibited the highest internal bond strength. However, addition of rice stalk fiber and rice husk ash reduced the internal bond strength. Moreover, boards made with rice stalk fiber had superior properties compared to boards made with a mixture of hardwood fibers.

Several meteorological conditions are known to cause anomalous propagation (AP) of microwave radiation. The effect of AP in weather radar measurements my be important as spurious echoes from distant ground targets may appear as precipitation leading to wrong rainfall estimations. AP may also affect dramatically the quality of clear air radar observations. In this study, more than one hundred radiosonde ascents are examined to evaluate the occurrence of AP at the coastal site of Barcelona (Spain). Temperature and humidity profiles are used to calculate refractivity gradients and to estimate the existence of ducting layers. Ducts represent the worst case of super refraction and within them microwaves travel trapped like in a waveguide. To detect thin AP features a vertical resolution higher than that given by standard operational radiosonde data is desirable. For this reason, radiosonde data recorded every 10 s have been used. Results are compared against standard operational radiosonde analysis revealing a significantly higher number of AP layers. The output of a mesoscale numerical weather prediction model is also used to derive refractivity gradients. The ability of the model to simulate the propagation conditions is overviewed in order to assess the feasibility of an operational diagnostic AP product.